"uuid","repository link","title","author","contributor","publication year","abstract","subject topic","language","publication type","publisher","isbn","issn","patent","patent status","bibliographic note","access restriction","embargo date","faculty","department","research group","programme","project","coordinates"
"uuid:a992351f-c72e-4b7f-9162-f625eed0dcdd","http://resolver.tudelft.nl/uuid:a992351f-c72e-4b7f-9162-f625eed0dcdd","Radar remote sensing of wind vector and turbulence intensity fields from raindrop backscattering","Oude Nijhuis, A.C.P. (TU Delft Microwave Sensing, Signals & Systems)","Yarovyi, O. (promotor); Russchenberg, H.W.J. (promotor); Krasnov, O.A. (copromotor); Delft University of Technology (degree granting institution)","2019","Scanning radars are promising sensors for atmospheric remote sensing, giving potential to retrieve parameters that characterize the local air dynamics during rain. For the observation of air motion, radars are relying on the backscatter of particles, which can, for example, be raindrops or insects. To measure wind vectors and turbulence intensities remotely during rain the radar is a common choice. This is mainly because the radar signals are not attenuated too much by the rain itself, which is the case for instruments operating at other frequencies, such as lidars. There is, however, a problem with measuring air dynamics from raindrops. Raindrops are not perfect tracers of the air motion. It may thus be necessary to make some corrections when air-dynamics parameters are estimated with a radar during the rain, and account for that raindrops are imperfect tracers of the air motion. This dissertation focuses on this problem. In addition, existing radar-based wind vector and turbulence intensity retrieval techniques are assessed for when they are applied during the rain, and they have been further developed.","Radar; remote sensing; turbulence; wind vectors; rain; inertia effect","en","doctoral thesis","","978-94-6384-004-0","","","","","","","","","","","",""
"uuid:351268eb-e24d-4c0d-8459-a45e74d5e132","http://resolver.tudelft.nl/uuid:351268eb-e24d-4c0d-8459-a45e74d5e132","Experimental study of the flow in the wake of a stationary sphere immersed in a turbulent boundary layer","van Hout, R.L.H. (Technion); Eisma, H.E. (TU Delft Fluid Mechanics); Elsinga, G.E. (TU Delft Fluid Mechanics); Westerweel, J. (TU Delft Fluid Mechanics)","","2018","In many applications, finite-sized particles are immersed in a turbulent boundary layer (TBL) and it is of interest to study wall effects on the instantaneous shedding of turbulence structures and associated mean velocity and Reynolds stress distributions. Here, 3D flow field dynamics in the wake of a prototypical, small sphere (D+=50, 692<ReD<959) placed in the TBL's outer, logarithmic, and buffer layer, were measured using time-resolved tomo-PIV. Increasing wall proximity increasingly tilted the mean recirculating wake away from the wall implying a negative lift force. Mean velocity deficit recovery scaled with the mean wake length with minor effects of wall proximity. Farthest from the wall, streamwise Reynolds normal stresses encircled the mean wake as an axisymmetric tubular ""shell,"" while transverse and wall-normal stresses extended off its tip as axisymmetric tapered cones. Wall proximity removed axisymmetry and attenuated values near the wall. Reynolds shear stresses were distributed as antisymmetric lobes extending off the mean wake displaying increasing values with reducing sphere-wall gap. Instantaneous snapshots revealed a wake densely populated by ""archlike"" vortices with shedding frequencies lower than for a sphere in uniform flow except in the buffer layer. Tilting of the wake away from the wall resulted from self-induced motion of shed hairpinlike vortices whose symmetry plane was increasingly wall-normal oriented with reduced sphere-wall gap.","boundary layers; fluid-particle interactions; turbulence; wakes & jets; fluid mechanics","en","journal article","","","","","","","","","","","","","",""
"uuid:d6097e3a-1cdd-4845-a71c-90f469d28b7a","http://resolver.tudelft.nl/uuid:d6097e3a-1cdd-4845-a71c-90f469d28b7a","Extreme gusts and their role in wind turbine design","Bos, R. (TU Delft Wind Energy)","van Bussel, G.J.W. (promotor); Bierbooms, W.A.A.M. (copromotor)","2017","Wind turbines are the largest rotating machines on the planet, operating in some of the most remote and hostile areas. During a lifespan of several decades, they have to withstand storms, waves, and gusts (and preferably produce electricity in the process). Yet, designers cannot make them too strong. Every additional kilogram of mass has to be manufactured, transported, and installed, and has a price tag that is directly added to the cost of energy. Optimizing structures, however, is very challenging. It requires designers to predict the highest loads on wind turbines, after they have spent decades in a turbulent wind climate. This PhD research focused on extreme wind gusts, which are among the most severe events that a wind turbine can encounter. The concept of a gust is extended from its common perception to a complete mathematical model of three-dimensional velocity fields. These fields have proven to be very valuable to the design process, since they offer a unique insight into the conditions in which high loads are triggered. Moreover, they can be used to efficiently predict the long-term loads with low uncertainty. At the same time, this physical gust model helps to answer an important question for future wind turbines: what happens when rotor blades outgrow the size of the most severe gusts in the atmosphere?","wind energy; wind gusts; extreme loads; turbulence; Monte Carlo methods; importance sampling","en","doctoral thesis","","978-94-92516-60-2","","","","","","","","","","","",""
"uuid:270b3cd3-da79-4a0f-b0ec-e60098cee1c7","http://resolver.tudelft.nl/uuid:270b3cd3-da79-4a0f-b0ec-e60098cee1c7","Measuring Tree Properties and Responses Using Low-Cost Accelerometers","van Emmerik, T.H.M. (TU Delft Water Resources); Steele-Dunne, S.C. (TU Delft Water Resources); Hut, R.W. (TU Delft Water Resources); Gentine, Pierre; Guerin, MF; Oliveira, Rafael; Wagner, Jim; Selker, John; van de Giesen, N.C. (TU Delft Water Resources)","","2017","Trees play a crucial role in the water, carbon and nitrogen cycle on local, regional and global scales. Understanding the exchange of momentum, heat, water, and CO 2 between trees and the atmosphere is important to assess the impact of drought, deforestation and climate change. Unfortunately, ground measurements of tree properties such as mass and canopy interception of precipitation are often expensive or difficult due to challenging environments. This paper aims to demonstrate the concept of using robust and affordable accelerometers to measure tree properties and responses. Tree sway is dependent on mass, canopy structure, drag coefficient, and wind forcing. By measuring tree acceleration, we can relate the tree motion to external forcing (e.g., wind, precipitation and related canopy interception) and tree physical properties (e.g., mass, elasticity). Using five months of acceleration data of 19 trees in the Brazilian Amazon, we show that the frequency spectrum of tree sway is related to mass, canopy interception of precipitation, and canopy–atmosphere turbulent exchange.","hydrology; tree physiology; tree sway; water stress; wind; canopy; Amazon; drag coefficient; climate change; turbulence; interceprtion","en","journal article","","","","","","","","","","","Water Resources","","",""
"uuid:421786ea-244a-4dc9-87d2-1e9d1ccce997","http://resolver.tudelft.nl/uuid:421786ea-244a-4dc9-87d2-1e9d1ccce997","The scope of inducing natural air supply via the façade","van den Engel, P.J.W. (TU Delft Building Services; Deerns Consulting Engineers); Kurvers, S.R. (TU Delft Indoor Environment)","","2017","An overview is given of recent developments in the use of a system of inducing natural air supply via the façade in the Netherlands. This is followed by a review of the results of measurements from climate chamber experiments of its inducing ventilation performance and detailed insights gained from related experiments of climate chamber measurements for a school and a hospital. Finally, lessons learned from practical experience gained in a newly built office and two schools are outlined. These studies of different systems of natural air supply via the facade are used to inform a scoping review of options for use in the design of new buildings using such systems in the future. Because turbulence is an important comfort-parameter, having a positive as well as negative influence on comfort and with physical principles that are, in relation to a number of parameters, still unknown, the issue of turbulence within such systems is discussed in more detail.","airflow; draught rate; induction; Natural air supply; thermal comfort; turbulence","en","journal article","","","","","","","","","","","","","",""
"uuid:b5e1b250-8134-418e-8354-c75c86c37a9c","http://resolver.tudelft.nl/uuid:b5e1b250-8134-418e-8354-c75c86c37a9c","Small-scale motions in turbulent boundary-free shear flows","Fiscaletti, D.","Westerweel, J. (promotor)","2016","The present work is an experimental and numerical investigation of the small-scale motions in turbulent free-shear flows. In the far-field turbulence of a jet at high Reynolds number (Re? = 350) hot-wire anemometry (HWA) is applied to measure time series of flow velocity. By filtering these time series, large- and small-scale velocity fluctuations are obtained. Both the amplitude and the frequency of the small-scale signals are locally stronger (weaker) for positive (negative) fluctuations of the large-scale signal, which is refered to as amplitude and frequency modulation. The local amplitude and frequency of the small-scale signals increase monotonically with the strength of the large-scale velocity fluctuations. The same flow is also investigated with long-range ?PIV (microscopic Particle Image Velocimetry). The measurement is validated against the HWA signals by comparison of the turbulence statistics. A validation based on the topological content is also performed. The coherent structures of vorticity and of intense dissipation are adequately resolved, and their characteristic size is assessed. It is found that the size of the vortical structures does not change significantly when conditioned on strongly-positive or strongly-negative large-scale velocity fluctuations. Using the PIV results the amplitude and frequency modulation observed from HWA signals is explained as an inhomogeneous distribution of the small-scale structures within the flow. In particular, the analysis of ?PIV data reveals that the intense vortical and dissipation structures tend to be preferentially located in high-velocity regions, hence they are characterized by convection velocities higher than the mean velocity of the flow. Furthermore, the spatially resolved velocity vector fields allow to quantify amplitude modulation directly in physical space. From this direct estimation in physical space, amplitude modulation is only 25% of the value measured from hot-wire anemometry. The remaining 75% comes from the fixed spectral band filter used to obtain the large- and the small-scale signals, which does not consider the local convection velocity (Taylor hypothesis of frozen turbulence). A very similar overestimation of amplitude modulation when quantified in the time-frame is also confirmed analytically. Based on the experimental analysis on the jet an explanation for amplitude and frequency modulation is developed, which can be extended to other free-shear flows. The validity of this interpretation is assessed based on the analysis of Direct Numerical Simulations of a mixing layer, at the Reynolds number based on the Taylor microscale (Re? =) of 250. The local vorticity rms, taken as a measure of the small-scale activity, is found to be modulated by the large-scale velocity fluctuations depending on the position within the flow. In particular, on the low-speed side of the mixing layer, positive large-scale velocity fluctuations correspond to a stronger vorticity rms, whereas on the high-speed side, they correspond to a weaker vorticity rms. This is consistent with previous studies on a mixing layer. Important differences are found in the strength of the scale interaction from time series and in physical space, consistent with the predictions developed from the analysis of the jet. On the high-speed side of the mixing layer, amplitude modulation from time series largely underestimates the value obtained from spatial series, and overestimates it on the low-speed side. Therefore, the interaction between large-scale velocity fluctuations and small scales is dependent on the flow position within the mixing layer, similar to a turbulent boundary layer. Nonetheless, when the vorticity rms is correlated with the large-scale shear velocity gradients, the correlation coefficient is found to be nearly constant throughout the mixing layer, and close to unity. This reveals that the large and the small scales present a strong interaction independent of the position when the large-scale shear velocity gradients are considered, instead of the large-scale velocity fluctuations, as in the existing literature on amplitude modulation. The strong correlation between the large-scale gradients and the small scales suggests to investigate possible evidence of the so called “scale invariance” (Meneveau and Katz 2000). The alignment between the local vorticity and the large-scale vorticity is examined within the vortical tubes. It is found that the vorticity from unfiltered (representing the small scales) and from low-pass-filtered velocity vector fields (representing the larger scales) tend to be aligned within the vortical tubes. This suggests that the direction of vorticity does not vary significantly across the scales. Therefore, the anisotropy of the large scales is partially preserved at the small-scale level, which is in contrast with the Kolmogorov’s hypothesis of local isotropy.","turbulence","en","doctoral thesis","","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:2878cdc3-f12c-4916-903d-7544d8746d2c","http://resolver.tudelft.nl/uuid:2878cdc3-f12c-4916-903d-7544d8746d2c","A critical assessment of methodologies for operations and safety evaluations of freeway turbulence","van Beinum, A.S.; Farah, H.; Wegman, F.C.M.; Hoogendoorn, S.P.","","2016","Turbulence in traffic is a commonly known phenomenon, but the exact characteristics of this phenomenon are not yet clear. It reflects individual changes in speed, headways, and lanes in the traffic stream. The currently used freeway design guidelines prescribe different measures for handling turbulence, such as sufficient ramp spacing, and spacing between road discontinuities. In situations where the available space between discontinuities is scarce, it might be necessary to make a trade-off between costs and safety/operation. For a valid trade off more insight is needed on the safety and operations effects when one deviates from the guidelines. A lot of research was done on the different causes of turbulence and their effect on safety and operation. This paper proposes a theoretical framework for turbulence phenomenon that facilitates the comparison of the available methodologies that can be used to evaluate a freeway design on the matter of turbulence and its impact on traffic operations and safety. The main finding of this review is that the currently available methodologies lack the ability to evaluate the impact of freeway turbulence on operations and safety simultaneously. Different recommendations to overcome limitations of current methodologies and further research possibilities to improve these methodologies are given.","traffic safety; operations; turbulence; surrogate safety measures; freeway design","en","conference paper","TRB","","","","","","","","Civil Engineering and Geosciences","Transport & Planning","","","",""
"uuid:be865e1e-4643-4239-950f-ccbb53ded8c7","http://resolver.tudelft.nl/uuid:be865e1e-4643-4239-950f-ccbb53ded8c7","Helical mode interactions and spectral energy transfer in magnetohydrodynamic turbulence","Linkmann, M.F.; Berera, A.; McKay, M.E.; Jäger, J.","Linkmann, M.F. (author); Berera, A. (author); McKay, M.E. (author); Jäger, J. (author)","2015","Spectral transfer processes in magnetohydrodynamic (MHD) turbulence are investigated by decomposition of the velocity and magnetic fields in Fourier space into helical modes. In 1992, Waleffe (Phys. Fluids A, 4:350 (1992)) used this decomposition to calculate triad interactions for isotropic hydrodynamic turbulence and determined whether a given triad contributed to forward or reverse energy transfer depending on the helicities of the interacting modes. The problem becomes more difficult in MHD due to the need to treat a coupled system of partial differential equations and the energy transfers between the magnetic and velocity fields. This requires the development of techniques that extend Waleffe's work, which are subsequently used to calculate the direction of energy transfer processes originating from triad interactions derived from the MHD equations. In order to illustrate the possible transfer processes that arise from helical mode interactions, we focus on simplified cases and putting special emphasis on interactions resulting in reverse spectral energy transfer. This approach also proves to be helpful in determining the nature of certain energy transfer processes, where transfer of energy between different fields and between the same field can be distinguished. Reverse transfer of magnetic energy was found if the helicities of two modes corresponding to the smaller wavenumbers are the same, while for reverse transfer of kinetic energy Waleffe's result is recovered. Reverse transfer of kinetic to magnetic energy is facilitated if the interacting magnetic field modes are of opposite helicity, and no reverse transfer of magnetic to kinetic energy was found. More generally, the direction of energy transfer not only depends on helicity but also on the ratio of magnetic to kinetic energy. For the magnetically dominated case reverse transfer occurs of all helicities are the same, the kinetically dominated case two modes need to have the same helicity while the third mode is of opposite helicity to allow reverse transfer.","spectral methods; magnetohydrodynamics; turbulence; helical mode decomposition","en","conference paper","","","","","","","","","","","","","",""
"uuid:bc93e79e-74d0-4443-9e72-db20fc544a3b","http://resolver.tudelft.nl/uuid:bc93e79e-74d0-4443-9e72-db20fc544a3b","Stratocumulus transitions in present-day and future climate","Van der Dussen, J.J.","Siebesma, A.P. (promotor); De Roode, S.R. (promotor)","2015","Clouds have a strong net cooling effect on our planet, as they reflect a large part of the incident solar radiation. To be able to make accurate forecasts of the global climate, cloudiness should therefore be correctly represented by climate models. Currently, however there are large differences in the forecasted temperature increase among climate models. One of the most important causes of these differences is the uncertainty in the representation of clouds, in particular of stratocumulus clouds. Stratocumulus clouds are low clouds that often form an almost completely closed cloud deck. Only little sunlight passes through them, so that they are often associated with grey and dull weather. Stratocumulus clouds are frequently found over oceans in the subtropics, where they can cover enormous areas of several millions of square kilometers. When they are advected by the trade winds from the subtropics toward the equator, a transition typically occurs during which the stratocumulus slowly thins and eventually breaks up. Simultaneously, cumulus clouds appear that have a much lower cloud cover and therefore reflect less sunlight. Hence, stratocumulus transitions cause an abrupt decrease of the cloud-induced net cooling effect, which makes them particularly important for climate models. At the same time, the representation of stratocumulus clouds is extremely challenging for climate models, since their development strongly depends on the transport of among others moisture by small-scale turbulence. Due to their coarse resolution, climate models are unable to explicitly simulate processes with typical sizes of a hundred kilometers or less. Hence, turbulent transport, together with other cloud related processes, is represented in a simplified statistical manner by parameterizations, which introduces much uncertainty. During this thesis project we have simulated stratocumulus clouds and their transitions with a numerical model that, in contrast to climate models, is capable of representing the interaction between turbulence and clouds in detail. In chapter two we compare the results of six of these so-called large-eddy simulation models with measurements that had been gathered during a stratocumulus transition. All models are shown to be capable of correctly representing the main features of the transition, including the slow thinning of the stratocumulus and the simultaneous development of cumulus clouds. The simulations yield a wealth of data on the three-dimensional structure of the atmosphere, which is impossible to obtain from measurements. These data allow us to investigate the causes of the thinning and breaking up of stratocumulus clouds during transitions in detail. One of these causes can be sought in the change of the turbulent structure of the atmospheric boundary layer in which the stratocumulus resides. At the start of the transition, the boundary layer is still rather shallow, allowing turbulence to vertically mix the air in it relatively well. Hence, the moisture that evaporates from the ocean surface can easily reach the cloud layer, thereby feeding and maintaining it. As the transition progresses, the boundary layer becomes deeper and the distance between the clouds and the surface increases. It has been suggested that eventually turbulence will not be sufficiently strong anymore to maintain the well-mixed structure of the boundary layer. This so-called decoupling would cause the moisture transport to the stratocumulus to be almost completely cut off, causing it to rapidly dry and dissolve. However, in chapter two we show from the model results that decoupling has less effect on the humidity transport than was originally thought. Another process that is often held responsible for the breaking up of stratocumulus clouds is entrainment. In this process, air from the relatively warm and dry free troposphere is mixed into the boundary layer. Hence, entrainment causes drying and warming of the stratocumulus cloud and is therefore associated with its thinning. In chapter three we derive an equation that describes the change with time of the total amount of condensed water in the cloud. Using this equation we argue that entrainment is indeed an important cause for the thinning of stratocumulus clouds during a transition. On the other hand, we also show that other processes, such as the supply of moisture from the sea surface, can be strong enough to diminish this thinning, even for conditions for which earlier studies predicted an unconditional breakup of the cloud. In the second part of this thesis, we investigate the effect of the warming of the climate on stratocumulus clouds. In chapter four, we perform a set of simulations of stratocumulus clouds for conditions that are representative for the current climate. In a second set of simulations we mimic a future climate by increasing the temperature of the atmosphere and of the sea. This idealized climate perturbation causes a decrease of the thickness and hence the reflectivity of the stratocumulus clouds in all simulations. This suggests that, in a future climate, more solar radiation will be able to reach the Earth's surface than in the current climate. This way, stratocumulus clouds will enhance the warming of the climate. As a result of a warming of the climate, the large-scale atmospheric circulation between the equator and the subtropics, the so-called Hadley circulation, will weaken. In chapter five we show that a weakening of the Hadley circulation delays the breakup of stratocumulus clouds during transitions. Effectively, this leads to an increase of the amount of stratocumulus clouds in a future climate. This mechanism therefore counteracts the reduction of the amount of stratocumulus clouds in response to a climate warming that we found in chapter four, but will likely not be strong enough to completely compensate for it.","stratocumulus; numerical simulation; turbulence; climate change","en","doctoral thesis","","","","","","","","2015-06-22","Civil Engineering and Geosciences","Geoscience and Remote Sensing","","","","34, 25"
"uuid:55649654-2924-42bc-aff9-eb174399b2e2","http://resolver.tudelft.nl/uuid:55649654-2924-42bc-aff9-eb174399b2e2","Efficiency of hanging silt curtains in cross-flow","Radermacher, M.; De Wit, L.; Uijttewaal, W.S.J.; Winterwerp, J.C.","","2015","When dredging in sensitive environments, efforts have to be made to limit the free dispersal of suspended fine sediment from the dredging spill. Especially the use of hanging silt curtains as an environmental mitigation measure is widespread. Despite frequent application, their ability to reduce turbidity levels through vertical diversion of sediment-laden currents remains subject of debate. This paper addresses a series of laboratory measurements and numerical model simulations in order to determine the efficiency of hanging silt curtains, defining a new efficiency parameter. The model was validated against the laboratory experiments. Model simulations focusing on vertical diversion of the sediment-laden current suggest that hanging silt curtains do not have a favorable influence on the settling of suspended sediment when applied in cross-flow. Diversion of currents underneath the curtain causes flow separation and intense turbulent mixing, which counteracts settling of suspended sediment particles. The results imply that the widespread application of hanging silt curtains should be reconsidered from a physical point of view.","dredging; turbidity; environmental issues; turbulence; numerical models; laboratory tests","en","journal article","ASCE","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:51454527-14fa-447c-9ff7-11c64aa73150","http://resolver.tudelft.nl/uuid:51454527-14fa-447c-9ff7-11c64aa73150","The influence of an obstacle on flow and pollutant dispersion in neutral and stable boundary layers","Tomas, J.M.; Pourquie, M.J.B.M.; Jonker, H.J.J.","","2015","Predicting pollutant dispersion in urban environments requires accurate treatment of obstacle geometry, inflow turbulence and temperature differences. This paper considers both the influence of thermal stratification and the presence of a single obstacle on pollutant dispersion in turbulent boundary layers (TBLs). Turbulent flow over a fence with line sources of pollutant in its vicinity is simulated by means of Large-Eddy Simulations. Separate ‘driver’ simulations are done to generate the inflow TBL for several levels of stratification. Using these inflow TBLs the flow development and pollutant dispersion behind the fence, up to 100 fence heights, h, is investigated. It is shown that the decay of velocity and temperature deficit is independent of stability, while the decay of Reynolds stress and concentration excess decreases with increasing stability. For neutral cases the influence of the obstacle is gone after approximately 75h, while for stable cases near the ground the flow is still accelerated compared to the undisturbed case. The fence does cause a local reduction of stratification and thereby increased pollutant dispersion. However, neglecting the effect of buoyancy results in an underestimation of pollutant concentration by a factor 2.5 at 75h downstream of the emission source for the most stable case.","stratification; boundary layer; obstacle; pollutant dispersion; turbulence; wake","en","journal article","Elsevier","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:08efee1b-d79e-45a2-9755-a3c96f3faf33","http://resolver.tudelft.nl/uuid:08efee1b-d79e-45a2-9755-a3c96f3faf33","Numerical simulation of turbulent heat transfer close to the critical point","Boersma, B.J.; Pecnik, R.; Nemati, H.; Peeters, J.W.R.","","2015","In this paper we discuss the effect of sharp property variations on the turbulent heat transfer in fluids close the critical point. The governing equations for this flow regime are discussed, a short description of the numerical tools that have been developed to study these flows is given. Finally, some results for supercritical heat transfer in developing turbulent pipe flow are presented.","turbulence; supercritical flows; heat transfer; direct numerical simulation","en","journal article","American Institute of Physics","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:78686573-dfe8-40c9-bbf7-da869bb39197","http://resolver.tudelft.nl/uuid:78686573-dfe8-40c9-bbf7-da869bb39197","Coherent Structures in Taylor-Couette Flow: Experimental Investigation","Tokgöz, S.","Westerweel, J. (promotor)","2014","Taylor-Couette flow is defined as the flow confined between two coaxial cylinders which can rotate independently. Several different flow states can be observed in the gap between the cylinders by changing the rotation speeds and the rotating directions of the cylinders. The aim of this thesis is to investigate the previously reported change of torque values with the rotation speeds of the cylinders and to study its relation to coherent turbulent flow structures. The flow structures are investigated using tomographic PIV, which is a fully volumetric measurement method that resolves all three velocity components. Different flow states, and their contribution to the Reynolds stresses are revealed. Initially, the Taylor-Couette setup was used to investigate the spatial resolution of tomographic PIV, by exploiting the fact that the power input to the system, as determined from the torque measurements and the cylinder rotation speed, is balanced by the viscous dissipation rate, which can be computed using the measured velocities. Next, time-resolved tomographic PIV measurements were performed at fully turbulent flow to demonstrate the capability of the measurement system and the flow geometry to study dynamic events in turbulence. Turbulent flow with an approximately zero mean velocity was created by rotating the cylinders in opposite directions with the same wall velocities. Using this idea, the observation times of the flow structures could be increased by an order of magnitude as compared to similar studies in turbulent boundary layers. Finally, the connection between turbulent flow structures and the change of the torque was made by using tomographic PIV. In order to compute the contribution of the large and the smaller-scale structures to the torque, the instantaneous flow was decomposed into the large and the smaller-scale motions by filtering. The Reynolds stresses associated with these structures indicate that the orientational change of the structures may be the mechanism responsible for the reported change of the torque scaling.","turbulence; rotating flows; Taylor-Couette; particle image velocimetry","en","doctoral thesis","","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:9dff055c-eb6d-4005-a052-fce8aaeea792","http://resolver.tudelft.nl/uuid:9dff055c-eb6d-4005-a052-fce8aaeea792","Numerical Methods for the Optimization of Nonlinear Residual-Based Sungrid-Scale Models Using the Variational Germano Identity","Maher, G.D.; Hulshoff, S.J.","","2014","The Variational Germano Identity [1, 2] is used to optimize the coefficients of residual-based subgrid-scale models that arise from the application of a Variational Multiscale Method [3, 4]. It is demonstrated that numerical iterative methods can be used to solve the Germano relations to obtain values for the parameters of subgrid-scale models that are nonlinear in their coefficients. Specifically, the Newton-Raphson method is employed. A least-squares minimization formulation of the Germano Identity is developed to resolve issues that occur when the residual is positive and negative over different regions of the domain. In this case a Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm is used to solve the minimization problem. The developed method is applied to the one-dimensional unsteady forced Burgers’ equation and the two-dimensional steady Stokes’ equations. It is shown that the Newton-Raphson method and BFGS algorithm generally solve, or minimize the residual of, the Germano relations in a relatively small number of iterations. The optimized subgridscale models are shown to outperform standard SGS models with respect to a L2 error. Additionally, the nonlinear SGS models tend to achieve lower L2 errors than the linear models.","subgrid-scale model; variational multiscale method; variational Germano identity; optimization; turbulence","en","conference paper","CIMNE","","","","","","","","Aerospace Engineering","Aerodynamics, Wind Energy & Propulsion","","","",""
"uuid:0b870201-4ae5-41c7-972b-21feb937f657","http://resolver.tudelft.nl/uuid:0b870201-4ae5-41c7-972b-21feb937f657","Modelling Vertical Variation of Turbulent Flow Across a Surf Zone Using SWASH","Zijlema, M.","","2014","This paper presents the application of the open source non-hydrostatic wave-flow model SWASH to propagation of irregular waves in a barred surf zone, and the model results are discussed by comparing against an extensive laboratory data set. This study focus not only on wave transformation in the surf zone, but also on the numerical prediction of undertow and vertical distribution of turbulence levels under broken waves. Present simulations demonstrate the overall predictive capabilities of the model in computing breaking surf zone waves.","surf zone; wave breaking; undertow; turbulence; modelling; SWASH; ICCE 2014","en","conference paper","Coastal Engineering Research Council","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:369f7fed-d18a-4227-a869-752b6d2da672","http://resolver.tudelft.nl/uuid:369f7fed-d18a-4227-a869-752b6d2da672","Turbulent Particle Transport and its Effect on Flotation","Warncke, N.G.W.","Westerweel, J. (promotor)","2014","This thesis studies the hydrodynamics of froth flotation, a process that is used in industry as a means of removing small inclusions from the treated liquid by gas bubble injection. This work is particularly motivated by the removal of aluminium and silicon oxides from liquid steel. Hydrodynamic aspects have a strong influence on the efficiency of froth flotation. The flow around a rising bubble, especially the turbulent flow in the near-wake, is of particular importance as it may cause preferential concentration of particles as well as significantly change the collision and attachment rates of particles with the surface of the bubble. Both effects are studied in this work. Preferential concentration of particles in the near-wake is a result of the time-averaged balance of inertial and pressure forces on a dispersed particle, resulting in a drift towards the wake. It can therefore be observed by an effective time-averaged particle slip velocity over the wake boundary. Likewise, a direct confirmation of preferential concentration is possible by measurements of the average concentration of particles in the wake. These measurements were done in the wake of a solid mock-up of a spherical-cap bubble in the VerMeer vertical water tunnel. The particle slip velocity was obtained from simultaneous two-phase Particle Image Velocimetry measurements and numerically integrated in post-processing. The particle concentration was obtained directly from the average scattered light intensity. Both experimental results are compared to the predictions of a model for the preferential concentration in the wake, derived from a local balance of inertial, gravitational and diffusive fluxes. The influence of turbulence on particle attachment rates was measured in the newly constructed DABuT (Dynamic Air Bubble Trap) facility, using bubbles of different shapes and volumes. The research was again focused on the semi-spherical bubbles, these were also found to have the highest attachment rates. The results of the measurements are compared to a model based on a turbulent attachment flux and the effective shielding of particle attachments by a mono layer of particles building up and finally covering the rear side of the bubble in proximity to the wake. In conclusion, both effects contribute to the overall efficiency of flotation. the results of this work suggest that spherical-cap bubbles — although potentially difficult to generate — have the highest potential for the optimisation of flotation processes in industry.","turbulence; particle transport; flotation; multiphase flows","en","doctoral thesis","","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:de683e0e-53c9-443c-ab0b-96b07195b6b9","http://resolver.tudelft.nl/uuid:de683e0e-53c9-443c-ab0b-96b07195b6b9","Droplet collisions in turbulence","Oldenziel, G.","Westerweel, J. (promotor)","2014","Liquid droplets occur in many natural phenomena and play an important role in a large number of industrial applications. One of the distinct properties of droplets as opposed to solid particles is their ability to merge, or coalesce upon collision. Coalescence of liquid drops is of importance in for example the food industry, where the shelf life of emulsion type of products is often elongated with absence of coalescence, rocket propulsion, where fuel and oxidizer are separately injected and ignite upon coalescence, the semiconductor industry, where droplets of molten tin are targeted by a laser to produce extreme ultraviolet light, and during the formation of rain. One might not realize it immediately, but the outcome of a droplet collision is not always coalescence, but also bouncing can occur. In most of the mentioned cases the droplet collisions do not take place in a stationary environment but in an environment that flows with respect to the droplets or is even turbulent. The aim of the current study is to gain insight into the effect of external turbulence on the outcome of droplet collisions. When two droplets collide it might appear that the drops are in contact, but in reality there will be a thin film of surrounding fluid in between the droplets. When this film becomes so thin that Van der Waals forces between the molecules of the two drops come into effect a hole will be initiated in the film and the droplets will coalesce. The associated thickness is in the order 60 nm. If the film thickness does not decrease to this value, the droplets will bounce. The time it takes from the start of drop interaction to film rupture is called the drainage time, denoted by t_drainage. The time during which the drops are in apparent contact is called the interaction time t_interaction. A droplet collision will thus lead to coalescence when t_{drainage} < t_{interaction}, otherwise the collision results in bouncing. To investigate film drainage an experiment was performed where a droplet of silicone oil was released in a layer of water below a layer of silicone oil. A film of water will form between the drop and the top liquid. Because the interaction time t_interaction is always larger than the drainage time t_drainage, the film will eventually always rupture. Refractive index matching and laser induced fluorescence (LIF) were used to image the film and determine its thickness and rupture location. The measurements show that the film attains a specific geometry. The film is thick at the center and thin at an off center ring. This geometry is termed a 'dimple'. The measurements also show that the film ruptures faster than simple analytical models predict and that the film thins asymmetrically. The rupture locations correlate with the region of minimal thickness. After the investigation of film drainage, investigation of droplet collisions with realistic values for the interaction time become of interest together with the influence of external turbulence. In order to investigate both phenomena a cylindrical tank was designed with discs at the top and bottom which could counter rotate at fixed frequency. The flow between counter rotating discs is referred to as Von Karman flow. At the center of the flow facility a small volume exists where the average flow is approximately zero, but the turbulent velocity fluctuations are finite. Nozzles were mounted in the Von Karman flow facility to inject droplets to produce collisions at this location. Droplets could be collided in a stationary flow and in a turbulent flow and using three high speed cameras the droplets could be tracked in 3D. The Reynolds number based on the Taylor micro scale and the turbulent fluctuations, denoted as Re_l, is zero in case of a stationary flow Re_l = 0 and for the turbulent case it holds that Re_l = 141. The collisions are characterized using the Weber number We = rho_d U^2 D/sigma based on the drop density rho_d, the relative drop velocity U, the drop diameter D and the interfacial tension sigma. The Weber number is an indicator for the balance of inertial forces over surface tension forces. In case of droplet collisions in a stationary liquid (Re_l = 0) for low values of the Weber number the outcome of collision events is bouncing. For large values of the Weber number the outcome is coalescence and for the largest achievable values of the Weber number the droplets would coalesce, the coalesced drop would form a flat disc which would eventually break up into two separate drops and one small satellite drop. This behavior with increasing values of the Weber number was shown before for droplets in air by Qian and Law, see (1). Droplet collisions in a liquid are thus similar to droplet collisions in air. In case of a stationary outer liquid the value of the Weber number at the boundary between bouncing and coalescence is approximately Wen_(II,III) = 20. In case of bouncing the interaction time was found to be equal to the theoretical drop oscillation period as derived by Miller and Scriven (2), and the drainage time was found to decrease with increase of the Weber number. For droplet collisions in a turbulent outer flow (Re_l = 141) on average collisions only lead to coalescence for values of the Weber number larger than 30 (We_(II,III) > 30). The external flow is thus of significant influence on the outcome of droplet collisions in terms of bouncing or coalescence. A possible explanation for this hindered coalescence is the fact that a compressional flow in the direction of the collision axis induces an internal drop flow which counteracts film drainage. If this explanation is valid the measured effect is not due to turbulence. References (1) Qian and Law, Regimes of coalescence and separation in droplet collision. J. Fluid Mech 331, 1997. (2) Miller and Scriven, The oscillations of a fluid droplet immersed in another fluid. J. Fluid Mech 32, 1968.","droplet; collisions; turbulence; coalescence; bouncing","en","doctoral thesis","","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:6e1a39dd-1581-4d87-b623-97d1dc39fb78","http://resolver.tudelft.nl/uuid:6e1a39dd-1581-4d87-b623-97d1dc39fb78","Micro Ramps in Supersonic Turbulent Boundary Layers: An experimental and numerical study","Sun, Z.","Scarano, F. (promotor)","2014","The micro vortex generator (MVG) is used extensively in low speed aerodynamic problems and is now extended into the supersonic flow regime to solve undesired flow features that are associated with shock wave boundary layer interactions (SWBLI) such as flow separation and associated unsteadiness of the interaction system. Numerous experimental and numerical studies have shown that despite their small size, such devices can alter the boundary layer properties very efficiently, when compared to the conventional vortex generators. In order to assist a more efficient design of MVGs, fundamental studies have been carried out to understand the associated wake properties such as the increased boundary layer mixing and the structure and stability of the induced vortex system. The present work is conducted in the framework of such fundamental studies. The micro ramp is among the most commonly used MVG devices and has been selected for the present investigations. The research is based both on wind tunnel experiments and numerical simulations in order to build a more comprehensive and detailed understanding of the flow behind a micro ramp immersed in a supersonic turbulent boundary layer. The choice of the experimental approach is justified by the fact that the incoming turbulent boundary layer exhibits a high Reynolds number (Re?=13,600), which makes it too challenging for extensive CFD investigation by using LES or DNS approaches. Variants of the micro ramp configuration as well as the attendant SWBLI can be studied efficiently by wind tunnel experiments adopting PIV as velocity field diagnostics. The use of numerical simulations by the implicit large eddy simulation (ILES) technique for one specific case enables the detailed inspection of the flow field that adds to the understanding of the flow development in regions or aspects where the experimental method provides limited access. Finally, there is general interest to know that till what extent numerical simulations can correctly identify the governing mechanisms of the boundary layer flow manipulation by micro ramps. Tomographic PIV is used as three-dimensional flow diagnostic technique in the investigation of flow organization in the micro ramp near wake (x/h?9~15). From the experimental data it is observed that the mean flow features a conical wake containing a pair of steady vortices aligned in streamwise direction. This is considered to be the basic mechanism of the boundary layer flow manipulation, whereas the wall-normal velocity component features a central focussed upwash with downwash motions at the sides. Simultaneously, a deficit region of streamwise velocity is produced in the center of the wake. The shear layer surrounding the wake is subject to Kelvin-Helmholtz (K-H) type instability and the instantaneous flow organization exhibits the formation of coherent K-H vortices that are arc shaped and dominate the velocity field fluctuations across the shear layer. Conditional averaging of the 3D velocity field yields the salient features of the interaction between the streamwise vortices and the K-H vortices whereas the former are found to be weakened at the generated of K-H vortices. The downstream decay of the flow features that are introduced by the micro ramp is relevant to its positioning with respect to the point of interaction between shock wave and boundary layer, indicating the relevance of investigating the further downstream development. Therefore experiments are conducted with large format PIV camera to study the decay in the center plane of the micro ramp far wake (x/h?12~32). In order to find a proper scaling parameter of the micro ramp wake, two geometrically similar micro ramps with different sizes are employed. Both streamwise and wall-normal velocity components exhibit a power-law decay in agreement with theories for the fully developed turbulent flow regime. The wall-normal velocity decays faster, approximately at a rate 2.5 times of the momentum deficit. The self-similarity of the velocity profiles is also examined. The streamwise velocity exhibits a good degree of self-similarity in the upper and lower shear layer, while the wall-normal component has overlapped upwash profiles. Concerning the turbulent properties, a strong anisotropy of velocity fluctuations is observed at upstream locations (x/h<20), nonetheless both fluctuation components decay to a similar magnitude when approaching the downstream end of the measurement domain (x/h>20). The organization of instantaneous vortical field is also investigated in the attempt to better understand their effect on the wake decay. Spatial auto-correlation of the instantaneous velocity fields yields the streamwise evolution of the average distance between vortices. Vortex pairing is identified in the range x/h=18~22 through an increase of such distance. The detection of counter-rotating vortices in the lower part of the wake suggests that the K-H vortices produced in the upper region of the shear layer propagate into the region close to the wall after vortex pairing, which eventually gives rise to ring-vortex formation in the later stage of the wake. A numerical study using ILES with high order scheme is carried out in collaboration with the University of Texas at Arlington. In order to establish a fair comparison with the experimental data, the flow conditions are made as similar as possible, matching the free stream Mach number and the ratio between micro ramp height and boundary layer thickness. The attendant limitations on computational resources limit the Reynolds number based on boundary layer momentum thickness to about one-third of that in the experiments. The comparison covers the most relevant quantities, such as the streamwise and wall-normal velocity and the peak vorticity. An overall good agreement is observed. A noticeable discrepancy involves underestimation of upwash motion: the wall-normal velocity amounts to 70% of the measured data. In the observation of instantaneous flow, vortex pairing is also identified and the spatial-temporal evolution of the K-H vortex is studied by tracking, which confirms the flow model conjectured from the planar PIV study in the center plane.","turbulence; flow control; Particle Image Velocimetry; Large Eddy Simulation; supersonic flow","en","doctoral thesis","","","","","","","","","Aerospace Engineering","Aerodynamics Wind Energy and Propulsion","","","",""
"uuid:bf295221-4e6e-4663-b046-79da13325243","http://resolver.tudelft.nl/uuid:bf295221-4e6e-4663-b046-79da13325243","Bulk Dynamics of Droplets in Liquid-Liquid Axial Cyclones","Van Campen, L.J.A.M.","Mudde, R.F. (promotor); Hoeijmakers, H.W.M. (promotor)","2014","Separation of oil and water is an essential step in the treatment of the production streams from fossil oil wells. Settling by gravity is a robust though voluminous process and therewith expensive method at remote locations, leading to a need for smaller separation equipment. In this thesis, we describe the research performed on the development of an inline axial cyclone for oil/water separation. This work is part of ISPT project OG-00-004 and has an experimental nature: a flowrig has been constructed to test different cyclones at flow rates up to 60 m3/h in a 10 cm diameter tube in which brine and low-viscosity lubricant oil can be mixed in almost any proportion. Results are compared with numerical datasets resulting from the same ISPT project. Three different swirl elements have been developed for this project: a strong swirl element and a weak swirl element with 10 cm diameter, and one element with a 26 cm diameter in combination with a tapered tube section. For all three swirl elements, the velocity profile of water has been measured with Laser Doppler Anemometry (LDA). The strong swirl element has a swirl number of 3.7, the weak of 2.3 and the large diameter element of 3.9. The axial velocity profile normalized with the bulk velocity shows vortex breakdown (upstream flow in the center), where the severeness of the breakdown normalized with the upstream bulk velocity shows proportionality with the swirl number. For the azimuthal velocity, the velocity profile was proportional to the bulk velocity. The non-dimensional azimuthal velocity was similar for all three swirl elements in the region |r/D| < 0.2. Outside that region the relative velocity is strongly influenced by the swirl element. Time series obtained with single phase LDA studies were used to estimate the effect of turbulent dispersion on droplet trajectories. A simplified equation of motion based on centrifugal buoyancy, drag and turbulent dispersion was solved for many fictitious droplet paths. The measured, chaotic axial velocity time series was used to mimic the radial component of the velocity fluctuations. With this model, we can predict the smallest droplet size that can be separated with a certain cyclone and the largest droplet size before it is broken by the flow. Model results show good agreement with overall bulk data obtained in the experimental flow rig. With an intrusive endoscope technique, we measured the droplet size distribution at various positions in the axial cyclone. From this, Hinze’s theory for the droplet size in turbulent pipe flow is confirmed. Furthermore, the inverse correlation between azimuthal velocity and median droplet size is shown and quantified: a lower velocity allows larger droplets to survive. Different designs were tested to understand which parameters have a large influence on the industrially relevant parameter of separation performance. This question is answered by variation of the swirl element, swirl tube length, pickup tube diameter, flow rate and droplet size. Changes that affect the droplet size have a severe effect on separation, these are the swirl element and flow rate. Changes that increase the droplet size lead to better phase separation. The other geometrical changes can be used to optimize performance, but are not identified as parameters leading to breakthrough improvements. Two non-dimensional numbers can be used to explain the behavior of the cyclone: the Weber number (We) based on the droplet size upstream of the swirl element and the maximum velocity obtained in the gaps of the swirl element, and the Reynolds number (Re_?) for the droplets downstream of the swirl element based on their median diameter and the azimuthal liquid velocity. Separation is better for a smaller We number, because droplets are less vulnerable for breakup under that condition. A large Re? number is beneficial since the droplets then experience a large centrifugal acceleration which is larger than turbulent dispersion. Both trends are confirmed with experimental data obtained in this project. We propose that there is a function for the maximum possible separation efficiency based on both non-dimensional numbers. The inverse coupling between We and Re_? via the azimuthal velocity makes optimization of separation efficiency difficult. Application of a large diameter swirl element (low velocity and therefore limited droplet breakup) in combination with a gradual tapering of the tube (increasing the azimuthal velocity) is a possibility to obtain both a large We and Re? number. Another option is to place multiple axial cyclones in series, with a stepwise increase of the swirl strength in each subsequent cyclone. In such a configuration, each step is capable of separating smaller droplets than the previous step, without immediate breakup of large droplets. This method should increase the overall quality of the phase separation.","swirling flow; cyclones; oil; turbulence; experimental methods; separation; multiphase flows","en","doctoral thesis","","","","","","","","2014-01-08","Applied Sciences","Chemical Engineering","","","",""
"uuid:cac43094-6191-49d4-a53b-131f6f940885","http://resolver.tudelft.nl/uuid:cac43094-6191-49d4-a53b-131f6f940885","Forced convection mass deposition and heat transfer onto a cylinder sheathed by protective garments","Ambesi, D.; Kleijn, C.R.; Den Hartog, E.A.; Bouma, R.H.B.; Brasser, P.","","2013","In chemical, biological, radiological, and nuclear protective clothing, a layer of activated carbon material in between two textile layers provides protection against hazardous gases. A cylinder in cross flow, sheathed by such material, is generally used to experimentally test the garment properties. This is, however, complicated and predictive models are needed. We present a computational fluid dynamics model for the initial phase in which the carbon filter material is not yet saturated. The textiles are modeled as chemically inactive porous layers, the carbon filter particles have been resolved explicitly. The model has been validated against experimental data. We demonstrate that (1) computational fluid dynamics simulations can be used for the efficient design and optimization of protective garments, and (2) the addition of a highly porous active carbon layer highly increases the chemical protection capabilities, while having relatively little negative impact on the thermal comfort of protective garments.","chemical; biological; radiological; and nuclear protective garments; cylinder; active carbon filter; heat transfer; mass transfer; computational fluid dynamics; turbulence; time-dependent Reynolds-averaged Navier-Stokes","en","journal article","Wiley","","","","","","","","Applied Sciences","ChemE/Chemical Engineering","","","",""
"uuid:5cb8276a-7aa1-430c-a759-026a7b925da0","http://resolver.tudelft.nl/uuid:5cb8276a-7aa1-430c-a759-026a7b925da0","Residual sediment fluxes in weakly-to-periodically stratified estuaries and tidal inlets","Burchard, H.; Schuttelaars, H.M.; Rockwell Geyer, W.","","2013","In this idealized numerical modeling study, the composition of residual sediment fluxes in energetic (e.g., weakly or periodically stratified) tidal estuaries is investigated by means of one-dimensional water column models, with some focus on the sediment availability. Scaling of the underlying dynamic equations shows dependence of the results on the Simpson number (relative strength of horizontal density gradient) and the Rouse number (relative settling velocity) as well as impacts of the Unsteadiness number (relative tidal frequency). Here, the parameter space given by the Simpson and Rouse numbers is mainly investigated. A simple analytical model based on the assumption of stationarity shows that for small Simpson and Rouse numbers sediment flux is down estuary and vice versa for large Simpson and Rouse numbers. A fully dynamic water column model coupled to a second-moment turbulence closure model allows to decompose the sediment flux profiles into contributions from the transport flux (product of subtidal velocity and sediment concentration profiles) and the fluctuation flux profiles (tidal covariance between current velocity and sediment concentration). Three different types of bottom sediment pools are distinguished to vary the sediment availability, by defining a time scale for complete sediment erosion. For short erosion times scales, the transport sediment flux may dominate, but for larger erosion time scales the fluctuation sediment flux largely dominates the tidal sediment flux. When quarter-diurnal components are added to the tidal forcing, up-estuary sediment fluxes are strongly increased for stronger and shorter flood tides and vice versa. The theoretical results are compared to field observations in a tidally energetic inlet.","channel flows; coastal flows; mixing; transport; turbulence; single column models","en","journal article","American Meteorological Society","","","","","","","2014-03-30","Electrical Engineering, Mathematics and Computer Science","Applied Mathematics","","","",""
"uuid:bc5304fa-dfe8-41af-97f3-30015ade9b55","http://resolver.tudelft.nl/uuid:bc5304fa-dfe8-41af-97f3-30015ade9b55","Dust emission modelling around a stockpile by using computational fluid dynamics and discrete element method","Derakhshani, S.M.; Schott, D.L.; Lodewijks, G.","","2013","Dust emissions can have significant effects on the human health, environment and industry equipment. Understanding the dust generation process helps to select a suitable dust preventing approach and also is useful to evaluate the environmental impact of dust emission. To describe these processes, numerical methods such as Computational Fluid Dynamics (CFD) are widely used, however nowadays particle based methods like Discrete Element Method (DEM) allow researchers to model interaction between particles and fluid flow. In this study, air flow over a stockpile, dust emission, erosion and surface deformation of granular material in the form of stockpile are studied by using DEM and CFD as a coupled method. Two and three dimensional simulations are respectively developed for CFD and DEM methods to minimize CPU time. The standard ?-? turbulence model is used in a fully developed turbulent flow. The continuous gas phase and the discrete particle phase link to each other through gas-particle void fractions and momentum transfer. In addition to stockpile deformation, dust dispersion is studied and finally the accuracy of stockpile deformation results obtained by CFD-DEM modelling will be validated by the agreement with the existing experimental data.","air pollution; computational fluid dynamics; deformation; dust; finite element analysis; granular materials; turbulence; two-phase flow","en","conference paper","American Institute of Physics","","","","","","","","Mechanical, Maritime and Materials Engineering","Marine and Transport Technology","","","",""
"uuid:9e32b3d5-eae0-4012-9020-7b32079b4787","http://resolver.tudelft.nl/uuid:9e32b3d5-eae0-4012-9020-7b32079b4787","A Feature Tracking Velocimetry algorithm to determine the velocities in Negatively Buoyant Jets","Ferrari, S.; Badas, M.G.; Besalduch, L.A.; Querzoli, G.","","2013","We present a novel algorithm, namely Feature Tracking Velocimetry (FTV), which is less sensitive to the appearance and disappearance of particles and to high velocity gradients than classical Particle Image Velocimetry (PIV). The basic idea of FTV is to compare windows only where the motion detection may be successful, that is where there are high luminosity gradients. The FTV algorithm is suitable in presence of different seeding densities, where other techniques produce significant errors, due to the non-homogeneous seeding at the boundary of a flow. The FTV algorithm has been tested for the analysis of laboratory experiments on simple jets (SJs) and negatively buoyant jets (NBJs), both issuing from a sharp-edged orifice. Among the others, the velocity and Turbulent Kinetic Energy profiles, orthogonal to the jet axis, the mean streamwise centerline velocity decay and the integral Turbulent Kinetic Energy along the jet axis have been measured and analyzed. These quantities have been employed to study the differences between simple jets and NBJs, and to investigate how the increase in buoyancy affects the NBJ behavior. Moreover, mean velocity fields have been used to study the geometrical dimensions of the jet, while second order statistics, such as Turbulent Kinetic Energy, have been analyzed to characterize the turbulence structure governing the mixing processes.","Feature Tracking Velocimetry (FTV); simple jet; negatively buoyant jet; turbulence","en","conference paper","","","","","","","","","","","","","",""
"uuid:05820775-31c5-4aa5-8d68-cfd1103f99fc","http://resolver.tudelft.nl/uuid:05820775-31c5-4aa5-8d68-cfd1103f99fc","Stereo-PIV Measurement of Turbulence Shear Stress in a Stirred Flow Mixer","Shekhar, C.; Nishino, K.; Iso, Y.","","2013","The turbulence dissipation rate and turbulence shear stress are estimated inside a cylindrical, stirred flow mixer by carrying out Stereo PIV measurements in twelve vertical and three horizontal planes. The flow domain is vertically oriented, filled with the water. A commercially-available, three-blade impeller, HR-100, is used as the agitator. The impeller is mounted near the tip of a thin, rigid shaft, which is aligned along the central axis of the flow domain. The impeller rotates with the constant angular speed of 150RPM, and the Reynolds number based on the impeller diameter and the blade's tip-velocity is equal to 59400. The turbulence statistics in the vertical measurement planes are reported before (Shekhar C, Nishino K, Yamane Y and Huang J, Stereo-PIV measurement of turbulence characteristics in a flow mixer Journal of Visualization 15 (2012) pp.293~308), which revealed that the rotation induces a downward, as well as tangential, bulk flow motion, which convects the turbulence generated at the blade-water interface, causing the turbulence level below the impeller to be much higher than the level above it. The present study is the second part of the same project, and reports the turbulence statistics in the horizontal measurement planes. The results show that the turbulence level is high in the area swept by the rotating impeller blades and underneath. However, in the outside region, the turbulence damps down and becomes negligible. The vertical and horizontal measurement results are also combined to estimate the production, convection, viscous diffusion, and turbulence dissipation terms of the turbulence kinetic energy's budget equation, along with the turbulence shear stress, along the lines where the different vertical and horizontal planes intersect.","stirred mixer; stereo PIV; turbulence; budget equation; dissipation rate; shear stress","en","conference paper","","","","","","","","","","","","","",""
"uuid:6c26c923-1518-4e53-bd22-f66c6b49ae7c","http://resolver.tudelft.nl/uuid:6c26c923-1518-4e53-bd22-f66c6b49ae7c","Identification of wind energy systems","Van der Veen, G.J.","Verhaegen, M. (promotor); Van Wingerden, J.W. (promotor)","2013","In the next decades wind energy is expected to secure a firm share of the total energy production capacity in many countries. To increase competitiveness of wind power with other power sources it is essential to lower the cost of wind energy. Given the design of a turbine, this objective can be attained in several ways, for instance by increasing the energy production of a wind turbine and by lowering loads on the wind turbine in order to reduce maintenance costs. Research performed in recent years has shown that feedback control plays an important role in these two aspects. Refined control design can increase power production, for instance by using feedforward information about the wind field. At the same time, control can reduce wear of the turbine by mitigating fatigue and extreme loads. For the design process of new and advanced control concepts which meet these objectives, detailed models are essential. System identification on the basis of experimental data can provide such models and help to understand differences between the behaviour of theoretical models and the real wind turbine. In this thesis we address specific challenges related to the application of system identification techniques to wind turbines and similar systems. The result is a set of methods which can be applied to wind turbines in practical situations.","system identification; wind turbines; turbulence; systems and control theory","en","doctoral thesis","","","","","","","","2013-04-16","Mechanical, Maritime and Materials Engineering","Delft Center for Systems and Control","","","",""
"uuid:cd6fe4c6-c420-4e88-b711-efd5d86714db","http://resolver.tudelft.nl/uuid:cd6fe4c6-c420-4e88-b711-efd5d86714db","Three-dimensional vortex analysis and aeroacoustic source characterization of jet core breakdown","Violato, D.; Scarano, F.","","2013","The three-dimensional behavior of jet core breakdown is investigated with experiments conducted on a free water jet at Re = 5000 by time-resolved tomographic particle image velocimetry (TR-TOMO PIV). The investigated domain encompasses the range between 0 and 10 jet diameters. The characteristic pulsatile motion of vortex ring shedding and pairing culminates with the growth of four primary in-plane and out-of-plane azimuthal waves and leads to the formation of streamwise vortices. Vortex ring humps are tilted and ejected along the axial direction as they are subjected to higher axial velocities. By the end of the potential core, this process causes the breakdown of the vortex ring regime and the onset of streamwise filaments oriented at 30°-45° to the jet axis and “C” shaped peripheral structures. The latter re-organize further downstream in filaments oriented along the azimuthal direction at the jet periphery. Instead, in the vicinity of the jet axis the filaments do not exhibit any preferential direction resembling the isotropic turbulent regime. Following Powell's aeroacoustic analogy, the instantaneous spatial distribution of the acoustic source term is mapped by the second time derivative of the Lamb vector, revealing the highest activity during vortex ring breakdown. A three-dimensional modal analysis of velocity, vorticity, Lamb vector, and Lamb vector second time derivative fields is conducted by proper orthogonal decomposition (POD) within the first 10 modes. The decomposed velocity fluctuations describe a helical organization in the region of the jet core-breakdown and, further downstream, jet axis flapping and precession motions. By the end of the potential core, vorticity modes show that vortex rings are dominated by travelling waves of radial and axial vorticity with a characteristic 40°-45° inclination to the jet axis. The Lamb vector and the Lamb vector second time derivative modes exhibit similar patterns for the azimuthal component, whereas the vortex ring coherence is described by the radial and the axial components. While velocity, vorticity, and Lamb vector modes are typically associated with Strouhal numbers (St) smaller than 0.9, the modes of the Lamb vector second time derivative are also related to higher frequencies (1.05 ? St ? 1.9) ascribed to the three-dimensional travelling waves. Far-field acoustic predictions are obtained on the basis of direct evaluation of Powell's analogy with TR-TOMO PIV data. The spectral analysis returns peaks at pairing (St = 0.36) and shedding (St = 0.72) frequency. A broader distribution with a hump between St = 1 and 2.25 is observed, which corresponds to the breakdown of ring vortices.","aeroacoustics; flow visualisation; jets; pulsatile flow; turbulence; vortices; water waves","en","journal article","American Institute of Physics","","","","","","","","Aerospace Engineering","Aerodynamics, Wind Energy & Propulsion","","","",""
"uuid:5691f357-5468-430e-8d49-d330350967a9","http://resolver.tudelft.nl/uuid:5691f357-5468-430e-8d49-d330350967a9","Experimental infrared measurements for hydrocarbon pollutant determination in subterranean waters","Lay-Ekuakille, A.; Palamara, I.; Caratelli, D.; Morabito, F.C.","","2013","Subterranean waters are often polluted by industrial and anthropic effluents that are drained in subsoil. To prevent and control pollution, legislations of different developed countries require an online monitoring measurement, especially for detecting organic solvents (chlorinated and unchlorinated ones). Online measurements include both real-time and no real-time measurements. In general, it is difficult to implement real-time measurements in stricto sensu for online acquisitions on aqueous effluents since they need to be processed by a modeling. This research presents an experimental measurement system based on infrared (IR) spectroscopy for aqueous effluents containing hydrocarbons and capable of displaying excellent values of pollutant concentrations even in instable conditions; the system is able to detect pollutants either in laminar or turbulent flow. The results show the possibility of avoiding the use of “Pitot tube” that is employed to create a stagnation point in order to convert kinetic energy into potential one. This conversion allows the transformation of a turbulent flow in a laminar flow making easy measurement of pollutants included in an aqueous effluent. Obviously, “Pitot tube” is also used for other fluid effluents. The obtained results have been compared with those produced by means of sophisticated IR instrumentation for laboratory applications.","effluents; geochemistry; geophysical fluid dynamics; hydrological techniques; infrared spectroscopy; laminar flow; soil; solvents (industrial); turbulence; water pollution control","en","journal article","American Institute of Physics","","","","","","","","Electrical Engineering, Mathematics and Computer Science","Microelectronics","","","",""
"uuid:98cbc8db-865f-429d-bff8-86b531ff0bed","http://resolver.tudelft.nl/uuid:98cbc8db-865f-429d-bff8-86b531ff0bed","Turbulence Transition in Shear Flows: Chaos in High-Dimensional Spaces","Eckhardt, B.E.","","2012","The study of the transition to turbulence in parallel shear flows without linear instability of the laminar profile has profited immensely from the application of dynamical systems ideas. Studies of the transition in plane Couette flow and pipe flow, in particular, have shown that the transition is connected with the appearance of 3-d coherent structures that form a chaotic saddle which shows up in a transient turbulent dynamics. It is remarkable that these concepts, initially developed for low-dimensional systems, also work in such a high-dimensional setting. The present note contains a brief summary of key features and a short list of references for further reading.","turbulence; transition","en","journal article","Elsevier","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:294e3244-28d5-40b9-8c3a-030f36930231","http://resolver.tudelft.nl/uuid:294e3244-28d5-40b9-8c3a-030f36930231","A conceptual framework for shear flow–induced erosion of soft cohesive sediment beds","Winterwerp, J.C.; Van Kesteren, W.G.M.; Van Prooijen, B.C.; Jacobs, W.","","2012","This paper proposes a conceptual framework for erosion of cohesive sediment beds. We focus on cohesive beds, distinguishing between floc erosion, surface erosion, and mass erosion. By (our) definition, surface erosion is a drained soil mechanical process, whereas mass erosion occurs under undrained conditions. The eroding shear stress is modeled through a probability density function. This yields a continuous description of floc erosion and surface erosion as a function of mean bed shear stress. Furthermore, we assume a distribution for the bed strength. The mean values of the bed strength are derived from soil mechanical theory, assuming that the surface erosion rate is limited by the swelling rate from the undrained shear strength in the bed to its drained value at its surface. The rate of erosion then relates to the undrained shear strength of the soil, and its consolidation (swelling) coefficient. The critical shear stress for erosion is slightly larger than the true cohesion of the bed, i.e., the drained strength, and follows a power law relation with the plasticity index. The conceptual framework proposed herein has been validated against a limited number of experimental data, and has a series of advantages above other methods of direct measuring erodibility, as it is inexpensive and can be used to attain space-covering information on the sediment bed. Moreover, the use of bulk soil mechanical parameters accounts implicitly for the effects of organic material, though the role of, e.g., macrophytobenthos mats and/or bioturbation is difficult to capture a priori.","cohesive sediment; erosion; soil properties; turbulence","en","journal article","American Geophysical Union","","","","","","","2013-04-24","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:d6229817-08eb-415b-bd6a-fbb590f366ba","http://resolver.tudelft.nl/uuid:d6229817-08eb-415b-bd6a-fbb590f366ba","On the effect of turbulence on bubbles: Experiments and numerical simulations of bubbles in wall-bounded flows","Harleman, M.J.W.","Westerweel, J. (promotor); Van Terwisga, T.J.C. (promotor)","2012","Previous research has shown that the addition of gas bubbles to a turbulent boundary layer in water can reduce the local skin-friction drag by up to 80%. Application of this technique to ships seems promising, but to date no significant drag reductions are obtained on full-scale ships. More knowledge about the drag reduction mechanism is required to better understand its restrictions and potential. The dispersion of small bubbles (d+ of about 1) in horizontal turbulent channel flow is studied by numerical simulations and by experiments with simultaneous PIV and bubble shadowgraphy. It is shown that the equilibrium bubble concentration distribution is described by a Rouse-profile, based on a gradient-diffusion hypothesis. The combination of turbulent mixing and a wall-normal concentration gradient results in a preferential concentration, so despite the small Stokes number (St<<1) the bubbles are not ideal flow tracers. Bubbles are driven away from the wall by turbophoresis, which may create a bubble-free layer that reduces drag reduction effectiveness. Finally, the drag reducing effect of electrolysis bubbles is determined from measurements of the velocity profile. In the literature high local drag reductions are obtained with very low concentrations of small bubbles, but in our experiments neither local or global drag reductions are measured. Therefore, the bubble drag reduction effectiveness is limited and seems to be essentially caused by a density-reduction effect.","turbulence; bubbles; drag reduction","en","doctoral thesis","Uitgeverij BOXPress 's-Hertogenbosch","","","","","","","2012-07-02","Mechanical, Maritime and Materials Engineering","Proces and Energy","","","",""
"uuid:f112b25d-bfc6-4771-8201-2ad5860cf373","http://resolver.tudelft.nl/uuid:f112b25d-bfc6-4771-8201-2ad5860cf373","Macrophytes in estuarine gradients: Flow through flexible vegetation","Dijkstra, J.T.","Stive, M.J.F. (promotor); Uijttewaal, W.S.J. (promotor)","2012","Aquatic plants –or macrophytes- are an important part of coastal, estuarine and freshwater ecosystems worldwide, both from an ecological and an engineering viewpoint. Their meadows provide a wide range of ecosystem services: forming a physical protection of the shoreline, enhancing water quality and harbouring many other organisms. Unfortunately, these vegetations such as salt marshes, seagrasses or mangroves have been on the decline as a result of anthropogenic pressure and climate change, despite costly conservation and restoration efforts. The low success rate of these efforts might partially be due to a lack of understanding of the complex bio-physical interactions between plant properties, plant growth, hydro- and morphodynamics and water quality. The capability of plants to alter their abiotic environment via these interactions is referred to as ‘ecosystem engineering’. Many experimental studies, both in the field and in laboratory flumes, have been performed to unravel these interactions. Since such experiments are always hampered by practical limitations such as flume dimensions, available time, or uncontrolled conditions, this knowledge cannot always be generically applied. Therefore, the primary objective of this study is to develop a generically applicable model for feedbacks between flexible macrophytes and their physical environment. To warrant this general applicability under the various circumstances occurring in estuaries, the model development follows a process based approach; a data-orientated approach is merely applicable to known conditions. Modelling starts out on the scale of one plant to finish at the scale of a meadow. The focus is on seagrass, as seagrasses are well studied, highly flexible, have a relatively simple shape and are among the most productive as well as threatened ecosystems. The first step was to create the numerical model called ‘Dynveg’, by combining a novel dynamic plant bending model based on a Lagrangian force balance to an existing 1DV k-? turbulence model (Chapter 2). The plant bending model is based on measurable biomechanical properties of plants: length, width, thickness, volumetric density and the elasticity modulus. Because very flexible plants can assume a position almost parallel to the flow direction, friction too needed to be incorporated rather than pressure drag alone. Flume measurements on strips of eelgrass-like proportions provided the actual values for drag- and friction coefficients, as well as validation data for predicted strip positions and forces. The effect of multiple plants on hydrodynamics was incorporated by assuming that all plants in a meadow do the same, and by defining two turbulence length scales: One for internally generated turbulence, related to the wakes behind individual stems, and one for larger eddies created in the shear layer above, penetrating the canopy depending on the space between the stems. Dynveg compared favourably with the measurements of hydrodynamic characteristics in mimicked eelgrass by Nepf & Vivoni [2000]. Next, Dynveg was combined with the large-scale hydro- and morphodynamic model Delft3D to simulate two-dimensional spatial processes in and around meadows of flexible macrophytes (Chapter 3). The leading principle for this integration is the conditional similarity between flow characteristics in flexible vegetation and those in rigid vegetation: If the rigid vegetation has i) the same height as the deflected vegetation, ii) its plant volume redistributed over the vertical accordingly and iii) a drag coefficient representative of the streamlined shape, the flow is practically analogous for a range of plant properties and hydrodynamic conditions. This modelling method was validated by comparing model results with flume experiments on two seagrass species, showing good agreement for canopy height, flow velocity profile and flow adaptation length. A field measurement campaign in a French macrotidal bay bordered by an eelgrass meadow provided validation data for application to real meadows (Chapter 5). Along with a detailed bathymetry survey by jetski, time-series of flow velocity and sediment dynamics inside a meadow and over a bare adjacent area were measured over two tidal periods. The applied sediment transport formula [van Rijn, 1993] deals with vegetation effects on sediment pick-up and transport via the effects of plants on hydrodynamics. Vegetation-specific interactions such as particle trapping by blades or flow intensification directly around shoots were not taken into account. Nevertheless, the three-dimensional numerical model was able to reproduce the main features of the observations, indicating that the processes of vegetation bending in non-stationary flow and sediment transport through vegetated areas are incorporated correctly. Thus, the objective of making a model for feedbacks between flexible macrophytes and their physical environment has been met. The model can be applied as a tool in conservation and restoration studies or in long-term biogeomorphological feedback studies. Recommended extensions are the incorporation of plant-wave interactions, more intricate plant morphologies and a vegetation-specific transport formula. The second objective of this thesis was to use the developed model(s) as a tool to learn more about biophysical interactions under different conditions. In Chapter 4, Dynveg and the two-dimensional model were used to assess the ecosystem engineering capacities of three plant species that partly co-occur in temperate intertidal areas: the stiff Spartina anglica, the short flexible seagrass Zostera noltii and the tall flexible seagrass Zostera marina. The flow velocity inside the canopy, the canopy flux and the bed shear stress were used as proxies for the species’ ability to respectively absorb hydrodynamic energy, the supply of nutrients or sediment and the ability to prevent erosion. This analysis showed that a species’ eco-engineering capacities depend on its spatial density, its size, its structural rigidity and its buoyancy, but also on environmental conditions. Therefore, biomass, leaf area index or other lumped parameters that neglect structural properties are no good generic indicators of ecosystem engineering capacities. Rigid plants have more potential to trap sediment due to a higher canopy flux than flexible plants. This canopy flux showed to be inversely related to spatial density along the entire natural range. For flexible plants, the canopy flux is only related to density in relatively sparse meadows; in denser meadows the canopy flux is constant with increasing density. Flexible plants are better at preventing erosion because they are more efficient in reducing bed shear stresses than rigid plants. For very thin plants, buoyancy is the most important determinant of position in given flow conditions. For intermediate flexible plants, the structural rigidity is the most influential parameter, whereas for (nearly) rigid plants, the spatial density is dominant. In Chapter 6, the three-dimensional model of the macrotidal bay was used to study the effects of different types of macrophytes on (residual) sediment transport and light availability. The effects of the real, relatively sparse eelgrass meadow were compared to those of a meadow with rigid plants of the same spatial density, with a dense eelgrass meadow, and with a bare bed. Though the differences between these four vegetation scenarios were small –only a few percent- the consequences on long timescales can be considerable. In deep water, sparse flexible vegetation kept more sediment inside the bay than rigid or denser plants. When vegetation only occupies a small part of the water column, plants prevent erosion rather than promote deposition and they have more effect on bed-load transport than on the transport of suspended sediment. Stiff and denser plants affect the bed-load more than sparse flexible vegetation, thereby blocking the transport from outside to inside. The presence of dense or stiff macrophytes increased the light availability at the bed over a tidal cycle up to 7% with respect to a bare bed. The increase of light availability was less pronounced for the relatively open eelgrass meadow: up to 3%. Overall, this study has resulted in a widely applicable model for the interactions between flexible aquatic plants, flow and sediment transport and in more insight in some of these interactions. Other researchers are encouraged to use this tool complementary to fieldwork and laboratory experiments, and to extend it with other functionalities, e.g. for wave attenuation or vegetation development.","macrophyte; flexible vegetation; estuary; hydrodynamics; turbulence; ecology; sediment transport; light climate; seagrass","en","doctoral thesis","","","","","","","","2012-03-06","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:415fad86-a326-4898-a495-343b41ea033b","http://resolver.tudelft.nl/uuid:415fad86-a326-4898-a495-343b41ea033b","Influence of wind conditions on wind turbine loads and measurement of turbulence using lidars","Sathe, A.R.","Van Bussel, G.J.W. (promotor); Mann, J. (promotor)","2012","Variations in wind conditions influence the loads on wind turbines significantly. In order to determine these loads it is important that the external conditions are well understood. Wind lidars are well developed nowadays to measure wind profiles upwards from the surface. But how turbulence can be measured using lidars has not yet been investigated. This PhD thesis deals with the influence of variations in wind conditions on the wind turbine loads as well as with the determination of wind conditions using wind lidars. Part I of the thesis focuses on analysis of diabatic wind profiles, turbulence, and their influence on wind turbine loads. The diabatic wind profiles are analyzed using the measurements from two offshore sites, one in the Dutch North Sea, and the other in the Danish North Sea. Two wind profile models are compared, one that is strictly valid in the atmospheric surface layer, and the other that is valid for the entire boundary layer. The second model is much more complicated in comparison to the first. It is demonstrated that at heights more than 50 m above the surface, where modern wind turbines usually operate, it is advisable to use a wind profile model that is valid in the entire boundary layer. The influence of diabatic wind profiles under steady winds on the fatigue damage at the blade root is also demonstrated using the aero-elastic simulation tool Bladed. Furthermore, detailed analysis of the combined influence of diabatic wind profile and turbulence on the blade root flap-wise and edge-wise moments, tower base fore-aft moment, and the rotor bending moments at the hub is carried out using the aero-elastic simulation tool HAWC2. It is found that the tower base fore-aft moment is influenced by diabatic turbulence and a rotor bending moment at the hub is influenced by diabatic wind profiles. The blade root loads are influenced by diabatic wind profiles and turbulence, which results in averaging of the loads, i.e. the calculated blade loads using diabatic wind conditions and those calculated using neutral wind conditions are approximately the same. The importance of obtaining a site-specific wind speed and stability distribution is also emphasized since it has a direct influence on wind turbine loads. In comparison with the IEC standards, which generalize the wind conditions according to certain classes of wind speeds, the site-specific wind conditions are demonstrated to give significantly lower fatigue loads. There is thus a potential in reducing wind turbine costs if site-specific wind conditions are obtained. In this regard we then are faced with measurement challenges. The current industry standard for the measurement of wind speed is either the cup or the sonic anemometer. Both instruments require a meteorological mast to be mounted at the measurement site. For measuring the wind profile the instruments need to be mounted at several heights on the mast. To install a mast and set up these instruments is quite expensive, especially at offshore sites, where the cost of foundation increases significantly. Besides, there are problems with the flow distortion that have to be taken care of. In order to overcome these problems it would be ideal to have a remote sensing instrument that measures wind speed. Wind lidars are capable of doing that albeit with a price. Part II of the thesis deals with detailed investigations of the ability of wind lidars to perform turulence measurements. Modelling of the systematic errors in turbulence measurements is carried out using basic principles. Two mechanisms are identified that cause these systematic errors. One is the averaging effect due to the large sample volume in which lidars measure wind speeds, and the other is the contribution of all components of the Reynolds stress tensor. Modelling of turbulence spectra as measured by a scanning pulsed wind lidar is also carried out. We now understand in detail the distribution of turbulent energy at various wavenumbers, when a pulsed wind lidar measures turbulence. The lidar turbulence models have been verified with the measurements at different heights and under different atmospheric stabilities. Finally, a new method is investigated that in principle makes turbulence measurements by lidars possible. The so-called six beam method uses six lidar beams to avoid the contamination by all components of the Reynolds stress tensor. The theoretical calculations carried out demonstrates the potential of this method. In order to avoid averaging due to volume sampling, a different analysis method is required, which has not been investigated in this thesis. To summarize the entire thesis, it can be said that more work is required to ascertain the influence of atmospheric stability on wind turbine loads. In particular, comparing with the load measurements will go a long way in consolidating the understanding gained from the analysis in this thesis. If lidars are able to measure turbulence, there is a tremendous potential for performing site-specific wind turbine design and making the class based design of the IEC standards obsolete.","wind profiles; turbulence; wind turbine loads; lidar turbulence measurement","en","doctoral thesis","DUWIND Delft University Wind Energy Research Institute","","","","","","","2012-02-13","Aerospace Engineering","Wind Energy","","","",""
"uuid:3fa8d72d-c3aa-4786-a321-41433ad1df78","http://resolver.tudelft.nl/uuid:3fa8d72d-c3aa-4786-a321-41433ad1df78","Transport of suspended particles in turbulent open channel flows","Breugem, W.A.","Uijttewaal, W.S.J. (promotor); Stelling, G.S. (promotor)","2012","Two experiments are performed in order to investigate suspended sediment transport in a turbulent open channel flow. The first experiment used particle image velocimetry (PIV) to measure the fluid velocity with a high spatial resolution, while particle tracking velocimetry (PTV) was used to measure the velocity of individual sediment particles. The sediment particles were injected in the flume close to the free surface at different distances from the measurement section. In this way, the development of a sediment plume towards an equilibrium situation could be studied. The results were compared with direct numerical simulations, in which the particle equation of motion was used to calculate the movement of individual sediment particles. The second experiment used refractive index matching, in order to make the sediment particle invisible. In this way, a PIV experiment could be performed in order to determine changes in the flow and turbulence structure due to high sediment concentrations.","sediment transport; PIV; PTV; refractive index matching; coherent structures; turbulence; DNS; two way coupling","en","doctoral thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:9474c415-f726-48ed-ae85-91cfe103b5bb","http://resolver.tudelft.nl/uuid:9474c415-f726-48ed-ae85-91cfe103b5bb","An experimental study of transitional pulsatile pipe flow","Trip, R.; Kuik, D.J.; Westerweel, J.; Poelma, C.","","2012","The transitional regime of a sinusoidal pulsatile flow in a straight, rigid pipe is investigated using particle image velocimetry. The main aim is to investigate how the critical Reynolds number is affected by different pulsatile conditions, expressed as the Womersley number and the oscillatory Reynolds number. The transition occurs in the region of Re?=?2250-3000 and is characterized by an increasing number of isolated turbulence structures. Based on velocity fields and flow visualizations, these structures can be identified as puffs, similar to those observed in steady flow transition. Measurements at different Womersley numbers yield similar transition behavior, indicating that pulsatile effects do not play a role in the regime that is investigated. Variations of the oscillatory Reynolds number also appear to have little effect, so that the transition here seems to be determined only by the mean Reynolds number. For larger mean Reynolds numbers, a second regime is observed: here, the flow remains turbulent throughout the cycle. The turbulence intensity varies during the cycle, but has a phase shift with respect to the mean flow component. This is caused by a growth of kinetic energy during the decelerating part and a decay during the accelerating part of the cycle. Flow visualization experiments reveal that the flow develops localized turbulence at several random axial positions. The structures quickly grow to fill the entire pipe in the decelerating phase and (partially) decay during the accelerating phase.","flow visualisation; fluid oscillations; laminar to turbulent transitions; pipe flow; pulsatile flow; turbulence","en","journal article","American Institute of Physics","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:36a6086e-ced5-4396-8161-42e50a775a9e","http://resolver.tudelft.nl/uuid:36a6086e-ced5-4396-8161-42e50a775a9e","Three-dimensional evolution of flow structures in transitional circular and chevron jets","Violato, D.; Scarano, F.","","2011","The three-dimensional behavior of flow transition in circular and 6-chevron jets at Re?=?5000 is investigated with experiments conducted on a free water jet by time-resolved tomographic particle image velocimetry. The emphasis is on the unsteady organization of coherent flow structures, which play a role in the generation of acoustic noise. Shedding and pairing of vortices are the most pronounced phenomena observed in the near field of the circular jet. The first and second pairing amplify the axial pulsatile motion in the jet column and lead to the growth of azimuthal waves culminating in the breakup of the vortex ring. Streamwise vortices of axial and radial vorticity are observed in the outer region and move inward and outward under the effect of the vortex rings. In the jet with chevrons, the axisymmetric ring-like coherence of the circular jet is not encountered. Instead, streamwise flow structures of azimuthal vorticity emanate from the chevron apices, and counter-rotating streamwise vortices of axial and radial vorticity develop from the chevron notches. The decay of streamwise vortices is accompanied by the formation of C-shaped structures. The three-dimensional analysis allows quantifying the vortex stretching and tilting activity, which, for the circular jet exit, is related to the azimuthal instabilities and the streamwise vortices connecting the vortex rings. In the chevron jet, stretching and tilting peak during the formation of C-structures. Following Powell’s aeroacoustic analogy, the spatial distribution of the source term is mapped, evaluating the temporal derivative of the Lamb vector. The spatio-temporal evolution of such source term is visualized revealing that the events of highest activity are associated with the processes of vortex-ring pairing and vortex-ring disruption for the circular jet, and with the decay of streamwise instabilities and the formation of C-shaped structures for the chevron case.","acoustic noise; aeroacoustics; flow instability; flow visualisation; jets; spatiotemporal phenomena; turbulence; vortices","en","journal article","American Institute of Physics","","","","","","","","Aerospace Engineering","Aerodynamics & Wind Energy","","","",""
"uuid:89881656-c928-43de-ba5c-8018ba4f8b46","http://resolver.tudelft.nl/uuid:89881656-c928-43de-ba5c-8018ba4f8b46","An experimental study on the aeroacoustics of wall-bounded flows: Sound emission from a wall-mounted cavity, coupling of time-resolved PIV and acoustic analogies","Koschatzky, V.","Boersma, B.J. (promotor); Westerweel, J. (promotor)","2011","This thesis deals with the problem of noise. Sound is a constant presence in our lives. Most of the times it is something wanted and it serves a purpose, such as communication through speech or entertainment by listening to music. On the other hand, quite often sound is an annoying and unwanted by-product of some other activity necessary to us. This is what we usually refer to as noise. Noise does not simply create an unpleasant environment but can severely affect human health and have a serious impact on the natural environment as well. The noise pollution problem is addressed in many different ways: by clever urban planning, for instance, and by providing the heavily exposed buildings with acoustic barriers and insulation. Legislation also provides a limit on the maximum acceptable noise levels in many contexts such as working environments, discotheques and music headphones devices. An important effort in the fight against noise is obviously placed at the source of the problem itself, trying to make our tools and devices quieter. In order to do so it is essential to understand, and eventually be able to model, the processes through which sound is produced and propagated. The objective of this work is to develop and to employ experimental methods, based on the particle image velocimetry (PIV) measurement technique, for the estimate of the emitted sound from flow field measurements. In particular, the focus will be on the study of the noise that is generated by the unsteady loads on solid bodies immersed in the flow. The proposed technique, based on time-resolved PIV measurements, provides insight into the sound sources that is not possible to achieve with other established experimental methods. It also permits experimental investigation in those situations where other measurements would not be possible, for example, in the presence of a noisy environment or when a suitable anechoic tests facility is unavailable. In addition, it allows the estimation of the acoustic emission, coupled to the proper acoustic model, in the same fashion as done in hybrid computational approaches. This kind of experimental approach seems to be particularly suited for the study of aerodynamic sound from wall-bounded flows at low Mach number. The main limitation of a PIV-based method in general noise applications is in fact the poor temporal resolution currently available. However, for low enough velocities this is no longer a concern since the resolution is good enough to perform time resolved measurements. Moreover, an experimental technique might be advantageous with respect to computational studies for low Mach number and high Reynolds number flows with immersed solid bodies. The numerical simulation of the flow in the proximity of solid boundaries is in fact particularly complex. In this thesis, a rectangular wall-mounted shallow open cavity has been chosen as a test case, a series of experiments have been performed and different methods and solutions has been tested. In chapter 3 the main details of the experimental setup are given and Curle’s analogy is applied in its classical formulation. In this chapter we also give an estimate of the span-wise coherence of the flow, based on stereoscopic PIV measurements. Both the process of pressure calculation from PIV data and the application of Curle’s analogy are affected by several uncertainties, and simplifying hypotheses is necessary in the process. For this reason we also perform direct microphones measurements of the pressure fluctuations at the walls of the cavity and of the sound emitted. We can therefore compare those values with the estimated quantities from the PIV data in order to validate the results and to check the range of validity of the approach. We find that both the hydrodynamic pressure computed from the PIV data and the sound emission obtained applying Curle’s analogy have a frequency spectrum that is comparable to that of the direct microphones measurements. In section 3.7 we demonstrate that the larger flow structures, responsible for most of the sound, are rather two-dimensional and coherent in the span-wise direction. In chapter 4 we discuss the details of the implementation and solution of both Curle’s Analogy and the theory of vortex sound. In this chapter, in contrast with the implementation of the model performed in chapter 3, we take into account the presence of the non-compact wall in which the cavity is located by using the image principle in the derivation of the solution. The two methods are derived under the assumption of low Mach number and high Reynolds number and for a listener positioned in the far field. The two analogies perform quite well for the present test case and give very similar results, both in total intensity and in the spectral distribution of the emitted sound. In their application, however, they each have different strengths and weaknesses. The two solutions are in fact derived through quite different pathways, and the mathematical schemes used to solve the equations are sensitive to different factors. The choice for either of the two methods needs therefore to be carefully done in relation to the specific application. The use of the image principle seems to be crucial to properly estimate the sound emission for compact geometries placed in large non-compact surfaces. In chapter 5 we investigate the effect of the three-dimensional velocity fluctuations on the final result. We discuss the results obtained by time-resolved thin tomographic PIV measurements. These measurements provided enough velocity vectors in the span-wise direction to allow for the calculation of the differential quantities in that direction and therefore for the computation of the full source terms of both Curle’s analogy and Theory of Vortex Sound. Details about the new experimental setup and measurement technique were given as well. Results showed that the flow is indeed rather two-dimensional and that there is little difference between the analogies source terms computed two or three-dimensionally. Compared to two-dimensional measurements, volumetric PIV measurements are more expensive, require more complex setups and the obtained data requires a much longer processing time. Moreover, the quality of the data is often lower than that of planar PIV. At the same time thin tomographic measurements do not seem to add relevant information to the computation of the sound emission for our experimental case, that is representative of quasi two-dimensional wall-bounded flows where the main source of sound is determined by large span-wise coherent flow structures.","aeroacoustics; cavity; turbulence; PIV; particle image velocimetry","en","doctoral thesis","","","","","","","","2012-02-02","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:4ce7bf05-f252-48e8-abb3-1f26fb367849","http://resolver.tudelft.nl/uuid:4ce7bf05-f252-48e8-abb3-1f26fb367849","Localized turbulence in pipe flow","Kuik, D.J.","Westerweel, J. (promotor)","2011","In this thesis the transition to turbulence in pipe flow is investigated. At low Reynolds numbers, the flow returns to the laminar state spontaneously. At high Reynolds number a small perturbation causes the flow to suddenly become turbulent. In the intermediate regime localized turbulence is observed. This regime is investigated using experimental and numerical methods. It is shown that there is not a critical Reynolds number above which the turbulent state becomes an attractor, hence that it cannot return to the laminar state.","pipe flow; turbulence; transition","en","doctoral thesis","","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:1f22fa6a-3e73-4189-83a4-ed1179d92d23","http://resolver.tudelft.nl/uuid:1f22fa6a-3e73-4189-83a4-ed1179d92d23","Can wind lidars measure turbulence?","Sathe, A.; Mann, J.; Gottschall, J.; Courtney, M.S.","","2011","Modeling of the systematic errors in the second-order moments of wind speeds measured by continuouswave (ZephIR) and pulsed (WindCube) lidars is presented. These lidars use the conical scanning technique to measure the velocity field. The model captures the effect of volume illumination and conical scanning. The predictions are compared with the measurements from the ZephIR, WindCube, and sonic anemometers at a flat terrain test site under different atmospheric stability conditions. The sonic measurements are used at several heights on a meteorological mast in combination with lidars that are placed on the ground. Results show that the systematic errors are up to 90% for the vertical velocity variance, whereas they are up to 70% for the horizontal velocity variance. For the ZephIR, the systematic errors increase with height, whereas for the WindCube, they decrease with height. The systematic errors also vary with atmospheric stability and are low for unstable conditions. In general, for both lidars, the model agrees well with the measurements at all heights and under different atmospheric stability conditions. For the ZephIR, the model results are improved when an additional low-pass filter for the 3-s scan is also modeled. It is concluded that with the current measurement configuration, these lidars cannot be used to measure turbulence precisely.","lidars; turbulence; wind","en","journal article","American Meteorological Society","","","","","","","2012-02-28","Aerospace Engineering","Aerodynamics and Wind Energy","","","",""
"uuid:8af6d7c4-77ec-4da1-bfbc-bd29371cf17d","http://resolver.tudelft.nl/uuid:8af6d7c4-77ec-4da1-bfbc-bd29371cf17d","Electromagnetically driven dwarf tornados in turbulent convection","Kenjeres, S.","","2011","Motivated by the concept of interdependency of turbulent flow and electromagnetic fields inside the spiraling galaxies, we explored the possibilities of generating a localized Lorentz force that will produce a three-dimensional swirling flow in weakly conductive fluids. Multiple vortical flow patterns were generated by combining arrays of permanent magnets and electrodes with supplied dc current. This concept was numerically simulated and applied to affect natural convection flow, turbulence, and heat transfer inside a rectangular enclosure heated from below and cooled from above over a range of Rayleigh numbers. The large-eddy simulations revealed that for low- and intermediate-values of Ra, the heat transfer was increased more than five times when an electromagnetic forcing was activated. In contrast to the generally accepted view that electromagnetic forcing will suppress velocity fluctuations and will increase anisotropy of turbulence, we demonstrated that localized forcing can enhance turbulence isotropy of thermal convection compared to its neutral state.","confined flow; electrohydrodynamics; flow simulation; fluctuations; magnetohydrodynamics; natural convection; numerical analysis; pattern formation; turbulence; vortices","en","journal article","American Institute of Physics","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:2a71de36-6ce8-447b-b031-5ba4eb877422","http://resolver.tudelft.nl/uuid:2a71de36-6ce8-447b-b031-5ba4eb877422","Modeling the interaction between flow and highly flexible aquatic vegetation","Dijkstra, J.T.; Uittenbogaard, R.E.","","2010","Aquatic vegetation has an important role in estuaries and rivers by acting as bed stabilizer, filter, food source and nursing area. However, macrophyte populations worldwide are under high anthropogenic pressure. Protection and restoration efforts will benefit from more insight into the interaction between vegetation, currents, waves and sediment transport. Most aquatic plants are very flexible, implying that their shape and hence their drag and turbulence production depend on the flow conditions. We have developed a numerical simulation model that describes this dynamic interaction between very flexible vegetation and a time-varying flow, using the seagrass Zostera marina as an example. The model consists of two parts: an existing 1DV k-? turbulence model simulating the flow combined with a new model simulating the bending of the plants, based on a force balance that takes account of both vegetation position and buoyancy. We validated this model using observations of positions of flexible plastic strips and of the forces they are subjected to, as well as hydrodynamic measurements. The model predicts important properties like the forces on plants, flow velocity profiles and turbulence characteristics well. Although the validation data are limited, the results are sufficiently encouraging to consider our model to be of generic value in studying flow processes in fields of flexible vegetation.","flexible aquatic vegetation; seagrass; turbulence; drag coefficient measurements; modelling","en","journal article","American Geophysical Union","","","","","","","2011-06-01","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:ef2c2b63-c4e4-4e3f-bed2-57ed98e4454c","http://resolver.tudelft.nl/uuid:ef2c2b63-c4e4-4e3f-bed2-57ed98e4454c","Dynamics of shallow lateral shear layers: Experimental study in a river with a sandy bed","Sukhodolov, A.N.; Schnauder, I.; Uijttewaal, W.S.J.","","2010","Shallow lateral shear layers forming between flows with different velocities, though essential for mixing processes in natural streams, have been examined only in laboratory settings using smooth, fixed?bed channels. This paper reports the results of an experimental study of a shear layer in a straight reach of a natural river where the layer, in contrast to the two?dimensional structure observed in the laboratory, is highly three?dimensional. The study included pronounced transverse pressure gradients, which influenced shear layer structure compared to flume experiments. It also introduces an analysis that complements conventional theory on mixing layers. The lateral velocity gradient between the flows downstream from a splitter plate placed in the river, the principal controlling factor, was adjusted for three experimental runs to determine the influence of different gradients on shear?layer dynamics. In each run, detailed three?dimensional measurements of mean and turbulent characteristics were obtained at five cross sections downstream from the splitter plate. Although experimental results agreed with conventional mixing?layer theories with respect to turbulence, the dynamics of the shear layers were dominated by the mean lateral fluxes of momentum. After re?examining the governing equations, we developed a parabolic equation describing the shear layer evolution and several scaling relations for essential terms of the energy budget: mean and turbulent lateral fluxes of momentum, turbulent kinetic energy, and dissipation rates. The study also provides insight into the spectral dynamics of turbulence in the shear layer and clarifies previous observations reported for confluences in natural streams.","rivers; confluences; shear layers; turbulence","en","journal article","American Geophysical Union","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:7a429d4d-ebf2-4582-88cb-c440974cb021","http://resolver.tudelft.nl/uuid:7a429d4d-ebf2-4582-88cb-c440974cb021","Long-term unsteadiness and large-scale structures in Rayleigh-Bénard convection with and without electromagnetic forcing","Verdoold, J.","Hanjalic, K. (promotor)","2010","This dissertation focuses on turbulent thermal convection, which occurs in a wide range of (geo)physical situations, like in the atmosphere, the oceans, the interior of stars or planets, and engineering applications, like metal casting or crystal growth processes. In this work, a special type of thermal convection, Rayleigh-Bénard convection, is studied with and without an additional electromagnetic body force. Experiments are performed in a rectangular RB convection cell of aspect-ratio ?=4 filled with water. Particle image velocimetry (PIV), laser Doppler anemometry (LDA), and liquid crystal thermography (LCT) measurements are used to obtain velocity and temperature information. The first part of this dissertation describes an investigation of the large-scale circulation (LSC) in Rayleigh-Bénard convection. In the second part, a study of the boundary layers in Rayleigh-Bénard convection is described. The last part reports on the control and enhancement of the heat transfer in Rayleigh-Bénard convection using an additional electromagnetic body force.","turbulence; thermal plume; buoyancy; natural convection; Rayleigh-Bénard convection; heat transfer; lda; lct; piv; magnetohydrodynamics; large-scale circulation; Karhunen-Loève analysis","en","doctoral thesis","Wöhrmann Print Service, Zutphen","","","","","","","2010-10-27","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:19920864-606e-4cb1-9f67-9a278f62a1f8","http://resolver.tudelft.nl/uuid:19920864-606e-4cb1-9f67-9a278f62a1f8","Fast reconstruction and prediction of frozen flow turbulence based on structured Kalman filtering","Fraanje, P.R.; Rice, J.; Verhaegen, M.; Doelman, N.","","2010","Efficient and optimal prediction of frozen flow turbulence using the complete observation history of the wavefront sensor is an important issue in adaptive optics for large ground-based telescopes. At least for the sake of error budgeting and algorithm performance, the evaluation of an accurate estimate of the optimal performance of a particular adaptive optics configuration is important. However, due to the large number of grid points, high sampling rates, and the non-rationality of the turbulence power spectral density, the computational complexity of the optimal predictor is huge. This paper shows how a structure in the frozen flow propagation can be exploited to obtain a state-space innovation model with a particular sparsity structure. This sparsity structure enables one to efficiently compute a structured Kalman filter. By simulation it is shown that the performance can be improved and the computational complexity can be reduced in comparison with auto-regressive predictors of low order.","probability theory, stochastic processes, and statistics; active or adaptive optics; turbulence; wave-front sensing; active or adaptive optics","en","journal article","Optical Society of America","","","","","","","","Mechanical, Maritime and Materials Engineering","Delft Center for Systems and Control","","","",""
"uuid:c015e479-8d7d-41d9-b9e9-9f6029712a4d","http://resolver.tudelft.nl/uuid:c015e479-8d7d-41d9-b9e9-9f6029712a4d","Large-eddy simulation of a curved open-channel flow over topography","Van Balen, W.; Uijttewaal, W.S.J.; Blanckaert, K.","","2010","Large-eddy simulation (LES) is performed of a curved open-channel flow over topography based on the laboratory experiment by Blanckaert [“Topographic steering, flow circulation, velocity redistribution and bed topography in sharp meander bends,” Water Resour. Res., doi:10.1029/2009WR008303 (in press)] . In the experiment, the large-scale bed topography had developed to a more or less stationary shape which was prescribed in the LES model as boundary conditions neglecting the small-scale dune forms by means of a straightforward immersed boundary scheme in combination with a simple wall-modeling approach. The small-scale dunes are accounted for in the numerical model by means of parametrization. Sensitivity of the flow to this roughness parametrization is examined by simulating the flow for three different roughness heights. It was found that, notwithstanding the coarse method of representing the dune forms, the qualitative agreement of the experimental results and the LES results is rather good. Comparison of the LES results with the Reynolds averaged numerical simulation results of Zeng et al. [“Flow and bathymetry in sharp open-channel bends: Experiments and predictions,” Water Resour. Res. 44, W09401, doi:10.1029/2007WR006303 (2008)] reveals surprisingly good agreement. This good agreement is explained by the minor importance of turbulence stress gradients in the contribution to the transverse and streamwise momentum balance. Moreover, it is found that in the bend the structure of the Reynolds stress tensor shows a tendency toward isotropy which enhances the performance of isotropic eddy viscosity closure models of turbulence. This observation is remarkable since highly anisotropic turbulence might well be expected considering the complex nature of the geometry. Furthermore, the LES results reveal a pronounced recirculation zone near the convex inner bank of the flume due to the shallowness of the flow and strong curvature of the flume. At the interface between the recirculation zone and the main flow, a curved mixing layer is identified as well as strong upwelling flow motion that is accompanied with large production of turbulent kinetic energy.","channel flow; flow simulation; turbulence; viscosity","en","journal article","American Institute of Physics","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:80878b38-c405-4567-8af9-cd582d6713ec","http://resolver.tudelft.nl/uuid:80878b38-c405-4567-8af9-cd582d6713ec","Curved open-channel flows - a numerical study","Van Balen, W.","Uijttewaal, W.S.J. (promotor); Stelling, G.S. (promotor)","2010","The flow of water through a river bend can be characterized as a turbulent open-channel flow that is dominated by streamline curvature. In order to adequately manage the planimetry of meandering rivers and the according safety aspects, the features of the flow and the associated turbulence have to be understood well. This thesis presents results of detailed numerical simulations of curved open-channel flows on a laboratory scale. These flows can basically be considered as downscaled river bend flows. The presented results provide a broad image of the behavior of curved open-channel flows in general and their characteristics with respect to the secondary flow, the bed shear stresses and turbulence in particular. Hereby, these results also facilitate further development of parameterizations of these key bend flow features for lower-dimensional modeling tools that are used in the engineering practice.","lLarge-eddy simulation; turbulence; river flow","en","doctoral thesis","VSSD","","","","","","","","Civil Engineering and Geosciences","Environmental Fluid Mechanics","","","",""
"uuid:23aa05e7-0762-4d13-bbe9-adccfc7e505a","http://resolver.tudelft.nl/uuid:23aa05e7-0762-4d13-bbe9-adccfc7e505a","On the scaling and unsteadiness of shock induced separation","Souverein, L.J.","Bakker, P.G. (promotor); Dussauge, J.P. (promotor)","2010","Shock wave boundary layer interactions (SWBLI) are a common phenomenon in transonic and supersonic flows. The presence of shock waves, induced by specific geometrical configurations, causes a rapid increase of the pressure, which can lead to flow separation. Examples of such interactions are found in amongst others rocket engine nozzles and on aerospikes, on re-entry vehicles, in supersonic and hypersonic engine intakes, and at the tips of compressor and turbine blades in jet engines. The interactions are important factors in vehicle development. Both the separated flow and the induced shock have been shown to be highly unsteady, causing pressure fluctuations and thermal loading. This generally leads to a degraded performance and possibly structural failure. The current work therefore aims to improve the physical understanding of the mechanisms that govern the interaction, with a special attention for the flow organisation and for the sources of the unsteadiness of the induced shock. In particular, the case of a reflecting incident shock is investigated, but the results find their application more generally in other configurations. Additionally, it is verified whether the interaction can be controlled by means of upstream fluid injection. To attain these aims, experiments were performed, comparing systematically several interactions for a range of shock intensities (producing incipiently separated and well separated flows) and under a number of flow conditions (Mach numbers of 1.7 and 2.3 and Reynolds numbers of 5,000 (‘low’) and 50,000 (‘high’)). This was done using the latest developments in the field of measurement techniques. A large amount of data was obtained for multiple interactions by means of a range of flow diagnostic techniques, yielding highly consistent results. A full field determination of the characteristic time scales by means of dual plane particle image velocimetry (Dual-PIV) has shown that the unsteadiness frequencies in the high Reynolds number incipient interaction span almost three orders of magnitude, demonstrating additionally the existence of low frequency dynamics of the reflected shock. The effect of control by means of air jet vortex generators (AJVGs) was thoroughly characterised, putting in evidence the generation of pairs of counter-rotating vortices of unequal strength that induce streaks of low and high speed fluid. These in their turn modify the separation bubble size without suppressing it. There is an inversely proportional relation between the reflected shock frequency and the bubble size. A scaling analysis was made, aimed at reconciling the observed discrepancies between interactions documented in literature. As part of this analysis, a separation criterion has been formulated that depends on the free-stream Mach number and the flow deflection angle only. In addition, a scaling approach has been derived for the interaction length based on the mass and momentum conservation. A conditional analysis has been performed based on the instantaneous separation bubble size. The generation and successive shedding of large coherent structures was found to be present also in absence of instantaneous flow separation. For the incipient cases, a link has been put into evidence between the separation region and the state of the upstream boundary layer. For the separated interactions, this link was absent and the shock unsteadiness seems to be mainly related to the separation bubble pulsation. The separation criterion in combination with the normalised interaction length represents a single trend line onto which all data for a large scope of documented interactions fall together with only a moderate scatter. This trend line predicts that the only way to effectively eliminate a separation bubble (without massive separation) by means of upstream control is by decreasing the displacement thickness of the incoming boundary layer. A scaling for the wall normal coordinate has been defined based on the interaction length with a correction for Mach number effects, producing a large resemblance in the geometric organisation of the mean and turbulent flow fields within the considered interactions. It can be concluded that multiple unsteadiness mechanisms are at work within the interaction, irrespective of the Mach number and the Reynolds number. It is proposed that the relative importance of the different mechanisms shifts with the imposed shock intensity. It seems that weak interactions without instantaneous flow separation should be governed by upstream effects only, with rather high shock frequencies. For incipient interactions, downstream effects start to occur; the region of high turbulence intensities displays mainly a lifting motion, producing a shock foot of varying strength and a shock unsteadiness that involves a time scales which can differ by at least one decade. Interactions with significant flow separation show mainly a translating motion, producing a low frequency unsteadiness and a shock foot of constant strength, which is in accordance with a free interaction behaviour. Concerning the Reynolds number and Mach number effects, it is concluded that for turbulent boundary layers, the onset of separation is Reynolds number independent. The interaction length is however governed by both the Reynolds number and the Mach number.","fluid dynamics; incident reflecting shock; compression ramp; shock wave; reflection; shock; turbulent; boundary layer; interaction; turbulence; compressible; supersonic; incipient; RSS; restricted; separation; unsteadiness; Mach; Reynolds; effect; Scaling; mass balance; control; air jet; vortex generator; PIV; Dual-PIV; time scales","en","doctoral thesis","","","","","","","","","Aerospace Engineering","Aerospace Design, Integration & Operations","","","",""
"uuid:b50eba10-ca07-4b52-9045-bb91bfef23be","http://resolver.tudelft.nl/uuid:b50eba10-ca07-4b52-9045-bb91bfef23be","The Micro-Pillar Shear-Stress Sensor MPS3 for Turbulent Flow","Grosse, S.; Schröder, W.","","2009","Wall-shear stress results from the relative motion of a fluid over a body surface as a consequence of the no-slip condition of the fluid in the vicinity of the wall. To determine the two-dimensional wall-shear stress distribution is of utter importance in theoretical and applied turbulence research. In this article, characteristics of the Micro-Pillar Shear-Stress Sensor MPS3, which has been shown to offer the potential to measure the two-directional dynamic wall-shear stress distribution in turbulent flows, will be summarized. After a brief general description of the sensor concept, material characteristics, possible sensor-structure related error sources, various sensitivity and distinct sensor performance aspects will be addressed. Especially, pressure-sensitivity related aspects will be discussed. This discussion will serve as ‘design rules’ for possible new fields of applications of the sensor technology.","fluid mechanics; turbulence; wall-shear stress; skin friction; micro-pillar; shear-stress sensor MPS3","en","journal article","MDPI AG","","","","","","","","Mechanical, Maritime and Materials Engineering","","","","",""
"uuid:f1d17514-77c0-4ed1-88ff-c46a1006f66d","http://resolver.tudelft.nl/uuid:f1d17514-77c0-4ed1-88ff-c46a1006f66d","Constrained stochastic simulation of wind gusts for wind turbine design","Bierbooms, W.A.A.M.","Van Bussel, G.J.W. (promotor); Van Kuik, G.A.M. (promotor)","2009","","wind energy; gust model; turbulence; extreme","en","doctoral thesis","Delft, DUWIND Delft University Wind Energy Research Institute","","","","","","","","Aerospace Engineering","","","","",""
"uuid:4108326c-ac6c-43bc-b94a-74f315c47a77","http://resolver.tudelft.nl/uuid:4108326c-ac6c-43bc-b94a-74f315c47a77","Analysis of particle-induced turbulence modulation and two-fluid closures by DNS channel flow","Bijlard, M.J.","Oliemans, R.V.A. (promotor); Ooms, G. (promotor)","2009","","turbulence; particles; particle-laden wall-bounded flow; Eulerian-Lagrangian simulation; particle-turbulence interaction; two-equation models; Direct Numerical Simulations (DNS)","en","doctoral thesis","","","","","","","","","Applied Sciences","","","","",""
"uuid:349c05b4-cb94-4dd1-bdd0-4edd03bf08b9","http://resolver.tudelft.nl/uuid:349c05b4-cb94-4dd1-bdd0-4edd03bf08b9","Unsteady Flow Organization of a Shock Wave/Boundary Layer Interaction","Humble, R.A.","Scarano, F. (promotor); Van Oudheusden, B.W. (promotor)","2009","A fundamental experimental study is carried out to investigate the unsteady flow organization of an incident shock wave/turbulent boundary layer interaction at Mach 2.1. Planar and tomographic particle image velocimetry (PIV) are used in combination with data processing using the proper orthogonal decomposition (POD), complemented with hot-wire anemometry (HWA) and nonlinear time series analyses. It is found that the global structure of the interaction region varies considerably in time and the mean flow-field is a simplified representation of a more complex instantaneous structure. An inter-relationship appears to exist between the incoming boundary layer, separated flow region, and reflected shock wave. The incoming boundary contains large-scale coherent motions, in the form of three-dimensional streamwise-elongated regions of relatively low- and high-speed fluid. The reflected shock wave region conforms to these regions as they enter the interaction, and may be viewed as a supposition of a streamwise translation and a spanwise rippling. The HWA results reveal that the reflected shock wave region contains energetic frequencies an order of magnitude lower than those found within the undisturbed boundary layer at the same distance from the wall. The time series is shown to be represented as a chaotic attractor in a limited dimensional state-space. This attractor has a rich, underlying structure, which contains the signatures of the low- and high-speed regions as they enter the interaction.","boundary layer; shock wave; interaction; unsteady; turbulence; particle image velocimetry; tomography; high-speed; proper orthogonal decomposition; chaos theory; dynamical systems","en","doctoral thesis","","","","","","","","","Aerospace Engineering","","","","",""
"uuid:84cec3f1-f84f-4b45-b23b-3e58cc3020f1","http://resolver.tudelft.nl/uuid:84cec3f1-f84f-4b45-b23b-3e58cc3020f1","Polymer flexibility and turbulent drag reduction","Gillissen, J.J.J.","","2008","Polymer-induced drag reduction is the phenomenon by which the friction factor of a turbulent flow is reduced by the addition of small amounts of high-molecular-weight linear polymers, which conformation in solution at rest can vary between randomly coiled and rodlike. It is well known that drag reduction is positively correlated to viscous stresses, which are generated by extended polymers. Rodlike polymers always assume this favorable conformation, while randomly coiling chains need to be unraveled by fluid strain rate in order to become effective. The coiling and stretching of flexible polymers in turbulent flow produce an additional elastic component in the polymer stress. The effect of the elastic stresses on drag reduction is unclear. To study this issue, we compare direct numerical simulations of turbulent drag reduction in channel flow using constitutive equations describing solutions of rigid and flexible polymers. When compared at constant or2, both simulations predict the same amount of drag reduction. Here o is the polymer volume fraction and r is the polymer aspect ratio, which for flexible polymers is based on average polymer extension at the channel wall. This demonstrates that polymer elasticity plays a marginal role in the mechanism for drag reduction.","channel flow; drag reduction; elasticity; friction; molecular configurations; polymer solutions; turbulence","en","journal article","American Physical Society","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:4c53dce6-0c51-4490-ac7c-28ae8158779c","http://resolver.tudelft.nl/uuid:4c53dce6-0c51-4490-ac7c-28ae8158779c","Numerical simulation of fibre-induced drag reduction in turbulent channel flow","Gillissen, J.J.J.","Boersma, B.J. (promotor); Andersson, H.I. (promotor)","2008","","turbulence; direct numericals imulation; drag reduction","en","doctoral thesis","","","","","","","","","Mechanical Maritime and Materials Engineering","","","","",""
"uuid:41f590a7-a3a9-4d28-a9f3-27fe6dccc209","http://resolver.tudelft.nl/uuid:41f590a7-a3a9-4d28-a9f3-27fe6dccc209","Dynamics of prolate ellipsoidal particles in a turbulent channel flow","Mortensen, P.H.; Andersson, H.I.; Gillissen, J.J.J.; Boersma, B.J.","","2008","The dynamical behavior of tiny elongated particles in a directly simulated turbulent flow field is investigated. The ellipsoidal particles are affected both by inertia and hydrodynamic forces and torques. The time evolution of the particle orientation and translational and rotational motions in a statistically steady channel flow is obtained for six different particle classes. The focus is on the influence of particle aspect ratio ? and the particle response time on the particle dynamics, i.e., distribution, orientation, translation, and rotation. Both ellipsoidal and spherical particles tend to accumulate in the viscous sublayer and preferentially concentrate in regions of low-speed fluid velocity. The translational motion is practically unaffected by the aspect ratio, whereas both mean and fluctuating spin components depend crucially on ?. The ellipsoids tend to align themselves with the mean flow direction and this tendency becomes more pronounced in the wall proximity when the lateral tilting of the elongated particles is suppressed.","channel flow; flow instability; turbulence; two-phase flow","en","journal article","American Institute of Physics","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:b5aa165b-e896-42de-b31b-80fb1e72ac24","http://resolver.tudelft.nl/uuid:b5aa165b-e896-42de-b31b-80fb1e72ac24","Tomographic particle image velocimetry and its application to turbulent boundary layers","Elsinga, G.E.","Bakker, P.G. (promotor); Scarano, F. (promotor)","2008","Tomographic Particle Image Velocimetry is a new experimental method developed to study three-dimensional motion in turbulent flows. The technique is an extension of standard PIV and makes use of several simultaneous views of illuminated tracer particles and their three-dimensional reconstruction as a light intensity distribution by means of tomography. The reconstructed tomogram pair is then analyzed by means of 3D cross-correlation returning the three-component velocity vector distribution over the measurement volume. The principles and details of the tomographic algorithm are discussed and a parametric study is carried out by to identify the most important parameters governing the experimental setup and to show their effect on the reconstruction accuracy. The capability of the technique in real experimental conditions is assessed with the measurement of the turbulent flow in the near wake of a circular cylinder. Next, this new technique has been applied to study the three-dimensional coherent structures in turbulent boundary layers. Quantitative visualizations of the individual (hairpin) vortices as well as the large-scale structures in both a low speed turbulent boundary layer and a high Reynolds number supersonic boundary layer have been obtained. The high Reynolds number data also suggests a very-large-scale flow organization exists not only in streamwise direction but also in spanwise direction. These very-large scale motions appear to consist of large-scale hairpins, which display a preferential alignment in streamwise direction and in the spanwise direction along the 45 degrees diagonal with the respect to the streamwise direction. Moreover, the time evolution of the flow structures is visualized in an experiment, in which the Tomographic-PIV technique is applied to nearly time-resolved image sequences recorded at 1.5 kHz.","piv; aerodynamics; measurement technique; turbulence; turbulent boundary layer; coherent structures","en","doctoral thesis","","","","","","","","","Aerospace Engineering","","","","",""
"uuid:a723a28e-2784-43be-8690-bc707af3eb1d","http://resolver.tudelft.nl/uuid:a723a28e-2784-43be-8690-bc707af3eb1d","Wind and boundary layers in Rayleigh-Bénard convection. I: Analysis and modeling","Van Reeuwijk, M.; Jonker, H.J.J.; Hanjali?, K.","","2008","","boundary layers; convection; Rayleigh-Benard instability; turbulence","en","journal article","American Physcial Society","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:3babfaee-062a-455a-8aff-7018fa30799d","http://resolver.tudelft.nl/uuid:3babfaee-062a-455a-8aff-7018fa30799d","Wind and boundary layers in Rayleigh-Bénard convection. II: Boundary layer character and scaling","Van Reeuwijk, M.; Jonker, H.J.J.; Hanjali?, K.","","2008","","boundary layers; convection; Rayleigh-Benard instability; turbulence","en","journal article","American Physcial Society","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:496e1cd3-2c8e-415f-bb57-08c98a576fe4","http://resolver.tudelft.nl/uuid:496e1cd3-2c8e-415f-bb57-08c98a576fe4","PDF modelling and particle-turbulence interaction of turbulent spray flames","Beishuizen, N.A.","Roekaerts, D.J.E.M. (promotor)","2008","Turbulent spray flames can be found in many applications, such as Diesel engines, rocket engines and power plants. The many practical applications are a motivation to investigate the physical phenomena occurring in turbulent spray flames in detail in order to be able to understand, predict and optimise them. Turbulent spray flames are a two-phase flow system consisting of a turbulent reacting continuum phase and a dispersed liquid phase contained in the continuum phase. In this thesis the interaction between these two phases is being studied, and specifically the effects of vaporisation and the effects of the presence of droplets on the turbulence characteristics of the continuum phase (the two-way coupling effect). The main goal of the research described in this thesis is to develop and evaluate stochastic Lagrangian models for turbulent spray flames that are characterised by significant or dominant two-way coupling effects. In the first part of the thesis, isolated droplet vaporisation models are studied and several models are evaluated. The problem of droplet interaction is briefly explained using some simple simulations and general theoretical ideas and the concept of group combustion is introduced. In the second part of the thesis the hybrid Lagrangian-Lagrangian method is presented. The continuum phase is described by the transport equation for the joint mass density function (MDF) of velocity and composition, which is solved indirectly using a Monte Carlo particle method where Langevin type equations are solved. This method is augmented by a finite volume method where the mean Eulerian transport equations for momentum, Reynolds stresses and turbulence dissipation are solved, making it a hybrid approach for the continuum phase. The dispersed phase is described solely by the droplet MDF which is also solved using a Monte Carlo method. A novel modification of the turbulence model for the pressure rate of strain in the Eulerian transport equation for the Reynolds stresses is presented that takes into account the effect of the presence of particles and the effect of mass transfer. The models in the Langevin equation in the Lagrangian formulation are then modified in order to be consistent with the p ressure rate of strain model in the Eulerian formulation. The modification of the diffusion term in the Langevin equation implies a modification of Kolmogorov's hypothesis regarding the first order Lagrangian structure function. An investigation of the transport equation for the scalar dissipation rate shows that Lagrangian mixing models are affected by liquid particle vaporisation. Other theoretical developments are the extension of the Generalised Gradient diffusion model and the Daly Harlow model for the triple correlations and an exact expression for the effect of mass transfer on the turbulence dissipation. In the third part of this thesis the results of numerical simulations of two test cases are presented. In the first test case an Eulerian-Lagrangian approach was used and the influence of the modification of the model for the pressure rate of strain by the presence of particles is investigated for an axially symmetric non-burning turbulent dispersed spray and compared with experimental data. Small improvements in turbulence quantities over the conventional turbulence and two-way coupling model are noticed when the novel modification of the pressure rate of strain is being used. In the second test case, simulations of an axially symmetric turbulent spray flame are performed to illustrate the performance of the complete hybrid method proposed in this thesis. A good agreement with experimental data was found using the hybrid method. In the mass transfer controlled spray flame studied in this thesis, taking into account the reduction of the drag coefficient and the heat and mass transfer coefficient due to mass transfer turned out to be crucial to the performance of the method.","turbulence; pdf method; spray flame; two way coupling","en","doctoral thesis","","","","","","","","","Applied Sciences","","","","",""
"uuid:3d6e7fe0-1849-4110-8e5e-5297702226be","http://resolver.tudelft.nl/uuid:3d6e7fe0-1849-4110-8e5e-5297702226be","The stress generated by non-Brownian fibers in turbulent channel flow simulations","Gillissen, J.J.J.; Boersma, B.J.; Mortensen, P.H.; Andersson, H.I.","","2007","Turbulent fiber suspension channel flow is studied using direct numerical simulation. The effect of the fibers on the fluid mechanics is governed by a stress tensor, involving the distribution of fiber position and orientation. Properties of this function in channel flow are studied by computing the trajectories and orientations of individual particles, referred to as the particle method. It is shown that, due to computer restrictions, the instantaneous stress in channel flow cannot be simulated directly with the particle method. To approximate the stress we compute the second-order moment of the fiber distribution function. This method involves an unknown subgrid term, which is modeled as diffusion. The accuracy of the moment approximation is studied by comparing Reynolds averaged stress to results obtained from the particle method. It is observed that the errors are ? 1% for y+>20, and ? 20% for y+<20. The model is improved by applying a wall damping function to the diffusivity. The moment approximation is used to simulate drag-reduced channel flow. A simplified model for fiber stress is introduced as fiber viscosity times rate of strain, where fiber viscosity is defined as the ratio of Reynolds averaged dissipation due to fiber stress and Reynolds averaged dissipation due to Newtonian stress. Fluid velocity statistics predicted by the simple model compare very well to those obtained from the moment approximation. This means that the effect of fibers on turbulent channel flow is equivalent to an additional Reynolds averaged viscosity.","channel flow; diffusion; drag reduction; fibres; flow simulation; stress analysis; turbulence","en","journal article","American Institute of Physics","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:b80dfdd2-1fe6-49ec-bb40-d402f5577c09","http://resolver.tudelft.nl/uuid:b80dfdd2-1fe6-49ec-bb40-d402f5577c09","Efficient computation of steady water flow with waves","Wackers, J.; Koren, J.","","2007","","steady water waves; turbulence; volume-of-fluid; compressive limiter; multigrid; defect correction","en","journal article","Wiley Interscience","","","","","","","","Aerospace Engineering","","","","",""
"uuid:da77cd4c-6fe8-41cb-87c5-0e274401559d","http://resolver.tudelft.nl/uuid:da77cd4c-6fe8-41cb-87c5-0e274401559d","Unsteady flow organization of compressible planar base flows","Humble, R.A.; Scarano, F.; Van Oudheusden, B.W.","","2007","The unsteady flow features of a series of two-dimensional, planar base flows are examined, within a range of low-supersonic Mach numbers in order to gain a better understanding of the effects of compressibility on the organized global dynamics. Particle image velocimetry is used as the primary diagnostic tool in order to characterize the instantaneous near wake behavior, in combination with data processing using proper orthogonal decomposition. The results show that the mean flowfields are simplified representations of the instantaneous flow organizations. Generally, each test case can be characterized by a predominant global mode, which undergoes an evolution with compressibility, within the Mach number range considered. (The term “global mode” is defined herein as an organized global dynamical behavior of the near wake region, recognizing that the near wake dynamics may be describable in terms of several global modes.) At Mach 1.46, the predominant global mode can be characterized by a sinuous or flapping motion. With increasing compressibility, this flapping mode decreases, and the predominant global mode evolves into a pulsating motion aligned with the wake axis at Mach 2.27. These global modes play an important role in the distributed nature of the turbulence properties. The turbulent mixing processes become increasingly confined to a narrower redeveloping wake with increasing compressibility. Global maximum levels of the streamwise turbulence intensity and the kinematic Reynolds shear stress occur within the vicinity of the mean reattachment location, and show no systematic trend with compressibility. In contrast, the global maximum level of the vertical turbulence intensity moves upstream from the redeveloping wake toward the mean reattachment location. The vertical turbulence intensity decays thereafter more slowly than the other turbulence quantities. Overall, the local maximum levels of the turbulence properties decrease appreciably with increasing compressibility.","compressible flow; supersonic flow; flow visualisation; wakes; turbulence","en","journal article","American Institute of Physics","","","","","","","","Aerospace Engineering","Aerodynamics & Wind Energy","","","",""
"uuid:c241f58e-c948-4d59-9e9b-7134beafbfda","http://resolver.tudelft.nl/uuid:c241f58e-c948-4d59-9e9b-7134beafbfda","Stone stability under decelerating open-channel flow","Hoan, N.T.; Booij, R.; Hofland, B.; Stive, M.J.F.; Verhagen, H.J.","","2007","The current research is aimed at finding a proper relation between flow forces acting on the bed and the bed response. To this end, experiments were carried out in which both the bed response (quantified by a dimensionless entrainment rate) and the flow field (velocity and turbulence intensity distributions) are measured. The three available stability parameters, which are used to quantify for the flow forces, were evaluated using the measured data. The focus of the evaluation is on the correlation of these stability parameters with the measured bed damage expressed in terms of the dimensionless entrainment rate. The experimental results confirm that the Shields stability parameter fails to predict bed damage for non-uniform flow conditions (R2=0.18). In contrast, the stability parameters of Jongeling et al. (2003) and Hofland (2005) give better damage predictions (R2 = 0.77). The results confirm the strong influence of the velocity and turbulence intensity distributions on the stability of bed material.","bed protection; stone stability; decelerating flow; turbulence; Shields","en","conference paper","World Scientific","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:8ffe1fb3-a0cd-4f7e-b4ea-9752aad913f4","http://resolver.tudelft.nl/uuid:8ffe1fb3-a0cd-4f7e-b4ea-9752aad913f4","Direct simulation and regularization modeling of turbulent thermal convection","van Reeuwijk, M.","Hanjalic, K. (promotor)","2007","This dissertation focuses on turbulent thermal convection, which occurs in a wide range of geophysical and engineering situations, such as the atmosphere, the sun, the earth's mantle, indoor climates etc. The first part of the thesis comprises a fundamental study of Rayleigh-BÉnard convection in a domain with aspect-ratio four. An ensemble of independent realizations, obtained by direct simulation, is processed with symmetry-accounted ensemble-averaging in order to distinguish the 'wind' from the fluctuations. Using this decomposition, the role of the wind and the behavior of the boundary layers is studied in detail. The second part is a study of the Leray-alpha model for wall-bounded flows. It is shown that the Leray-alpha model does not guarantee the filtered velocity field to be divergence-free in the presence of no-slip boundary conditions. Then, the Leray-alpha model is validated for three generic wall-bounded flows, i.e. plane channel flow, Rayleigh-BÉnard convection and the side-heated vertical channel. In the last part, the effects of truncation errors of finite difference approximations are included as low-pass filter operations in the Navier-Stokes equations. The resulting modified equations of the ""numerical"" fluid are studied to quantify the effect of discretizations on the turbulent energy spectrum.","turbulence; thermal; plume; buoyancy; regularization; regularisation; dns; leray; simulation; Rayleigh-Bénard; convection","en","doctoral thesis","","","","","","","","","Applied Sciences","","","","",""
"uuid:a103b1f8-af47-4f35-88ca-221341c91b3e","http://resolver.tudelft.nl/uuid:a103b1f8-af47-4f35-88ca-221341c91b3e","On the performance of the moment approximation for the numerical computation of fiber stress in turbulent channel flow","Gillissen, J.J.J.; Boersma, B.J.; Mortensen, P.H.; Andersson, H.I.","","2007","Fiber-induced drag reduction can be studied in great detail by means of direct numerical simulation [ J. S. Paschkewitz et al., J. Fluid Mech. 518, 281 (2004) ]. To account for the effect of the fibers, the Navier-Stokes equations are supplemented by the fiber stress tensor, which depends on the distribution function of fiber orientation angles. We have computed this function in turbulent channel flow, by solving the Fokker-Planck equation numerically. The results are used to validate an approximate method for calculating fiber stress, in which the second moment of the orientation distribution is solved. Since the moment evolution equations contain higher-order moments, a closure relation is required to obtain as many equations as unknowns. We investigate the performance of the eigenvalue-based optimal fitted closure scheme [ J. S. Cintra and C. L. Tucker, J. Rheol. 39, 1095 (1995) ]. The closure-predicted stress and flow statistics in two-way coupled simulations are within 10% of the “exact” Fokker-Planck solution.","turbulence; flow simulation; channel flow; drag reduction; Navier-Stokes equations; Fokker-Planck equation; eigenvalues and eigenfunctions","en","journal article","American Institute of Physics","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:c1b00eb3-66a2-40be-ab8c-7417f9c2476d","http://resolver.tudelft.nl/uuid:c1b00eb3-66a2-40be-ab8c-7417f9c2476d","Stone stability under non-uniform flow","Hoan, N.T.; Booij, R.; Hofland, B.; Stive, M.J.F.; Verhagen, H.J.","","2007","The current research is aimed at finding a dimensionless stability parameter for non-uniform flow in which the effect of turbulence is incorporated. To this end, experiments were carried out in which both the bed response (quantified by a dimensionless entrainment rate) and the flow field (velocity and turbulence intensity distributions) are measured. A new stability parameter is proposed, which together with those of Shields [1], Jongeling et al. [2] and Hofland [3] was evaluated using the measured data. The focus of the evaluation is on the correlation of these stability parameters with the measured bed damage expressed in terms of the dimensionless entrainment rate. The experimental results confirm that the Shields stability parameter fails to predict bed damage for non-uniform flow conditions (R2=0.18). In contrast, Jongeling et al. [2], Hofland [3] and our new proposed stability parameters give better damage predictions (R2 = 0.77÷0.81). The results confirm the strong influence of the velocity and turbulence intensity distributions on the stability of bed material.","stone stability; bed protection; non-uniform flow; turbulence; Shields","en","conference paper","Arizona State University","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:fe01c539-598f-42dd-91e9-726c433bccd0","http://resolver.tudelft.nl/uuid:fe01c539-598f-42dd-91e9-726c433bccd0","Modelling hydrodynamics in Eelgrass (Zostera Marina)","Dijkstra, J.T.; Uittenbogaard, R.E.; Stive, M.J.F.","","2006","In many areas around the world, there is a large interest in the protection and restoration of aquatic vegetation, like eelgrass (Zostera marina), but little is known about the interaction of such vegetation with its environment. To improve this knowledge, a model has been developed that simulates this interaction between highly flexible vegetation and hydrodynamics. The model consists of two parts: a 1DV k-? turbulence model that simulates the flow, and a model that simulates the movement of the vegetation, based on a Langrangian force balance. This model has been validated against our own measurements on positions and forces of flexible plastic strips, as well as hydrodynamic measurements from literature. It performs well in these situations, but the validation data is limited. Nevertheless, it can be considered to be a very useful and generic tool in studying flow processes in fields of flexible vegetation.","flexible vegetation; seagrass; turbulence; flume experiments","en","conference paper","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:9cf3de30-3337-468d-8204-b7f898c9d746","http://resolver.tudelft.nl/uuid:9cf3de30-3337-468d-8204-b7f898c9d746","Stability of stones under non-uniform flow","Hoffmans, G.J.C.M.","Rijkswaterstaat","2006","In the Netherlands hydraulic engineering structures are not only designed, but are also managed and maintained (M&M). In the nineteen-nineties in the United States during periods of high water scores of bridges collapsed, because American law forbids the use of bed protection around bridge piers and moreover, no maintenance plans were available. Despite the fact that in Korea there are always strong tides recently during periods of high water, various bridge piers and abutments have been undermined with dramatic consequences. This emphasises the need for good M&M plans for both bed and bank protection. This publication contributes to the International Conference on Scour and Erosion (ICSE-3) in Amsterdam (1 -3 November 2006) and contains information on the loads imposed on bed protection by currents and on the amount of damage that may occur in normative situations","stability of stones; non-uniform flow; turbulence; Shields","en","report","Rijkswaterstaat, DWW","","","","","","","","","","","","KWP-collection",""
"uuid:c8dcce2e-159d-45be-ae23-c84f7a022d65","http://resolver.tudelft.nl/uuid:c8dcce2e-159d-45be-ae23-c84f7a022d65","Numerical modeling for the accurate computations of arc-heater flows","Lee, J.I.; Kim, C.; Kim, K.H.","","2006","The purpose of this paper is to develop an accurate analysis code for the flow of arc-heater by employing advanced numerical models. Governing equations are hyperbolic-type axisymmetric Navier-Stokes equations which include joule heating by arc, radiation and turbulent transport effect. Joule heating is simply calculated by Ohms law with the given distribution of current. Radiation is computed by the three-band model which accounts for self-absorption and is consistent with the detailed line-by-line radiation model. Turbulence effect is incorporated by two-equation turbulence models which can describe the transport of turbulence. In order to assess the performance of the newly developed code, AHF and IHF are calculated in various operating conditions. And, it is confirmed that two-equation turbulence models combined with the three-band radiation model simulate the flow physics in arc-heater more accurately than any other previous models and the influence of the turbulence is as much as or bigger than radiation effect.","plasma wind tunnel; arc-heater; joule heating; radiation; turbulence","en","conference paper","","","","","","","","","","","","","",""
"uuid:5e85afbc-e6c8-47c7-b39c-6dfe8652d04c","http://resolver.tudelft.nl/uuid:5e85afbc-e6c8-47c7-b39c-6dfe8652d04c","A new Eulerian Monte Carlo method for the joint velocity-scalar PDF equations in turbulent flows","Soulard, O.; Sabel'nikov, V.","","2006","In the field of turbulent combustion, Lagrangian Monte Carlo (LMC) methods (Pope, 85) have become an essential component of the probability density function (PDF) approach. LMC methods are based on stochastic particles, which evolve from prescribed stochastic ordinary differential equations (SODEs). They are used to compute the one-point statistics of the quantities describing the state of a turbulent reactive flow: namely, the velocity field and the reactive scalars (species mass fractions and temperature). Numerous publications document the convergence and accuracy of LMC methods. They have been used in many complex calculations (including LES), and for several years now, they have been implemented in commercial CFD codes. Nonetheless, the development of a new Eulerian Monte Carlo (EMC) method is useful and stimulating, since the competition between LMC and EMC methods could push both approaches forward. EMC methods have already been proposed by Sabel'nikov and Soulard (2006) in order to compute the one-point PDF of turbulent reactive scalars. EMC methods are based on stochastic Eulerian fields, which evolve from prescribed stochastic partial differential equations (SPDE) statistically equivalent to the PDF equation. The extension of EMC methods to include velocity still remains to be done. Thus, the purpose of this article is to derive SPDEs allowing to compute a modeled one-point joint velocity-scalar PDF. To achieve this objective, we start from existing Lagrangian stochastic models. The latter are described by SODEs, which can be considered as modeled Navier-Stokes equations written in Lagrangian variables. Then, the idea is to transform these Lagrangian SODEs into Eulerian SPDEs, in the same way one transforms the Lagrangian Navier-Stokes equations into Eulerian equations, in classical hydrodynamics. However, our case differs from the classical one. Indeed, the stochastic velocity does not respect an instantaneous continuity constraint, but only a mean one. To account for this difference between the stochastic and the physical system, one must introduce a stochastic density, different from the physical density. As a result of this procedure, we eventually obtain hyperbolic conservative SPDEs giving the evolution of a stochastic velocity, of stochastic scalars, and of a stochastic density. In addition to the main result, an alternative EMC method for the scalar PDF is also derived as a special case of the full velocity-scalar method.","turbulence; combustion; probability density functions; stochastic partial differential equations; Eulerian Monte Carlo Method","en","conference paper","","","","","","","","","","","","","",""
"uuid:2b90abcc-3488-41f4-b70e-f2afdd8c3a0e","http://resolver.tudelft.nl/uuid:2b90abcc-3488-41f4-b70e-f2afdd8c3a0e","Particle Sedimentation in Wall-Bounded Turbulent Flows","Cargnelutti, M.; Breugem, W.A.; Portela, L.M.; Mudde, R.F.; Uijttewaal, W.S.J.; Stelling, G.S.","","2006","In this work, a comparison between the results of point-particle direct numerical simulations and PIV/PTV experiments of a particle-laden horizontal channel flow is presented. The numerical simulations were preformed trying to mimic as much as possible the experimental conditions. The accuracy of the point-particle approach was evaluated by comparison of the concentration, velocity and velocity fluctuation profiles. The agreement was good, both qualitatively and quantitatively, in the central part of the channel. However, in the near-wall region some differences were found. This can be explained by the lack of resuspension present in the simulations, because we considered only the fluid-particle interaction (one-way coupling) and neglected both the particle-fluid interaction (two-way coupling) and the particle-particle interaction (collisions).","particle-laden flows; turbulence; DNS; PIV","en","conference paper","","","","","","","","","","","","","",""
"uuid:39a5176c-4f4b-4fa9-8589-5fe7c7279544","http://resolver.tudelft.nl/uuid:39a5176c-4f4b-4fa9-8589-5fe7c7279544","Non reflecting boundary conditions for reacting flows","Prosser, R.","","2006","In this paper we explore the specification of time dependent boundary conditions suitable for the simulation of low Mach number reacting flows. The standard treatments used to date are based on the method of characteristics, and essentially set to zero the incoming characteristics; this practise has been shown to have deleterious effects on the flow evolution. The new approach, while still based on the method of characteristics, circumvents this problem through the application of a double expansion in terms of an appropriately defined Mach number. In the method, a low Mach number expansion of the dependent variables is coupled to a two-length scale decomposition. Through the double expansion, it is possible to separate inertial events (i.e. those moving at the local convection velocity) from acoustic events (those moving at the local sound speed). The paper highlights why previous treatments have encountered difficulties in turbulent flows, and provides a method by which heat release effects can be incorporated into a non-reflecting boundary condition framework. The accuracy of the method is demonstrated using a curved stagnating flame, in which the reaction zone crosses the boundary.","characteristics; turbulence; boundary conditions","en","conference paper","","","","","","","","","","","","","",""
"uuid:d6379127-c85d-4fe5-b05f-63c5667f6a09","http://resolver.tudelft.nl/uuid:d6379127-c85d-4fe5-b05f-63c5667f6a09","Particle Sedimentation in Wall-Bounded Turbulent Flows","Cargnelutti, M.; Breugem, W.A.; Portela, L.M.; Mudde, R.F.; Uijttewaal, W.S.J.; Stelling, G.S.","","2006","In this work, a comparison between the results of point-particle direct numerical simulations and PIV/PTV experiments of a particle-laden horizontal channel flow is presented. The numerical simulations were preformed trying to mimic as much as possible the experimental conditions. The accuracy of the point-particle approach was evaluated by comparison of the concentration, velocity and velocity fluctuation profiles. The agreement was good, both qualitatively and quantitatively, in the central part of the channel. However, in the near-wall region some differences were found. This can be explained by the lack of resuspension present in the simulations, because we considered only the fluid-particle interaction (one-way coupling) and neglected both the particle-fluid interaction (two-way coupling) and the particle-particle interaction (collisions).","particle-laden flows; turbulence; DNS; PIV","en","conference paper","Delft University of Technology; European Community on Computational Methods in Applied Sciences (ECCOMAS)","","","","","","","","Applied Sciences","","","","",""
"uuid:b4f624be-50fa-475d-9615-15027916da4c","http://resolver.tudelft.nl/uuid:b4f624be-50fa-475d-9615-15027916da4c","Regularization modeling for large-eddy simulation of diffusion flames","Geurts, B.J.","","2006","We analyze the evolution of a diffusion flame in a turbulent mixing layer using large-eddy simulation. The large-eddy simulation includes Leray regularization of the convective transport and approximate inverse filtering to represent the chemical source terms. The Leray model is compared to the more conventional dynamic mixed model. The location of the flame-center is defined by the `stoichiometric' interface. Geometrical properties such as its surface-area and wrinkling are characterized using an accurate numerical level-set quadrature method. This allows to quantify flame-properties as well as turbulence modulation effects due to coupling between combustion and turbulent transport. We determine the active flame-region that is responsible for the main part of the chemical conversion in the flame and compare direct and large-eddy simulation predictions.","turbulence; combustion; large-eddy simulation; regularization modeling; iso-surface analysis","en","conference paper","","","","","","","","","","","","","",""
"uuid:9375fecd-4428-4a29-a4e6-8a0a1f3cc4be","http://resolver.tudelft.nl/uuid:9375fecd-4428-4a29-a4e6-8a0a1f3cc4be","Influence of the nozzle outlet face on near flow field mixing","Khan, I.M.; Gilbert, T.; Barigou, M.","","2006","The shape of the nozzle geometry is increasingly attractive in heating, ventilation and air conditioning applications. However an important consideration in the design of nozzle geometry is its effect on the dynamics of near flow field jet and to minimise the manufacturing cost for practical applications. In this investigation the effect of an identical 3d contraction of the nozzle geometry is investigated numerically with circular, square and rectangular outlets of similar effective area. Comparisons of the axial mean streamwise velocity decay, turbulence, entrainment and the temperature distribution were reported for a circular, square and rectangular outlet sections of the nozzle to evaluate its performance in the space. From the analysis of data, it was found that enhanced mixing between the jet flow and the still surrounding fluid was noticed for the JETs existing nozzle geometry with circular outlet section which generated relatively higher turbulence kinetic energy in the near flow field, which implies better diffusion of temperature for air conditioning and ventilation applications.","nozzle; turbulence; diffusion; near flow field","en","conference paper","","","","","","","","","","","","","",""
"uuid:c7f5a261-e698-4d69-8da9-3c2a17be505d","http://resolver.tudelft.nl/uuid:c7f5a261-e698-4d69-8da9-3c2a17be505d","Direct numerical simulation of the interaction between unsheared turbulence and a free-slip surface","Campagne, G.; Cazalbou, J.B.; Joly, L.; Chassaing, P.","","2006","In this paper, direct numerical simulation is used to study the interaction between turbulence and a free surface. The configuration is original due to the fact that a random force generates turbulence in the vicinity of a plane parallel to the free surface. Turbulence is therefore statistically steady and nearly isotropic at some distance of the surface. A detailed description of the flow is provided, including second-order statistics and full Reynolds-stress budgets. It is shown that the results obtained in this configuration can help the understanding of intercomponent energy-transfer mechanisms.","turbulence; direct numerical simulation; free surface; random forcing","en","conference paper","","","","","","","","","","","","","",""
"uuid:dccf9895-423d-4070-ba7c-adf4d92c8427","http://resolver.tudelft.nl/uuid:dccf9895-423d-4070-ba7c-adf4d92c8427","Magnitude control of commutator errors","Geurts, B.J.","","2006","Non-uniform filtering of the Navier-Stokes equations expresses itself, next to the turbulent stresses, in additional closure terms known as commutator errors. These terms require explicit subgrid modeling if the non-uniformity of the filter is sufficiently pronounced. We derive expressions for the magnitude of the mean flux, the turbulent stress flux and the commutator error for individual Fourier modes. This gives rise to conditions for the spatial variations in the filter-width and the filter-skewness subject to which the magnitude of the commutator errors can be controlled. These conditions are translated into smoothness requirements of the computational grid, that involve ratios of first -, second - and third order derivatives of the grid mapping.","turbulence; large-eddy simulation; non-uniform filter; commutator errors","en","conference paper","","","","","","","","","","","","","",""
"uuid:e055f69a-4edc-4f86-83fd-962465e71ee3","http://resolver.tudelft.nl/uuid:e055f69a-4edc-4f86-83fd-962465e71ee3","Multiscale Methods in Computational Fluid and Solid Mechanics","De Borst, R.; Hulshoff, S.J.; Lenz, S.; Munts, E.A.; Van Brummelen, E.H.; Wall, W.A.","","2006","The basic idea of multiscale methods, namely the decomposition of a problem into a coarse scale and a fine scale, has in an intuitive manner been used in engineering for many decades, if not for centuries. Also in computational science, large-scale problems have been solved, and local data, for instance displacements, forces or velocities, have been used as boundary conditions for the resolution of more detail in a part of the problem. Recent years have witnessed the development of multiscale methods in computational science, which strive at coupling fine scales and coarse scales in a more systematic manner. Having made a rigorous decomposition of the problem into fine scales and coarse scales, various approaches exist, which essentially only differ in how to couple the fine scales to the coarse scale. The Variational Multiscale Method is a most promising member of this family, but for instance, multigrid methods can also be classified as multiscale methods. The same conjecture can be substantiated for hp-adaptive methods. In this lecture we will give a succinct taxonomy of various multiscale methods. Next, we will briefly review the Variational Multiscale Method and we will propose a space-time VMS formulation for the compressible Navier-Stokes equations. The spatial discretization corresponds to a high-order continuous Galerkin method, which due to its hierarchical nature provides a natural framework for `a priori' scale separation. The latter property is crucial. The method is formulated to support both continuous and discontinuous discretizations in time. Results will be presented from the application of the method to the computation of turbulent channel flow. Finally, multigrid methods will be applied to fluid-structure interaction problems. The basic iterative method for fluid-structure interaction problems employs defect correction. The latter provides a suitable smoother for a multigrid process, although in itself the associated subiteration process converges slowly. Indeed, the smoothed error can be represented accurately on a coarse mesh, which results in an effective coarse-grid correction. It is noted that an efficient solution strategy is made possible by virtue of the relative compactness of the displacement-to-pressure operator in the fluid-structure interaction problem. This relative compactness manifests the difference in length and time scales in the fluid and the structure and, in this sense, the multigrid method exploits the inherent multiscale character of fluid-structure-interaction problems.","multiscale methods; fluid flow; turbulence; fluid-structure interaction; multigrid methods","en","conference paper","","","","","","","","","","","","","",""
"uuid:15a56eea-69bc-46e9-b24a-502fc697e9cc","http://resolver.tudelft.nl/uuid:15a56eea-69bc-46e9-b24a-502fc697e9cc","Application of Local Defect Correction in a 3d turbulent channel flow","De Hoogh, J.; Kuerten, J.G.M.","","2006","To investigate the behavior of the concentration of a passive scalar in a turbulent flow with realistic high Schmidt numbers, one needs a very fine grid to capture all length-scales that are present in the concentration. An efficient method to increase the spatial resolution without a drastic raise of the computational costs is to apply a Local Defect Correction method. To compute the velocity field of a turbulent channel flow, a Direct Numerical Simulation is used that consists of a pseudo-spectral method in two periodic directions and a Chebyshev expansion in the wall-normal direction. The concentration equation for the passive scalar is solved using a finite volume method. The main concern with an LDC method in a hyperbolic time-dependent problem is the interpolation of the concentration when the refinement area is moved. The interpolated solution has to be smooth and continuous to ensure numerical stability. Using the mean averaged radius and the total surface area of the concentration, the behaviour of several numerical methods can be investigated.","turbulence; local defect correction; passive scalar","en","conference paper","","","","","","","","","","","","","",""
"uuid:c2fb8e60-8b4e-446f-8535-c41372a3a9da","http://resolver.tudelft.nl/uuid:c2fb8e60-8b4e-446f-8535-c41372a3a9da","Multiscale Methods in Computational Fluid and Solid Mechanics","De Borst, R.; Hulshoff, S.J.; Lenz, S.; Munts, E.A.; Van Brummelen, E.H.; Wall, W.A.","","2006","The basic idea of multiscale methods, namely the decomposition of a problem into a coarse scale and a fine scale, has in an intuitive manner been used in engineering for many decades, if not for centuries. Also in computational science, large-scale problems have been solved, and local data, for instance displacements, forces or velocities, have been used as boundary conditions for the resolution of more detail in a part of the problem. Recent years have witnessed the development of multiscale methods in computational science, which strive at coupling fine scales and coarse scales in a more systematic manner. Having made a rigorous decomposition of the problem into fine scales and coarse scales, various approaches exist, which essentially only differ in how to couple the fine scales to the coarse scale. The Variational Multiscale Method is a most promising member of this family, but for instance, multigrid methods can also be classified as multiscale methods. The same conjecture can be substantiated for hp-adaptive methods. In this lecture we will give a succinct taxonomy of various multiscale methods. Next, we will briefly review the Variational Multiscale Method and we will propose a space-time VMS formulation for the compressible Navier-Stokes equations. The spatial discretization corresponds to a high-order continuous Galerkin method, which due to its hierarchical nature provides a natural framework for `a priori' scale separation. The latter property is crucial. The method is formulated to support both continuous and discontinuous discretizations in time. Results will be presented from the application of the method to the computation of turbulent channel flow. Finally, multigrid methods will be applied to fluid-structure interaction problems. The basic iterative method for fluid-structure interaction problems employs defect correction. The latter provides a suitable smoother for a multigrid process, although in itself the associated subiteration process converges slowly. Indeed, the smoothed error can be represented accurately on a coarse mesh, which results in an effective coarse-grid correction. It is noted that an efficient solution strategy is made possible by virtue of the relative compactness of the displacement-to-pressure operator in the fluid-structure interaction problem. This relative compactness manifests the difference in length and time scales in the fluid and the structure and, in this sense, the multigrid method exploits the inherent multiscale character of fluid-structure-interaction problems.","multiscale methods; fluid flow; turbulence; fluid-structure interaction; multigrid methods","en","conference paper","Delft University of Technology; European Community on Computational Methods in Applied Sciences (ECCOMAS)","","","","","","","","Aerospace Engineering","","","","",""
"uuid:ed4bd9db-f347-4173-8ccb-65634ac5fd25","http://resolver.tudelft.nl/uuid:ed4bd9db-f347-4173-8ccb-65634ac5fd25","Large-eddy Simulation of Isotropic Homogeneous Decaying Turbulence","Thornber, B.; Drikakis, D.","","2006","Simulations of three dimensional freely decaying homogeneous turbulence in a periodic cube have been used to examine in a detailed and quantitative manner the behaviour of a Large Eddy Simulation (LES) using implicit subgrid modelling. This paper details the form and behaviour of the implicit subgrid models for the Minmod and third order limiting methods at several mesh resolutions. It is shown that for simulations above 32^3 the decay of kinetic energy follows a power law with a decay exponent between 1:2 and 1:4, except in the case of turbulence with a constrained length scale for which the decay exponent is 2:1. This is in very good agreement with experimental data and theoretical analysis where the exponent ? 1:2 ? 1:4 unconstrained, and 2:0 when constrained. At a resolution of 32^3 the number of degrees of freedom are not sufficient to allow a turbulent flow, and velocity derivative statistics are Gaussian. The skewness of the velocity derivative is lower than existing explicit LES and Direct Numerical Simulation (DNS) simulations, but in good agreement with the most recent experimental results. There is a limited sub-inertial range with the three-dimensional Kolmogorov constant ? 1:9, also in agreement with DNS. The less dissipative nature of the third order limiter gives better skewness at a lower grid resolutions, however both give good results in terms of energy dissipation and growth of the length scales.","LES; implicit; isotropic; turbulence; decay; MUSCL","en","conference paper","","","","","","","","","","","","","",""
"uuid:e8c57b2f-74d8-402b-b3c6-e52d85344300","http://resolver.tudelft.nl/uuid:e8c57b2f-74d8-402b-b3c6-e52d85344300","On the required Reynolds-number dependence of variational multi-scale Smagorinsky models","Meyers, J.; Sagaut, P.","","2006","A theoretical analysis is presented on the dependence of the standard and variational multi-scale Smagorinsky models on the proximity of the LES filter width Delta to the integral length scale of turbulence L on the one hand, and to the Kolmogorov scale eta on the other hand. Moreover modifications of the models are formulated, which respond better to Delta/eta changes. Apart from a priori evaluations of L/Delta and Delta/eta effects, the quality of our proposed modifications to the models is further evaluated and corroborated based on LES of decaying homogeneous isotropic turbulence.","turbulence; modelling; large-eddy simulation; variational multi scale; Smagorinsky","en","conference paper","","","","","","","","","","","","","",""
"uuid:5e947fcf-c257-45c8-b750-0a942d5512d3","http://resolver.tudelft.nl/uuid:5e947fcf-c257-45c8-b750-0a942d5512d3","LES and URANS Unsteady Boundary Layer Strategies for Pulsating and Oscillating Turbulent Channel Flows Applications","Panara, D.; Porta, M.; Schoenfeld, T.","","2006","The use of wall functions has been investigated for LES and URANS numerical simulation in pulsating and oscillating channel flow applications. The results show that the wall function approach is accurate in the so-called quasi-steady regime but there are discrepancies with the experimental results in the intermediate frequency range. A special attention is given to the wall-shear stress prediction, and in particular on the wall-shear stress phase shift with respect to the free stream velocity. In order to capture such unsteady flow effects, the boundary layer needs to be resolved. Different approaches such as Low Reynolds Number near wall turbulence modeling (URANS) or the proposed Wall-Normal Resolved strategy (LES) seem to be suited for this purpose. The drawback is unfortunately the increasing of computational points in the boundary layer and consequently the higher computational costs.","fluid dynamics; turbulence; URANS; LES; pulsating flow; oscillating flow; near wall modeling; wall shear stress","en","conference paper","","","","","","","","","","","","","",""
"uuid:df2adbc6-0c02-4507-bf84-fbb7721916da","http://resolver.tudelft.nl/uuid:df2adbc6-0c02-4507-bf84-fbb7721916da","A Reinterpretation of the Non-Linear Galerkin Method as a Large Eddy Simulation Technique","Guermond, J.L.; Prudhomme, S.","","2006","The purpose of this paper is to show that the Fourier-based Nonlinear Galerkin Method (NLGM) constructs suitable weak solutions to the periodic Navier--Stokes equations in three dimensions. We re-interpret NLGM as a Large-Eddy Simulation technique (LES) and we rigorously deduce a relationship between the mesh size and the large-eddy scale.","NavierStokes equations; turbulence; large eddy simulation; nonlinear Galerkin method; suitable solutions","en","conference paper","","","","","","","","","","","","","",""
"uuid:6dd67142-c893-485a-b384-6ec500b51d72","http://resolver.tudelft.nl/uuid:6dd67142-c893-485a-b384-6ec500b51d72","Bubble columns: Structures or stability?","Harteveld, W.K.","Van den Akker, H.E.A. (promotor); Mudde, R.F. (promotor)","2005","The aim of the thesis is to contribute to the understanding of the hydrodynamics of the gravity driven bubbly flow that can be found in bubble columns. Special attention is paid to the large scale structures that have a strong impact on several key parameters such as the degree of mixing, mass and heat transfer. An experimental investigation has been performed in a bubble column with a needle sparger that allows for gas injection in well controlled patterns. Experimental teechniques such as Laser Doppler Anemometry (LDA) and glass fiber probes are employed, and their accuracy for application to bubbly flows is investigated and improved. Signal processing techniques are evaluated and improved, with special attention for techniques to determine turbulence power spectra for LDA signals obtained in bubbly flows. The hydrodynamics for uniform gas injection have been studied, with particular attention for the stability of the flow regime. The importance of the lift force for stability is discussed. The importance of non-uniformities in the gas injection on the flow is investigated, focusing on the behavior of both the entrance region and the bulk region. Finally, liquid velocity flucuations are studied: both the strength of the pseudo-turbulence and the turbulence power spectra are explored.","bubble column; bubbly flow; laser doppler anemometry; glass fiber probe; turbulence","en","doctoral thesis","","","","","","","","","Applied Sciences","","","","",""
"uuid:a5e99cea-454a-42ee-82d2-bb9b87edd25c","http://resolver.tudelft.nl/uuid:a5e99cea-454a-42ee-82d2-bb9b87edd25c","Effect of particle inertia and gravity on the turbulence in a suspension","Ooms, G.; Poesio, P.","","2005","A theoretical model is presented for the effect of particle inertia and gravity on the turbulence in a homogeneous suspension. It is an extension of the one-fluid model developed by L’vov, Ooms, and Pomyalov [Phys. Rev. E 67, 046314 (2003)] , in which the effect of gravity was not considered. In the extended model the particles are assumed to settle in the fluid under the influence of gravity due to the fact that their density is larger than the fluid density. The generation of turbulence by the settling particles is described, with special attention being paid to the turbulence intensity and spectra. A comparison is made with direct numerical simulation calculations and experimental data. Also a sensitivity study is carried out to investigate at which conditions the gravity effect becomes important.","gravity waves; turbulence; suspensions; two-phase flow","en","journal article","American Institute of Physics","","","","","","","","Mechanical, Maritime and Materials Engineering","Process and Energy","","","",""
"uuid:90796c07-7666-4550-b73b-2b8f70057768","http://resolver.tudelft.nl/uuid:90796c07-7666-4550-b73b-2b8f70057768","Rock and roll: Turbulence-induced damage to granular bed protections","Hofland, B.","Battjes, J.A. (promotor); d'Angremond, K. (promotor)","2005","Layers of stone or rock are often applied around hydraulic structures to protect the sand bed from scouring. Determining the size of the rocks is complicated by the fact that the flow over a bed protection usually is non-uniform. This thesis describes two experimental series in which the stability of a single stone under non-uniform flows is studied in detail. From these experiments more understanding in the process of damage to bed protections is obtained. It is seen that besides the classical form drag, also small-scale accelerations in the flow attribute to the stone movement. In the last chapter a new stability model is developed for practical use.","turbulence; bed protection; bed stability; piv; backward-facing step; turbulence wall pressures","en","doctoral thesis","","","","","","","","","Civil Engineering and Geosciences","","","","",""
"uuid:0690f5f1-c13a-45e9-bc40-6a7a12f6571b","http://resolver.tudelft.nl/uuid:0690f5f1-c13a-45e9-bc40-6a7a12f6571b","Detailed simulations of liquid and solid-liquid mixing: Turbulent agitated flow and mass transfer","Hartmann, H.","Van den Akker, H.E.A. (promotor)","2005","This thesis aims at a contribution to reliable and accurate predictions of complex, multi-phase processes. The reader is presented detailed simulations on liquid and solid-liquid mixing using large eddy simulations (LES) including scalar mixing and particle transport in a Rushton turbine stirred tank geometry. Large eddy simulations have been critically assessed against LDA (Laser Doppler Anemometry) experiments and a transient RANS (Reynolds-Averaged Navier-Stokes) simulation. Furthermore, attention is paid to the recovery of large-scale precessing vortices (i.e. a so-called macro-instability) by means of LES. A scalar mixing solver has been developed and assessed in order to predict the mixing performance (in terms of a mixing time) of the turbulent flow generated by a Rushton turbine in a stirred tank. A simple blending process is used as a reference case. For the ultimate simulation of a dissolution process, a particle transport solver has been coupled to the flow and scalar mixing solvers. The results provide insight in the complex interaction between the turbulent flow phenomena, particle transport, and mass transfer.","stirred tank; turbulence; scalar mxing; solids suspension; mass transfer","en","doctoral thesis","","","","","","","","","Applied Sciences","","","","",""
"uuid:7f3f0559-42a3-4c3b-9dbe-8331fc604a8a","http://resolver.tudelft.nl/uuid:7f3f0559-42a3-4c3b-9dbe-8331fc604a8a","Particle imaging velocimetry-based identification of coherent structures in normally impinging multiple jets","Geers, L.F.G.; Tummers, M.J.; Hanjali?, K.","","2005","To gain a better insight into the effects of eddy structure and their thermal imprint on the impinging surface, we applied several methods to identify coherent structures in two arrangements of multiple jets. High-resolution particle imaging velocimetry measurements of the instantaneous fluid velocity have been performed to provide data for structure identification. Several methods based on critical point theory, i.e., vorticity magnitude, kinematic vorticity number, and second invariant of the velocity gradient tensor provided similar and generally useful information. However, in the flow considered they all appear inferior as compared with the proper orthogonal decomposition (POD). Despite lacking in conspicuous high energy modes, the flow showed to be well suited for POD, which provided clear identification of the near wall dominating vortical structure.","jets; vortices; turbulence; heat transfer","en","journal article","American Institute of Physics","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:10486dfd-24f4-4044-bc1b-754dbc2bea70","http://resolver.tudelft.nl/uuid:10486dfd-24f4-4044-bc1b-754dbc2bea70","Scales and structures in bubbly flows. Experimental analysis of the flow in bubble columns and in bubbling fluidized beds","Groen, J.S.","Mudde, R.F. (promotor); Van den Akker, H.E.A. (promotor)","2004","In this project a detailed experimental analysis was performed of the dynamic flow field in bubbly flows, with the purpose of determining local hydrodynamics and scale effects. Measurements were done in gas-liquid systems (air-water bubble columns) and in gas-solid systems (air-sand bubbing fluidized beds) of different size. Amongst others with the help of lasers and glass fibres the behaviour was studied of single bubbles, bubble swarms and the continuous phase. These techniques were combined with advanced signal processing such as correlation analysis. A striking new phenomenon was the anisotropy or orientation dependence of experimental results. A detailed experimental study into the applicability of laser Doppler anemometry (LDA) in bubble columns showed that virtually only the liquid phase is recorded with LDA. The measurements show a pronouncedly turbulent flow field, but it is striking that this complex view can be characterized by a small number of fixed parameters. These parameters can also be used quite well for describing several physical phenomena and for scaling up these reactors.","bubble column; fluidized bed; bubbly flow; turbulence; laser doppler anemometry; glass fibre; coherent structures; dispersion; scale-up","en","doctoral thesis","","","","","","","","","Applied Sciences","","","","",""
"uuid:1409cb4d-167c-46da-aa82-1b4926fad230","http://resolver.tudelft.nl/uuid:1409cb4d-167c-46da-aa82-1b4926fad230","Energy transfers and spectral eddy viscosity in large-eddy simulations of homogeneous isotropic turbulence: Comparison of dynamic Smagorinsky and multiscale models over a range of discretizations","Hughes, T.J.R.; Wells, G.N.; Wray, A.A.","","2004","Energy transfers within large-eddy simulation (LES) and direct numerical simulation (DNS) grids are studied. The spectral eddy viscosity for conventional dynamic Smagorinsky and variational multiscale LES methods are compared with DNS results. Both models underestimate the DNS results for a very coarse LES, but the dynamic Smagorinsky model is significantly better. For moderately to well-refined LES, the dynamic Smagorinsky model overestimates the spectral eddy viscosity at low wave numbers. The multiscale model is in good agreement with DNS for these cases. The convergence of the multiscale model to the DNS with grid refinement is more rapid than for the dynamic Smagorinsky model.","flow simulation; turbulence; viscosity; convergence of numerical methods; differential equations","en","journal article","American Institute of Physics","","","","","","","","Civil Engineering and Geosciences","Design & Construction","","","",""
"uuid:e1de702f-1f8b-4623-9920-5252dd377dd0","http://resolver.tudelft.nl/uuid:e1de702f-1f8b-4623-9920-5252dd377dd0","A direct-numerical-simulation-based second-moment closure for turbulent magnetohydrodynamic flows","Kenjere, S.; Hanjali?, K.; Bal, D.","","2004","A magnetic field, imposed on turbulent flow of an electrically conductive fluid, is known to cause preferential damping of the velocity and its fluctuations in the direction of Lorentz force, thus leading to an increase in stress anisotropy. Based on direct numerical simulations (DNS), we have developed a model of magnetohydrodynamic (MHD) interactions within the framework of the second-moment turbulence closure. The MHD effects are accounted for in the transport equations for the turbulent stress tensor and energy dissipation rate—both incorporating also viscous and wall-vicinity nonviscous modifications. The validation of the model in plane channel flows with different orientation of the imposed magnetic field against the available DNS (Re = 4600,Ha = 6), large eddy simulation (Re = 2.9×104,Ha = 52.5,125) and experimental data (Re = 5.05×104 and Re = 9×104, 0 ? Ha ? 400), show good agreement for all considered situations.","turbulence; magnetohydrodynamics; damping; internal stresses; Boltzmann equation; Poisson equation; channel flow","en","journal article","American Institute of Physics","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:6b22f7e7-e3ae-4c2b-aacc-d864de5cee4b","http://resolver.tudelft.nl/uuid:6b22f7e7-e3ae-4c2b-aacc-d864de5cee4b","Multiple impinging jet arrays. An experimental study on flow and heat transfer","Geers, L.F.G.","Hanjalic, K. (promotor)","2004","Because of their high efficiency and their ability to provide high heat transfer rates, impinging jets are applied for rapid cooling and heating in a wide variety of industrial processes. However, the physical phenomena controlling the heat transfer from impinging jets are to a large degree unknown. The goal of the present project was to gain a better understanding of the interaction between the flow and heat transfer in impinging jet arrays. Experiments were performed in two different configurations: a single impinging jet and multiple impinging jet arrays. LDA and PIV velocity measurements in the single jet were compared: these measurements were aimed at serving as a reference for comparison of the multiple jet features. In a hexagonal and an in-line array of jets PIV was used to provide instantaneous velocity fields over the flow area of interest, what proved to be essential for detecting some salient features of the multiple-jet dynamics. These dynamics were investigated on the basis of POD filtered snapshots of the flow. In both arrays, large scale eddies in the development zone cause the impinging jets to break up or be severely displaced in the out-of-plane direction. A horse-shoe vortex appears around the outer jets of the hexagonal array, whereas the in-line array does not show this feature. This is most likely caused by the higher pitch in the in-line configuration. On the other hand, the flow field in the in-line arrangement appears to be diagonally asymmetrical, even though the symmetry of the nozzle arrangement would imply a symmetrical flow field. This flow asymmetry causes an elliptical heat transfer pattern in the impingement zone of the center jet. The flow asymmetry was also found in numerical simulations of a similar configuration, conducted in a parallel project. Heat transfer measurements were performed in the impingement surface for seven multiple jet arrays using LCT, which provided insight in the influence of different geometrical parameters of the arrays on the heat transfer. Additionally, a non-dimensional correlation was derived linking heat transfer to geometrical and flow parameters. Heat transfer profiles were subsequently compared to velocity and turbulence quantities just above the impingement plate to investigate the effect of the mean flow features on the heat transfer. The apparent correlation between the Nusselt number and the normal contribution to the production of turbulent kinetic energy is coincidental: the cause of the increased Nusselt number in the impingement points of the jets is a combination of an oscillating impingement position due to large vortical structures and a very strong acceleration of the fluid from the impingement point outwards. In the impingement point the boundary layer is very thin and the temperature gradient is very steep, enabling impinging vortices to encapsulate and remove heat in an effective way. The results presented in this thesis should aid in better understanding the role of turbulence and vortical structures in the interaction between adjacent impinging jets, and in the heat transfer from impinging jets. With the advent of a measurement technique capable of capturing instantaneous heat transfer distributions, a potential extension of the present work is a more detailed analysis of the influence of individual vortical structures on impingement heat transfer.","impinging jets; heat transfer; piv; lct; structure identification; pod; turbulence","en","doctoral thesis","","","","","","","","","Applied Sciences","","","","",""
"uuid:9cdda996-0ab8-4a35-a346-ce912d6e6a8a","http://resolver.tudelft.nl/uuid:9cdda996-0ab8-4a35-a346-ce912d6e6a8a","Sensitivity of the scale partition for variational multiscale large-eddy simulation of channel flow","Holmen, J.; Hughes, T.J.R.; Oberai, A.A.; Wells, G.N.","","2004","The variational multiscale method has been shown to perform well for large-eddy simulation (LES) of turbulent flows. The method relies upon a partition of the resolved velocity field into large- and small-scale components. The subgrid model then acts only on the small scales of motion, unlike conventional LES models which act on all scales of motion. For homogeneous isotropic turbulence and turbulent channel flows, the multiscale model can outperform conventional LES formulations. An issue in the multiscale method for LES is choice of scale partition and sensitivity of the computed results to it. This is the topic of this investigation. The multiscale formulation for channel flows is briefly reviewed. Then, through the definition of an error measure relative to direct numerical simulation (DNS) results, the sensitivity of the method to the partition between large- and small-scale motions is examined. The error in channel flow simulations, relative to DNS results, is computed for various partitions between large- and small-scale spaces, and conclusions drawn from the results.","turbulence; channel flow; flow simulation; numerical analysis; variational techniques","en","journal article","American Institute of Physics","","","","","","","","Civil Engineering and Geosciences","Design & Construction","","","",""
"uuid:feaa053c-1903-47ef-8564-38230a369c1d","http://resolver.tudelft.nl/uuid:feaa053c-1903-47ef-8564-38230a369c1d","Numerical Simulations of Competitive-Consecutive Reactions in Turbulent Channel Flow","Vrieling, A.J.","Nieuwstadt, F.T.M. (promotor)","2003","This thesis deals with mixing of passive scalars in a turbulent flow. The passive scalars are released in a turbulent plane channel flow and interpreted as either non-reactive components or reactive components that are involved in a competitive-consecutive reaction system. The evolution of these passive scalars under influence of turbulent mixing and chemical reaction is studied by means of Direct Numerical Simulation (DNS). For the non-reactive case, the mixing process behind two parallel line sources placed at a short distance from each other is studied and in particular the interference in the concentration statistics. The results of this study are interpreted as the concentration statistics resulting from two nearby stacks that release into the atmospheric boundary layer. For the reactive components, we study the downstream situation behind a source of species B placed in a background concentration of species A, where A and B are involved in the reaction system A+B => R, R+B => S and R is considered to be the desired reaction product. The rate of both reactions is determined by the combined influence of turbulent mixing and chemical kinetics. The primary influence on the course of this reaction system is found to be the hydrodynamics. The large scale turbulent fluctuations suppress the rate of the first reaction and enhance the rate of the second reaction. This effect could be an explanation for the anomalous high yields of S that are sometimes observed in experiments.","series-parallel reaction; turbulence; direct numerical simulation; passive scalar","en","doctoral thesis","","","","","","","","","Design, Engineering and Production","","","","",""
"uuid:5359c790-71b8-4467-a31c-b30533e7c833","http://resolver.tudelft.nl/uuid:5359c790-71b8-4467-a31c-b30533e7c833","Turbulent reactive mixing in process equipment: 3-D LIF experiments and FDF/LES modelling","van Vliet, E.","van den Akker, H.E.A. (promotor)","2003","","turbulence; mixing; chemical reactions; three-dimensional laser induced fluorescence; FDF/LES numerical simulations","en","doctoral thesis","Delft University Press","","","","","","","","Applied Sciences","","","","",""
"uuid:115c96a3-6c4e-48c1-a698-f831829d184c","http://resolver.tudelft.nl/uuid:115c96a3-6c4e-48c1-a698-f831829d184c","A linear approach for the evolution of coherent structures in shallow mixing layers","Van Prooijen, B.C.; Uijttewaal, W.S.J.","","2002","The development of large coherent structures in a shallow mixing layer is analyzed. The results are validated with experimental data obtained from particle tracking velocimetry. The mean flow field is modeled using the self-similarity of the velocity profiles. The characteristic features of the down-stream development of a shallow mixing layer flow, like the decrease of the velocity difference over the mixing layer, the decreasing growth of the mixing layer width, and the transverse shift of the center of the mixing layer layer are fairly well represented. It turned out that the entrainment coefficient could be taken constant, equal to a value obtained for unbounded mixing layers: ? = 0.085. Linearization of the shallow water equations leads to a modified Orr–Sommerfeld equation, with turbulence viscosity and bottom friction as dissipative terms. Growth rates are obtained for each position downstream, using the model for the mean flow field. For a given energy density spectrum at the inflow boundary, integration of the growth rates along the downstream direction yields the spectra at various downstream positions. These spectra provide a measure for the intensity and the length scale of the coherent structures (the dominant mode). The length scales found are in good agreement with the measured ones. The length scale of the most unstable mode appears much larger than the length scale of the dominant mode. Obviously, the longevity of the coherent structures plays a significant role. Three growth regimes can be distinguished: in the first regime the dominant mode is growing, in the second regime the dominant mode is dissipating, but other modes are still growing, and in the third regime all modes are dissipating. It is concluded that the development of the coherent structures in a shallow mixing layer can fairly well be described and interpreted by the proposed linear analysis.","rivers; channel flow; mixing; turbulence; flow instability; flow visualisation; hydrology; fractals","en","journal article","American Institute of Physics","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:13f30972-43e1-48d6-8275-293a1be71afa","http://resolver.tudelft.nl/uuid:13f30972-43e1-48d6-8275-293a1be71afa","Stability of coarse granular structures: Investigation on micro-scale","Hofland, B.","","2002","","Granular material; critical shear stress; bed protection; turbulence","en","report","Delft Cluster","","","","","","","","","","","","",""
"uuid:9cb2c3d3-988b-45a9-a09b-607f0baa6b66","http://resolver.tudelft.nl/uuid:9cb2c3d3-988b-45a9-a09b-607f0baa6b66","Elliptic blending model: A new near-wall Reynolds-stress turbulence closure","Manceau, R.; Hanjali?, K.","","2001","A new approach to modeling the effects of a solid wall in one-point second-moment (Reynolds-stress) turbulence closures is presented. The model is based on the relaxation of an inhomogeneous (near-wall) formulation of the pressure–strain tensor towards the chosen conventional homogeneous (far-from-a-wall) form using the blending function ?, for which an elliptic equation is solved. The approach preserves the main features of Durbin’s Reynolds-stress model, but instead of six elliptic equations (for each stress component), it involves only one, scalar elliptic equation. The model, called “the elliptic blending model,” offers significant simplification, while still complying with the basic physical rationale for the elliptic relaxation concept. In addition to model validation against direct numerical simulation in a plane channel for Re? = 590, the model was applied in the computation of the channel flow at a “real-life” Reynolds number of 106, showing a good prediction of the logarithmic profile of the mean velocity.","turbulence; channel flow","en","journal article","American Institute of Physics","","","","","","","","Applied Sciences","Multi-Scale Physics","","","",""
"uuid:469616a2-2fd9-4a92-b21f-2143e62dc721","http://resolver.tudelft.nl/uuid:469616a2-2fd9-4a92-b21f-2143e62dc721","Numerical simulation of particle-laden turbulent channel flow","Li, Y.; McLaughlin, J.B.; Kontomaris, K.; Portela, L.","","2001","This paper presents results for the behavior of particle-laden gases in a small Reynolds number vertical channel down flow. Results will be presented for the effects of particle feedback on the gas-phase turbulence and for the concentration profile of the particles. The effects of density ratio, mass loading, and particle inertia will be discussed. The results were obtained from a numerical simulation that included the effects of particle feedback on the gas phase and particle–particle collisions. The resolution of the simulation was comparable to the smallest scales in the particle-free flow, but the grid spacings were larger than the particle size. Particle mass loadings up to 2 and both elastic and inelastic collisions were considered. Particle feedback causes the turbulent intensities to become more anisotropic as the particle loading is increased. For small mass loadings, the particles cause an increase in the gas flow rate. It will be shown that the particles tend to increase the characteristic length scales of the fluctuations in the streamwise component of velocity and that this reduces the transfer of turbulent energy between the streamwise component of velocity and the components transverse to the flow. Particle–particle collisions greatly reduce the tendency of particles to accumulate at the wall for the range of mass loadings considered. This was true even when the collisions were inelastic.","numerical analysis; turbulence; channel flow; two-phase flow; fluctuations; flow simulation","en","journal article","American Institute of Physics","","","","","","","","Applied Sciences","","","","",""
"uuid:e99cfcfb-b520-4bc4-a3d9-071472daee49","http://resolver.tudelft.nl/uuid:e99cfcfb-b520-4bc4-a3d9-071472daee49","Random Taylor hypothesis and the behavior of local and convective accelerations in isotropic turbulence","Tsinober, A.; Vedula, P.; Yeung, P.K.","","2001","The properties of acceleration fluctuations in isotropic turbulence are studied in direct numerical simulations (DNS) by decomposing the acceleration as the sum of local and convective contributions (aL = ?u/?t and aC = u??u), or alternatively as the sum of irrotational and solenoidal contributions [aI = ??(p/?) and aS = ??2u]. The main emphasis is on the nature of the mutual cancellation between aL and aC which must occur in order for the acceleration (a) to be small as predicted by the “random Taylor hypothesis” [Tennekes, J. Fluid Mech. 67, 561 (1975)] of small eddies in turbulent flow being passively “swept” past a stationary Eulerian observer. Results at Taylor-scale Reynolds number up to 240 show that the random-Taylor scenario ?a2???aC2? ? ?aL2?, accompanied by strong antialignment between the vectors aL and aC, is indeed increasingly valid at higher Reynolds number. Mutual cancellation between aL and aC also leads to the solenoidal part of a being small compared to its irrotational part. Results for spectra in wave number space indicate that, at a given Reynolds number, the random Taylor hypothesis has greater validity at decreasing scale sizes. Finally, comparisons with DNS data in Gaussian random fields show that the mutual cancellation between aL and aC is essentially a kinematic effect, although the Reynolds number trends are made stronger by the dynamics implied in the Navier–Stokes equations.","turbulence; convection; acceleration; fluctuations","en","journal article","American Institute of Physics","","","","","","","","Aerospace Engineering","","","","",""
"uuid:19751801-3f4f-4d92-a740-ba663c12fe6a","http://resolver.tudelft.nl/uuid:19751801-3f4f-4d92-a740-ba663c12fe6a","Stone stability","Mosselman, E.; Akkerman, G.J.; Verheij, H.J.; Hoffmans, G.J.C.M.; Jongeling, T.H.G.; Petit, H.A.H.","","2000","","stortsteen; rubble; stabiliteit; stability; turbulentie; turbulence","en","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:41dbf079-7039-4bfe-bc9c-509125385325","http://resolver.tudelft.nl/uuid:41dbf079-7039-4bfe-bc9c-509125385325","Entraining Flow of a Concentrated Benthic Suspension: Computations with the Prandtl Mixing-Length Model","Kranenburg, C.","","1999","The Prandtl mixing-length model of turbulent exchange of mass and momentum is applied to calculate the entrainment of overlying water into a layer of suspended fine sediment at a horizontal bed. In the field the flow and turbulence in such a concentrated benthic suspension (CBS) are driven by a streamwise pressure gradient resulting from the tide. However, in this report a proposed laboratory experiment is simulated in which flow and turbulence in the CBS are driven by a movable bottom screen which is started instantaneously from rest. The aims of the calculations are to show the feasibility of this laboratory experiment, to carry out a sensitivity analysis and, in future work, to compare with experimental results. Damping functions accounting for the reduction in turbulent exchange caused by density stratification are calibrated using results of entrainment experiments with two-fluid systems reported in the literature. The adopted modelof the rheological behaviour of the CBS is of the Herschel-Bulkley type. In addition, the effects of hindered settling and sidewall friction are included. Entrainment rates are found to be particularly sensitive to the speed of the screen and the excess weight of the sediment, whereas the rheological model has little influence.","fine sediments; Prandtl; mixing-length model; entrainment; concentrated benthic suspension; CBS; experiment; hindered settling; turbulence; rheological behaviour; sidewall friction","en","report","TU Delft, Department of Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:f3534d8e-3f2a-4afe-ae1f-c48f72c9ca3a","http://resolver.tudelft.nl/uuid:f3534d8e-3f2a-4afe-ae1f-c48f72c9ca3a","Overview of turbulence models for external aerodynamics","Henkes, R.A.W.M.","","1998","An overview is given on the background of different turbulence models that can be used to compute boundary layers in external aerodynamics, such as for aircraft. The overview includes algebraic models (Cebeci-Smith, Baldwin-Lomax), a half-equation model (Johnson-King), two-equation models ( K-E-K-), and a differential Reynolds-stress model. The models were compared for boundary layer without and with streamwise pressure gradient. The models were also used to study the large-Reynolds number scalings (wall function and defect layer). The comparisson of models and the sealing analysis are described in three seperate journal contributions.","turbulence; turbulent flow; aerodynamics","en","book","Delft University Press","","","","","","","","Aerospace Engineering","","","","",""
"uuid:1a2a249b-80fe-4360-bbbc-689b6cc735cd","http://resolver.tudelft.nl/uuid:1a2a249b-80fe-4360-bbbc-689b6cc735cd","Prediction of bypass transition with differential Reynolds stress models","Westin, K.J.A.; Henkes, R.A.W.M.","","1998","Boundary layer transition induced by high levels of free stream turbulence (FSl), so called bypass transition, can not be predicted with conventional stability calculations (e.g. the en-method). The use of turbulence models for transition prediction has shown some success for this type of flows, and the present study is a further investigation on the use of low-Reynolds number, single-point closures for transition prediction. The work is focused on two differential Reynolds Stress Models (DSM), which are compared with the in previous studies more elaborately tested two-equation model by Launder Sharma. The results obtained with the DSM are relatively promising in some test cases, with a qualitatively correct description of the different normal stresses throughout the computational domain. However, the applicability of the models is very limited if a wider range of test cases are considered. Although the Differential Reynolds Stress Models have better prospects to describe the very anisotropic fluctuations that are observed in transitional boundary layers, the overall predictions of the location of the transition region are worse than obtained with the two-equation model. Detailed comparisons with results from Large Eddy Simulations have revealed significant shortcomings in the modelling of the dissipation. The DSM suffer from a large overprediction of the dissipation in the pre-transitional boundary layer, which also affects the modelling of the pressure strain terms. The present study also shows that some of the results reported in the literature may be too optimistic, since they seem to be affected by the implementation of the free stream boundary conditions. The predictions are sensitive to the FST-level in the vicinity of the boundary layer edge, and large variations in the predicted transition location can be obtained depending on how the free stream boundary is treated. The outcome of the present study indicates that, if single-point closures are going to be considered as a realistic transition prediction tool for industrial applications, some major improvements in the modelling are necessary.","turbulence; turbulent flow; gas boundary-layer flow","en","book","Delft University Press","","","","","","","","Aerospace Engineering","","","","",""
"uuid:a21087ed-1cb3-4662-8fba-08d69f85162e","http://resolver.tudelft.nl/uuid:a21087ed-1cb3-4662-8fba-08d69f85162e","A pilot study to test a method to enhance the turbulence of a water flow in a laboratory flume","Suastika, I.K.","","1998","In this report we present some results of a laboratory pilot study which has been conducted to test a method to artificially enhance the turbulence of a water flow in a laboratory flume. The purpose of this test is to provide data which will be used to verify whether the method is suitable for the study of the dissipative effects of turbulence on surface waves. In order to study the dissipative effects of turbulence on the surface waves, it is essential to enhance the turbulent intensity of the flow. On the other hand, it is desirable to enhance the shear rates of mean flow, and to generate turbulence of relatively large length scales. A strong interaction between the turbulent motion and the surface waves might be expected in cases where the turbulent length scale is of the same order as the wave length of the surface waves. In this study, we investigate the feasibility of a method to intensify the turbulence of the flow and to establish larger structures (""turbulence"") in the flow of length scale of order of the surface waves wave length. For this purpose, we disturb the flow in the flume of smooth walls by constructing a number of rectangular plates at the bottom of the flume. As first estimates, we use plates of height of about 20 percent of the water depth and width of about 20 percent of the flume width. The plates were fastened at the bottom on two rails by using T-shaped chords. It is possible to adjust the orientation of a particular plate by rotating it around the horizontal part of the T-chord and by rotating the T-chord itself on the rail around its vertical leg. Further, it is possible to adjust the distance between two subsequent plates on the rail. However, once the plates have been fastened on the rails, it is not possible to change their orientation and position during a particular test. In this series of experiments, we measured the flow velocity at a number of measurement points in a cross section of the flume. From the velocity measurement, a number of flow parameters are determined: mean velocity,standard deviation(r.m.s.-value of the velocity fluctuation),lateral shear stress and mean rate of shear. From the same measurement, we determine also the auto-correlation function of the flow velocity at each measurement point. In the analysis, these parameters and the plot of the auto-correlation function determined from the measurement in the presence of the plates are compared with those determined from a reference measurement, that is the measurement in the absence of the plates. CONCLUSIONS 1. The presence of the plates in the flume changes the mean longitudinal flow velocity; a decrease near the bottom and an increase at higher elevations. 2. The increase of turbulent intensity caused by the vertical plates is very much larger in the locations near the bottom, slightly smaller in the locations near the free surface and, over the whole cross section, very much larger than that caused by the tilted plates. 3. The increase of magnitude of the Reynolds stress caused by the vertical plates is larger than that caused by the tilted plates. 4. The increase of mean rates of shear caused by the vertical plates is smaller in the locations at higher elevations and larger in the locations near the bottom than that caused by the tilted plates. 5. The plates do not introduce turbulence of larger time scales.","turbulence; enhancement; laboratory study; water flow; flume; dissipative effects","en","report","TU Delft, Department of Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:c38e218b-e9e6-43f9-87f3-b4bc61686a9a","http://resolver.tudelft.nl/uuid:c38e218b-e9e6-43f9-87f3-b4bc61686a9a","Probabilistische analyse stabiliteitsvoorspellers","Akkerman, G.J.; Verheij, H.J.","","1998","","sedimenttransport; sediment transport; turbulentie; turbulence; korrels; grains; stabiliteit; stability","nl","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:67529e4e-3b4a-4933-a7fe-34b21ca72225","http://resolver.tudelft.nl/uuid:67529e4e-3b4a-4933-a7fe-34b21ca72225","Interactie van submerse vegetatie en fysische processen: Getijgootexperimenten","Boot, G.","","1998","","experimenteel onderzoek; experimental research; sedimentatie; sedimentation; ruwheid; roughness; turbulentie; turbulence; veldonderzoek; site surveying; biologie; biology","nl","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:09d5a5c7-616e-4e96-b557-4e0fbe920c0e","http://resolver.tudelft.nl/uuid:09d5a5c7-616e-4e96-b557-4e0fbe920c0e","Literatuurinventarisatie stabiliteitsvoorspellers en -gegevens","Akkerman, G.J.; Verheij, H.J.","","1998","","turbulentie; turbulence; korrels; grains; stabiliteit; stability; stroming; flow; sedimenttransport; sediment transport","nl","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:20338941-b8b3-4be9-83a0-a36ccb2dd536","http://resolver.tudelft.nl/uuid:20338941-b8b3-4be9-83a0-a36ccb2dd536","Turbulentie opwekking door drempels van steenbestorting: Berekeningen met Delft3D","Verwey, A.; Agtersloot, R.C.","","1998","","turbulentie; turbulence; stromingsmodellen; flow models; drempels; bars; steenstorting; stone dumping","nl","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:86fb0e20-8cd2-47a8-ba13-0083cc550498","http://resolver.tudelft.nl/uuid:86fb0e20-8cd2-47a8-ba13-0083cc550498","Turbulence schematization for stone stability assessment","Uittenbogaard, R.E.; Hoffmans, G.J.C.M.; Akkermans, R.J.","","1998","","verdedigingswerken; revetments; stortsteen; rubble; turbulentie; turbulence; debietmeting; discharge measurement","en","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:17ca4978-1fb8-4477-ac90-91efb5477807","http://resolver.tudelft.nl/uuid:17ca4978-1fb8-4477-ac90-91efb5477807","Turbulence, heat and mass transfer 2: Proceedings of the second international symposium on turbulence, heat and mass transfer. Delft, The Netherlands, June 9-12, 1997","Hanjalic, K.; Peeters, T.W.J.","","1997","","turbulence; heat transfer; mass transfer","en","book","Delft University Press","","","","","","","","","","","","",""
"uuid:2fa59b2d-8131-4608-9eb8-61b256603516","http://resolver.tudelft.nl/uuid:2fa59b2d-8131-4608-9eb8-61b256603516","Rotorcraft responses to atmospheric turbulence","Van Gool, P.C.A.","Mulder, J.A. (promotor)","1997","","rotorcraft; turbulence; simulation","en","doctoral thesis","Delft University Press","","","","","","","","Aerospace Engineering","","","","",""
"uuid:212ed7c8-e7c7-4c76-9e11-366f922930d7","http://resolver.tudelft.nl/uuid:212ed7c8-e7c7-4c76-9e11-366f922930d7","Recirculating steady flow in harbours: Comparison of numerical computations to laboratory measurements","Bijvelds, M.D.J.P.","","1997","In this report the water flow in harbours, that are situated on a river, are considered. Due to the flow velocity difference between the river and the harbour, a turbulent mixing layer develops at the harbour entrance. The shallowness of the water induces the coexistance of two disparate turbulent length scales in these regions. Besides the ""ordinary"" small-scale 3D turbulence, which is generated by bottom friction, large quasi 2D turbulent structures are generated by horizontal shear in the mixing layer. These large structures have a typical turbulent length scale that, in contrast with the 3D turbulence, is at least several times the water depth. The standard 3D k-? turbulence model, takes only one turbulent length scale into account and, as a consequence, the computed eddy viscosities and Reynolds stresses are too low, which results in an underprediction of the velocities in the gyre. Therefore, a new turbulence model, based on the standard k-? turbulence model, was developed that does take non-isotropic behaviour of the turbulence into account. This new model consists of two distinct turbulence models, that together model the 3D and quasi-2D turbulence: the vertical eddy viscosity that determines the vertical Reynolds stresses are computed with a 3D k-? turbulence model, in which the production of turbulent kinetic energy is determined by vertical shear only, i.e. bottom friction. The horizontal eddy viscosity that determines the horizontal Reynolds stresses is computed by a 2D depth averaged k-? model, in which the production of turbulent kinetic energy is dependent upon horizontal velocity gradients only. Direct interaction bet ween the two turbulence models, by means of energy transfer, is neglected. However, interaction via the mean-flow equations still exists. The standard 3D k-? turbulence model and the new two-length-scale model were tested for two different geometries. Besides earlier measurements in a 1xl m2 harbour, new measurements that were carried out in a more realistic geometry were used for model testing. The laser Doppler experiments carried out in the latter scale model, clearly revealed the existance of two disparate turbulent length scales by studying the autocorrelation functions and the turbulent power density spectra at positions in the mixing layer and the river. In both cases, results from computations with the two-length-scale model were in better agreement with measurements than the standard one-length-scale K-? model, supporting the necessity to account for the non-istrophy of the turbulence.","recirculation; harbour flow; turbulence","en","report","TU Delft, Department of Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:6a73f423-aa9e-4468-8a1e-3b7045d03920","http://resolver.tudelft.nl/uuid:6a73f423-aa9e-4468-8a1e-3b7045d03920","3D wave-current interaction: CL-vortex force, simulation of wave-flume experiments","Kester, J.A.T.M. van; Uittenbogaard, R.E.; Dingemans, M.W.","","1996","","numerieke modellen; numerical modelling; turbulentie; turbulence; wervelingen; vortices, swirls","en","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:fd3b621b-07e7-43cf-b89c-ab465ade709b","http://resolver.tudelft.nl/uuid:fd3b621b-07e7-43cf-b89c-ab465ade709b","Drag reduction by polymer additivers in a turbulent pipe experiments","Den Toonder, J.M.J.","Nieuwstadt, F.T.M. (promotor)","1996","","drag reduction; turbulence; rheology","en","doctoral thesis","","","","","","","","","Mechanical Maritime and Materials Engineering","","","","",""
"uuid:3f77b8ad-c562-4df6-9055-1277c75b473b","http://resolver.tudelft.nl/uuid:3f77b8ad-c562-4df6-9055-1277c75b473b","ERCOFTAC Workshop on Data Bases and Testing of Calculation Methods for Turbulent Flows","Rodi, W.; Bonnin, J.C.; Buchal, T.","TU Delft","1995","This document contains the calculation results for 5 test cases and the description of calculation methods submitted to the ERCOFTAC Workshop on Data Bases and Testing of Calculation Methods for Turbulent Flows held in Karlsruhe from April 3 to 7, 1995. The workshop was an outcome and marked the end of the research project ""Data Validation and Comparison in Fluid Mechanics"" financed by the CEC programme SCIENCE, in which 7 ERCOFTAC members participated to called experimental and numrical data on turbulent flows, to check their reliability and suitability fortest cases, to set up test cases and perform calculations with various turbulence models, and finally to create a data bank from which the data can be accessed. This project, which was restricted to incompressible single-phase flows without chemical reactions, has yielded data for about 70 flows, and 10 well documented test cases have been set up from which 5 were chosen for the present workshop. The workshop follows three previous ERCOF,TAC/IAHR Workshops on Refined Flow Modelling which have succeeded 14 IAHR workshops that were organised by the IAHR Working Group on Refined Flow Modelling in the years 1980 - 1991. Hence it is considered to be the 4th workshop in this series. The flows have been selected to range from relatively simple ones (Couette flow) to fairly complex 3D flows, to cover a number of different physical processes, and to be of practical relevance. Deliberately, only isothermal flows have been included; flows with heat transfer and buoyancy effects will be dealt with at future workshops. Initially 101 groups intended to submit results to the workshop; in the end 36 groups managed to meet the deadline, submitting altogether 129 solutions for the 5 test cases. At the workshop, some inconsistencies and errors were detected and some information on the turbulence models, wall functions and inflow conditions used was missing. The contributors were asked to resupply corrected results and additional information. This document is a revised version of the original proceedings and contains the corrected results. A paper giving an interpretation of the results is in preparation. The processing of the submitted data and the cross plotting required considerable effort. We are grateful to Dr. D. Laurence and his coworkers at EDF, Chatou, for helping us on this task by handling the originally submitted data for test cases 4 and 5.","turbulence; calibration; model tests","en","report","University of Karsruhe","","","","","","","","","","","","",""
"uuid:8de82382-1b74-4e2e-94c1-5115021ff0fd","http://resolver.tudelft.nl/uuid:8de82382-1b74-4e2e-94c1-5115021ff0fd","Flow measurements for an afterbody in a vertical wind tunnel","Skare, P.E.","","1995","","Aerodynamics; turbulence; boundary layer; hot-wire anemometry; afterbody; wake flow","en","report","Delft University of Technology","","","","","","","","Aerospace Engineering","","","","",""
"uuid:4b7cadbd-ddb5-4e31-b423-832b78984c92","http://resolver.tudelft.nl/uuid:4b7cadbd-ddb5-4e31-b423-832b78984c92","Influence of turbulence on the derivation of the drag from wake measurements","Aswatha Narayana, P.A.","","1995","","Aeronautics; aerodynamics; wake flows; turbulence","en","report","Delft University of Technology","","","","","","","","Aerospace Engineering","","","","",""
"uuid:05e1cb8f-721e-4ef6-9b73-3a6c23bdf2bc","http://resolver.tudelft.nl/uuid:05e1cb8f-721e-4ef6-9b73-3a6c23bdf2bc","Visualization of Turbulent Flow","Hin, A.J.S.","Jansen, F.W. (promotor)","1994","","scientific computing; visualization; turbulence","en","doctoral thesis","","","","","","","","","Electrical Engineering, Mathematics and Computer Science","","","","",""
"uuid:63a0ccd5-368a-48a8-b01f-16fffe3c59f7","http://resolver.tudelft.nl/uuid:63a0ccd5-368a-48a8-b01f-16fffe3c59f7","The importance of internal waves for mixing in a stratified estuarine tidal flow","Uittenbogaard, R.E.; Imberger, J.","","1993","","interne golven; internal waves; getijstromen; tidal currents; estuaria; estuaries; menging; mixing; dichtheidsstroming; density induced flow; gelaagdheid; stratification; turbulentie; turbulence; Rotterdamse Waterweg","en","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:0b8c7f05-2db3-417e-8048-9795a4183986","http://resolver.tudelft.nl/uuid:0b8c7f05-2db3-417e-8048-9795a4183986","Analysis of unsteady and stratified tidal flume experiments with emphasis on turbulence, mixing and on internal gravity waves","Uittenbogaard, R.E.","","1993","","getijstromen; tidal currents; experimenteel onderzoek; experimental research; modelonderzoek; modelling; turbulentie; turbulence; menging; mixing; interne golven; internal waves; goten; flumes","en","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:798bdbe4-cc5c-4dc1-b53e-0bbc3f8e1556","http://resolver.tudelft.nl/uuid:798bdbe4-cc5c-4dc1-b53e-0bbc3f8e1556","A study concerning the influence of the relative local turbulence on local-scour holes","Hoffmans, G.J.C.M.","Rijkswaterstaat","1993","Scour is the lowering of the sea or river-bed as a result of non-equilibrium sediment transport conditions and can be divided into several categories (Breusers and Raudkivi, 1991). Local scour, which may occur at the base of a structure because of the affected flow pattern, can severely endanger the stability of this structure. Many varieties of localscour systems downstream from hydraulic structures exist, each with its own particular geometry and hence local scour mechanism. Local scour is superimposed on general and constriction scour. The prediction of local-scour holes that develop downstream from hydraulic structures plays an important role in their design. Excessive local scour can progressively undermine the foundation of a structure. Because complete protection against scour is too expensive generally, the maximum scour depth and the upstream slope of the scour hole have to be predicted to minimize the risk of failure. In 1961 a systematical research with respect to scour holes started at Delft Hydraulics within the scope of the Dutch Delta works. After the catastrophic flood disaster in 1953 the Delta plan was made to protect the Rhine-Meuse-Scheldt delta for future disasters. Dams with large scale sluices were planned in some estuaries. The severe scour expected necessitated a better understanding of the scour process. To find detailed information about the physical processes playing a role in scour many experiments were carried out, in which various parameters of the f l ow and the scoured material were varied. From the results of experiments in flumes with all difficulties of scale effects and limitations in instrumentation some empirical relations were obtained, which describe the erosion process as function of time and place (Prins, 1963 and Breusers, 1966, 1967). In these empirical relations a not well defined turbulence coefficient was introduced. Up to now this coefficient was related to the geometry upstream of the scour hole, which relation was based on trial and error. Based on theoretical grounds an analytical relation for the depth-averaged turbulence intensity is derived. This relation, which implies a modification of the turbulence coefficient in the Breusers scour formula, is verified using approximately 300 experiments. The modified scour formula yields results that compare reasonably well to measured and computed developments of a scour hole in case of a uniform flow upstream of the scour hole corresponding with a large protected bed area. The computations were based on the two-dimensional Navier-Stokes and convection-diffusion equations (Hoffmans, 1992) . The present paper aims at extension of the domain of application of the scour formula to non-uniform flow conditions upstream.","scour; erosion; turbulence","en","report","Rijkswaterstaat, DWW","","","","","","","","","","","","",""
"uuid:9f5623f8-e6e9-4362-8303-340207c8ddbf","http://resolver.tudelft.nl/uuid:9f5623f8-e6e9-4362-8303-340207c8ddbf","Depth-Averaged Numerical Modeling for Curved Channels","Bernard, R.S.; Schneider, M.L.","TU Delft","1992","Without some correction for the interaction between lateral curvature and vertically nonuniform velocity, depth-averaged numerical models cannot predict the gradual migration of high velocity toward the outside of channel bends. Conventional depth-averaging does not account for the curvature-induced secondary flow that gives rise to the migration. To remedy this deficiency, a secondary flow correct ion (SFC) has been developed and incorporated in the two-dimensional STREMR code. The secondary governing equation contains two empirical coefficients, and these have been adjusted to make STREMR predictions agree with observed velocity profiles for a single 270-deg bend. Without further adjustment, the SFC yields comparable accuracy for multiple-bend predictions in two other channels with depth, curvature, and bottom friction different from the 270-deg case. This suggests that the SFC may be useful in general for bendways with small curvature, gentIe side slope, and moderate bottom friction.","modeling; numerical; bendway; open channel; secondary flow; turbulence; velocity distribution","en","report","USACE (US Army Corps of Engineers)","","","","","","","","","","","","",""
"uuid:664ef369-6ad3-44b8-b5f3-5e813e26fd90","http://resolver.tudelft.nl/uuid:664ef369-6ad3-44b8-b5f3-5e813e26fd90","Ein Algebrisches Turbulenzmodel für Ästuare","Lehfeldt, R.","Holz, K.P. (contributor); Sündermann, J. (contributor); Zielke, W. (contributor); TU Delft","1991","The density distribution within a fluid is of great importance for flow and transport phenomena in geophysical computational fluid dynamics. All turbulent exchange processes are influenced by buoyancy effects and must be adequately considered in numerical models. Parameterizations of the vertical eddy viscosity and eddy diffusivity are presented for application in estuarine simulations. This model is implemented in a 2D vertically structured finite difference scheme. The algebraic turbulence model is based on the mixing length assumption and damping functions. Schematic test cases and a long term simulation of three months for a salt wedge estuary show that this is an adequate parameterization of the turbulent exchange processes. Without any change of coefficients the model is applied to a well mixed estuary giving equally satisfactory results. A comparison of commonly used mixing length approaches for homogeneous and stratified flows shows differences near the surface which modify the flow field considerably and consequently effect the advective transport of scalar quantities. Simulations of laboratory experiments corroborate a mixing length formulation for the eddy viscosity which can be derived from free surface flow conditions assuming the logarithmic velocity profile and a linear shear stress distribution. The applied damping functions depend on the gradient Richardson number and comply with limiting conditions which are derived from energy redistribution considerations. The coefficients of these functions are determined from data of the atmospheric layer which is an environment quite different from the intended area of application, so as to avoid any system specific calibration. A comparison of model runs in a case-study of the Trave estuary demonstrates that constant eddy coefficients lead to systematic errors even within short simulation periods. With the algebraic turbulence model presented in this study simulations of estuaries which are primarily characterized by an equilibrium of horizontal advection and vertical diffusion reproduce even complex transport processes with sufficient accuracy.","estuary; mathematical model; turbulence; flow model","de","report","University of Hannover","","","","","","","","","","","","",""
"uuid:984b81e3-2bcb-4060-b2d1-1ab584188606","http://resolver.tudelft.nl/uuid:984b81e3-2bcb-4060-b2d1-1ab584188606","Characteristics of Turbulence Structure and Undertow in the Surf Zone","Okayasu, A.","TU Delft","1989","The characteristic motion of water under breaking waves and the turbulence structure in the surf zone were investigated through detailed two-dimensional velocity measurements in a wave flume. Significant difference was found between the breaking processes in the outer and inner regions of the surf zone. The velocity field in each region consists of steady current, periodic wave motion, organized vortex motion and turbulence. It was found that the organized vortex motion caused by wave breaking was an important fluid motion connecting the wave motion and the turbulence. The vertical profiles of the undertow, which is the steady current below the wave trough level, were investigated from velocity histories measured by hot-film and laser-Doppler-velocimeters. The turbulence generated in the upper layer by wave breaking prevents the development of the bottom boundary layer in the inner region. The vertical distribution of the mean Reynolds stress and the mean eddy viscosity coefficient in the inner region can be approximated by linear functions of the vertical elevation. The offshore-directed mean shear stress on the bottom is so large that it can not be neglected in the modeling of the undertow. The transition point which was the boundary between the outer and inner regions of the surf zone was defined as the offshore limit of the quasi-steady breaking region. The distance from the breaking point to the transition point was expressed in terms of the breaking water depth and the bottom slope. In order to describe the mechanism of the energy transfer during wave breaking accurately, a model was presented in which the organized large vortexes were taken into account as a transmitter of energy in the energy transfer process from wave motion to turbulence. The distribution of the turbulence energy calculated by this model agreed with the experimental results qualitatively. The mass and momentum fluxes by the organized large vortexes were also discussed. The mass transport by breaking waves was found to be induced by the wave motion and the organized large vortexes. By using the models of the energy distribution and the mass transport, a model was presented for the two-dimensional distribution of the undertow. The Reynolds stress and the eddy viscosity coefficient were quantitatively evaluated from the energy dissipation rate on the basis of the dimensional analysis. The variation of the mean water level in the surf zone was also predicted with a good accuracy by considering the momentum flux by the organized vortexes. The model can evaluate the distribution of the undertow on an arbitrary beach topography from the incident wave condition.","breaking waves; turbulence; surf zone; coastal hydraulics; undertow; vortex; eddy; energy transfer; energ distribution","en","report","","","","","","","","","","","","","",""
"uuid:8ca675f5-7387-459a-8672-f274de78e2bf","http://resolver.tudelft.nl/uuid:8ca675f5-7387-459a-8672-f274de78e2bf","Turbulence measurements above artificial dunes","Mierlo, M.C.L.M. van; Ruiter, J.C.C. de","","1988","","turbulentie; turbulence; rivierbodem; river bed; ribbels; ripples; duinen; dunes","en","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:e2c43d1a-0ed5-4f62-afc9-f00475aa5c88","http://resolver.tudelft.nl/uuid:e2c43d1a-0ed5-4f62-afc9-f00475aa5c88","Dynamic forces on a bed element in open channel flow with a backward facing step","Wang, X.K.","","1988","Experiments to measure forces on objects in open channel flow behind a bacward facing ste","backward facing step; open channel flow; turbulence","en","report","TU Delft","","","","","","","","Civil Engineering and Geosciences","Section Hydraulic Engineeering","","","",""
"uuid:3e53bc4e-4ccf-4497-a2d5-c829a8f4e103","http://resolver.tudelft.nl/uuid:3e53bc4e-4ccf-4497-a2d5-c829a8f4e103","Internal lee waves in a turbulent two-layer flow","Pietrzak, J.; Kranenburg, C.; Abraham, G.","","1988","","turbulentie; turbulence; gelaagdheid; stratification; dichtheidsstroming; density induced flow","en","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:e23c0e5b-1407-4239-8457-49f93abfe870","http://resolver.tudelft.nl/uuid:e23c0e5b-1407-4239-8457-49f93abfe870","Stormvloedkering Oosterschelde: Analyse turbulentiemetingen","Flokstra, C.","","1988","","Oosterschelde; Zeeland; turbulentie; turbulence; gegevensverwerking; data processing","nl","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:73b43904-c1dc-439f-a7a9-76669d189878","http://resolver.tudelft.nl/uuid:73b43904-c1dc-439f-a7a9-76669d189878","Local Scour: Damping of turbulence parameters in relaxation zone","Hoffmans, G.J.C.M.","","1988","The general purpose of this research project is to model mathematically the scour-hole downstream of a structure (2-D). The model has to simulate the development of the scour as a function of time. Basically two models are necessary namely a flow model and a morphological model. The latter has to describe the bed- and suspended load and the erosion of the bed. In the present study the behaviour of the turbulence in the relaxation zone of a local scour hole (figure 1) is discussed. The purpose of this note is to find a function (prescription) for the eddy viscosity in the relaxation zone, which will be used in the flow model DUCT, where the eddy viscosity is prescribed (Hoffmans, 1988b). The function for the eddy viscosity is based on the transport equations for the kinetic energy and its dissipation, in which the diffusion and gain (production) terms are neglected. Computations by means of the k-eps-model for the flow in a number of configurations (Hoffmans, 1988a), were used for calibration. CONCLUSIONS In the relaxation zone the diffusion and the dissipation do have a damping influence on the behaviour of the turbulence parameters, while the turbulence increases by the production of the Reynolds stresses. In the derivation of the analytical solutions of the damping functions the production and the diffusion are neglected in the transport equations for the kinetic energy and its dissipation. The agreement between the results of the perturbated functions is rather good.","local scour; turbulence; relaxation zone; flow model; morphology; diffusion; dissipation","en","report","TU Delft","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:eadc3880-04b3-4e89-947c-728f93b7e697","http://resolver.tudelft.nl/uuid:eadc3880-04b3-4e89-947c-728f93b7e697","Stratified flows: Turbulence and mixing in stratified flow: report on literature survey","Abraham, G.","","1986","","dichtheidsstroming; density induced flow; turbulentie; turbulence; menging; mixing","en","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:e37cd010-e809-4c27-a429-a5cf8b7ef311","http://resolver.tudelft.nl/uuid:e37cd010-e809-4c27-a429-a5cf8b7ef311","Stormvloedkering Oosterschelde: Oriënterend onderzoek naar de turbulentiestructuur in de monding van de Oosterschelde","Flokstra, C.","Deltares","1986","","estuaria; estuaries; flow models; stromingsmodellen; turbulence; turbulence models; turbulentie; turbulentiemodellen; Zeeland","nl","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:c7ae566d-416a-454f-82d4-d1f18c1e4fac","http://resolver.tudelft.nl/uuid:c7ae566d-416a-454f-82d4-d1f18c1e4fac","Large-Scale Structure of the Turbulent Wake Behind a Flat Plate","Jovic, S.; Ramaprian, B.R.","TU Delft","1986","An experimental study of the developing two-dimensional turbulent wake behind a streamlined flat-plate is reported. The purpose of this study was to understand the evolution of the wake structure from the one characteristic of the boundary layer to that characteristic of the far-wake, and to investigate whether organized motions are present in the wake flow just as in other free shear flows. The evolution of the flow and the mixing process in the developing wake was studied by using the well-known heat-tagging technique. This consisted of heating very slightly the boundary layer on one side of the plate, so that heat could be used as a passive tracer. The experiments were performed in a wind tunnel at a Reynolds number (based on the trailing-edge momentum thickness) of 1100. Simultaneous measurements of two components of velocity and local temperature were made. The data were obtained at eight stations over the distance range of 0, starting at the trailing edge, to 260 wake-momentum thicknesses and hence practically up to the far-wake. In addition to obtaining the usual conventionally averaged turbulence properties, special procedures such as conditional averaging and selective ensemble averaging were employed to analyse the data. The conditional averaging technique has been used to gain better insight into the dynamics of the interface separating the mixed and unmixed fluid in the developing wake. Finally, a selective ensemble averaging technique was successfully used to identify and educe organized large-scale structures present in the wake flow. Two types of structures were identified on one side of the wake, one originating from the same side of the wake centerline and the other one from the opposite side. The frequency of their occurrence and their dimensions, as well as all the properties associated with their organized and random motions are determined.","experimental; turbulence; turbulent wake; wake; flow measurements","en","report","Iowa Institute of Hydraulic Research","","","","","","","","","","","","",""
"uuid:62a0e84f-e1b9-4a72-8bb8-9e0b796247a9","http://resolver.tudelft.nl/uuid:62a0e84f-e1b9-4a72-8bb8-9e0b796247a9","Turbulentie en menging in inhomogene stromingen: Middellang Termijn Plan '86-'91","Uittenbogaard, R.E.","","1986","","turbulentie; turbulence; menging; mixing; turbulentiemodellen; turbulence models; meetmethoden; measuring methods","nl","report","Deltares (WL)","","","","","","","","","","","","",""
"uuid:18e869f3-cbd9-462e-a64c-be7c16a17e25","http://resolver.tudelft.nl/uuid:18e869f3-cbd9-462e-a64c-be7c16a17e25","Development of a Two-Scale Turbulence Model and Its Applications","Chen, C.J.; Singh, K.","TU Delft","1985","The use of second order closure turbulence model in predicting turbulent flows is known to be more successful than the classical mixing length model. However, it is found that if the turbulence constants are not altered or modified, the second order closure turbulence model is unable to predict satisfactorily f or some flows such as round jet and wake flows. In order to improve the predictability of the second order closure model, the present work proposes to consider two turbulent scales in the modelling of turbulent flows. One of these scales is based on using the turbulent kinetic energy, k, and its dissipation rate, epsilon, to characterize the large energy containing eddies. The other scale is based on the dissipation rate and the kinematic viscosity, nie, to characterize the small energy dissipating eddies. The second scale is based on the well known Kolmogorov hypothesis that dissipation of turbulent kinetic energy occurs primarily at small eddies. The turbulence model derived based on the concept of two different scales is called the two-scale turbulence model. The existing turbulence model which is modelled based on the one-scale concept of k and epsilon is called the one-scale turbulence model. The two-scale turbulence model is then applied to predict turbulent free shear flows and recirculating flows. The calculations were done in three parts. The first test case was nonbuoyant free shear flows which included round and plane jets in stagnant and moving streams, plane wakes and mixing layer. In the second part, the model was tested for plane and round buoyant jets having different Froude numbers. Finally, some results were obtained for recirculating flows, namely, backward facing step and flow past an obstruction. It is shown in the present study that the two-scale turbulence model performs significantly better than the one-scale turbulence model in all the cases concerned. The prediction capability of the two-scale turbulence model is shown since one does not need to alter or modify the turbulence constants as in the case of the one-scale turbulence model.","turbulence; turbulence model; two-scale model; one-scale model; turbulent flows; modelling; jet and wake flows; computational fluid dynamics","en","report","University of Iowa","","","","","","","","","","","","",""
"uuid:1cc9886b-23c8-43e3-bc3c-728375ba9966","http://resolver.tudelft.nl/uuid:1cc9886b-23c8-43e3-bc3c-728375ba9966","A k-Model for Stably Stratified Nearly Horizontal Turbulent Flows","Kranenburg, C.","","1985","A k-model is formulated that consists of the turbulent kinetic energy equation and an algebraic expression for the mixing length taking into account the influence of stratification. Applicability of the model is restricted to shallow, nearly horizontal flows. For local-equilibrium flows the model reduces to the well-known mixing-length hypothesis including a damping function depending on the gradient Richardson number. The model is applied to stratified turbulent Poiseuille flow between two parallel plates.","k-model; turbulence; turbulent kinetic energy equation; mixing length; stratification; horizontal flows; Poiseuille flow","en","report","TU Delft, Department of Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:0f4f9eec-a4bc-45be-ac90-fc42e0da7fb3","http://resolver.tudelft.nl/uuid:0f4f9eec-a4bc-45be-ac90-fc42e0da7fb3","The Turbulent Wave Boundary Layer: A theoretical study","Justesen, P.","TU Delft","1985","Close to the bottom the no-slip condition will retard the flow and cause a boundary layer to develop. In nature this boundary layer will for most practical purposes be turbulent and the bed will be rough. Usually the boundary layer is confined to a thin layer close to the bed having a typical thickness of 0.2 m under surface gravity waves. The turbulence intensity in this rather thin layer can be very high and strongly unsteady. The understanding of the hydrodynamics of this flow is not only of great academic/scientific interest. but it has also a wide range of applications in practical engineering. The object of this study is to investigate the use of turbulence modellinq in connection with the turbulent wave boundary layer. Two theoretical models are established and their results are checked against available measurements. A third model is constructed but not implemented so no results are thus being presented for this model. Finally, the effects of a refined flow model in connection with sediment transport computations are considered through a few examples. First , the mixing-length theory was used to construct the zero-equation model BL0BAK. Modelling of the turbulent kinetic energy budget was improved by, inclusion of a transport equation for turbulent kinetic energy which gave the one-equation model BL1PJ. Finally a two-equation model BL2PJ for which also a transport equation for the dissipation rate has been outlined but it has not been implemented. It can he concluded that the description of the velocity field in an oscillatory boundary layer can be obtained from BL0BAK or BL1PJ as well as from a constant eddy viscosity model like that of Myrhaug. The latter model requires, however, that the boundary layer thickness is knows a priory, whereas the present models do not have this limitation. In the theoretical models it has been possible to determine the energy loss factor. A good agreement with experimental data is observed. This applies to BL0BAK as well as to BL1PJ, the latter model yielding slightly better results. Perhaps the most important topic in this report is the calculation of the turbulent kinetic energy conditions in the boundary layer. Using the model BL1PJ it has been investigated how energy is produced, diffused, dissipated, and preserved from times with a production surplus to later times where dissipation is high. The calculations show that the production is very high just before the maximum ambient velocity culminates. This surplus of energy is then preserved in the turbulent motions and is spread towards areas in which the energy level is lower. The turbulent motions are of course sustained throughout the period of oscillation. The model shows that this effect is most pronounced for low values of a/kN. The time scale for the decay of turbulent energy is then comparable with the period of oscillation. Unfortunately we have not been able to check the theoretical findings against measurements duo to the lack of experimental data.","boundary layer; turbulence; BL0BAK; BL1PJ; turbulent wave; oscillatory flow; theoretical model","en","report","Institute of hydrodynamics and hydraulic engineering","","","","","","","","","","","","",""
"uuid:dfad7850-e361-4718-a490-f251b5224e33","http://resolver.tudelft.nl/uuid:dfad7850-e361-4718-a490-f251b5224e33","Reproduction of Velocity Profiles in Estuaries by some One-Dimensional Mathematical Models","Booij, R.","","1982","For the prediction of dispersion phenomena and of changes in the morphology of an alluvial bottom, a detailed description of the water flow is necessary. The flow in estuaries is a complicated one, partly because of the time-dependence. To isolate this aspect of tidal flow a simplifying one-dimensional (vertical) flow model 1S used. This one-dimensional model is obtained by the neglect of convective derivatives of the longitudinal velocity and the use of the rigid lid approximation, i.e. the replacement of the free surface by a flat frictionless plate. The error introduced by these approximations is not large for the flow in most tidal channels. Convective derivatives are generally of minor importance. The rigid lid approximation is inaccurate for tidal waves with a large ratio between wave height and water depth. Tidal flow is usually described by simple eddy viscosity models in which various simple distributions of the eddy viscosity are prescribed. Recently models with an eddy viscosity depending on the turbulence energy have gained wide acceptance for all kinds of boundary layer flow. In this investigation the k-model and the k-e:-model are compared to an eddy viscosity model with an appropriate distribution of the eddy viscosity and to the mixing-length model for the case of steady and of time dependent free surface flow. The time dependent free surface flows, considered, represent flows in a tidal channel without a nett discharge over the tidal period. The roughness values and the velocities are typical for tidal channels. The results of the various models differ hardly. The only appreciable difference is around slack water, where all models used are, however, less reliable. The close correspondence is explained by the short adjustment times of the turbulence energy and its dissipation compared to the tidal period and by the small relative roughness height. The flow in a tidal channel can be considered as slowly varying, showing almost logarithmic velocity profiles except around slack water. The hysteresis effect of the shear stress with respect to the surface velocity calculated with all these models is therefore small, in contradiction to the large hysteresis effect as found in some of the prototype measurements.","velocity profile; eddy viscosity; turbulence","en","report","TU Delft, Department of Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:6bca6b54-b2f6-41fc-a42e-adbd173489ee","http://resolver.tudelft.nl/uuid:6bca6b54-b2f6-41fc-a42e-adbd173489ee","Experiments on Unsteady Seperating Flow in an Open Channel","Koppel, T.","","1981","Experiments on unsteady separating flow past a sudden expansion in an open channel are described. To simulate tidal flow the flow rate in the approach channel varied as a half-sine with time. Different visualization techniques to obtain an insight into the behaviour of the separating vortex in time and space were used. Laser-Doppler velocimetry (LDV) was employed to measure velocities and turbulence characteristics. The data obtained from the LDV-measurements was processed on a computer using the ensemble-averaging technique. Results at different flow rates of the visualization experiments, measured mean-velocity fields, and characteristics of the turbulence - such as intensities, Reynolds stresses and turbulence energy - are shown. Some examples of velocity power-spectra and statistical distributions are presented also. The experiments also provide information about the generation of turbulence related to the separation mechanism and that owing to boundary-layer instability.","unsteady; seperating; flow; sudden expansion; open channel; experiments; turbulence; velocity power-spectra; statistical distribution; instability","en","report","TU Delft, Department of Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:b2476a3d-eacb-409e-a76b-48391ea5c5a7","http://resolver.tudelft.nl/uuid:b2476a3d-eacb-409e-a76b-48391ea5c5a7","Turbulence measurements using a small electromagnetic current meter in open channel flows","Atkins, R.","TU Delft","1980","The use of a small two-component electromagnetic current meter to record turbulent velocities in a laboratory flume is described. The results from these experiments are compared with results obtained previously using different measuring techniques.","turbulence; measurements; electromagnetic current meter; measurement equipment; open channel flow; laboratory experiment","en","report","Hydraulics Research Station, Wallingford","","","","","","","","","","","","",""
"uuid:343e27ca-3018-4b98-9315-5b79976fc677","http://resolver.tudelft.nl/uuid:343e27ca-3018-4b98-9315-5b79976fc677","Steady turbulent flow in curved rectangular channels","De Vriend, H.J.","","1979","After the study of fully developed and developing steady laminar flow in curved channels of shallow rectangular wet cross-section (see earlier reports in this series), steady turbulent flow in such channels is investigated as a next step towards a mathematical model of the flow in shallow river bends. A mathematical model is developed for this type of flow, using a turbulence model with a prescribed distribution of the turbulence viscosity and starting from the same similarity hypothesis as in the equivalent laminar flow models. The influence of various characteristics of the turbulence model and the admissability of the most important assumptions underlying the mathematical system that is solved are tested for the flow in a rather sharply curved flume with rather strong effects of curvature. The performance of the model is tested by comparing its results with experimental data from various laboratory flumes. The turbulence model appears to be of great importance to the secondary flow and the related quantities. The similarity hypothesis yields satisfactory results for the main flow, but it needs to be refined for the calculation of the magnitude and the direction of the bed shear stress. The model appears to be applicable to curved channels of not very sharp curvature (i.e. flows with a smallor intermediate equivalent Dean number), although the secondary flow intensity tends to be underestimated.","steady turbulent flow; turbulence; mathematical model; experiments; curved channels","en","report","TU Delft, Department of Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:a41f39c2-fce6-4647-bd7a-1d412c720ed7","http://resolver.tudelft.nl/uuid:a41f39c2-fce6-4647-bd7a-1d412c720ed7","Turbulent structure in open-channel flows","Nezu, I.","TU Delft","1977","Ph.D. thesis","turbulence; turbulent structure; open channel flow","en","report","Kyoto University","","","","","","","","","","","","",""
"uuid:2f988adf-70d1-4c78-afe0-a17ff5072167","http://resolver.tudelft.nl/uuid:2f988adf-70d1-4c78-afe0-a17ff5072167","Cursus Turbulentie","Vreugdenhil, C.B.","Deltares","1977","Interne cursus van het Waterloopkundig Laboratorium over turbulentie, bevat de onderwerpen: -basisvergelijkingen; -statistische beschrijving; -transportprocessen; -wandturbulentie; -tijdsafhankelijke grenslagen; -vrije turbulentie; -nieuwe turbulentietheorien.","turbulence","nl","report","Deltares (Waterloopkundig Laboratorium)","","","","","","","","","","","","",""
"uuid:f13a8efe-f8ec-4708-87d6-7a94622c36c3","http://resolver.tudelft.nl/uuid:f13a8efe-f8ec-4708-87d6-7a94622c36c3","Flow-induced gate vibrations: Prevention of sef-excitation computation of dynamic gate behaviour and the use of models","Kolkman, P.A.","Schoemaker, H.J. (promotor); Van Douwen, A.A. (promotor)","1976","The objective of this study is to develop design criteria for the dynamic behaviour of gates and valves. To this end, a resume of existing theories is given as well as an extended analysis of the added water mass, hydrodynamic rigidity and damping (also negative damping or self-excitation) and excitation by turbulent flow. New computation methods are presented for self-exciting vibrations: The ensuing introduction of an instability indicator permits the prediction of such vibrations in the design phase. Methods are described to calculate the added water mass and water damping in flowing water. Also treated are the instability of overflowing and falling water nappes, the response of a mass-spring system to noise excitation by turbulence, and the technique of hydroelastic models. Prior publications by the author on these subjects are to be found in the Appendices.","vibrations; weirs; gates; turbulence","en","doctoral thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:9e6bbf79-3a57-44dd-b4e8-8e83a8d5d883","http://resolver.tudelft.nl/uuid:9e6bbf79-3a57-44dd-b4e8-8e83a8d5d883","Gedaechtniseffekte in der Turbulenz","Hinze, J.O.","","1975","NOTE: REPORT IN GERMAN, ONLY THE ABSTRACT IS IN ENGLISH Memory effects in the flow of fluids are known long since. One has to reckon with such effects when studying the flow of non-Newtonian fluids, for instance when they exhibit a visco-elastic behaviour. Certain phenomena in turbulence also seem to point towards a visco-elastic behaviour. If, however, turbulence is considered as a hypothetical non-Newtonian fluid, there are essential differences with actual non-Newtonian fluids. First the requirement of objectivity concerning invariance of any constitutive equation against a time-dependent bodily rotation, has to be dropped. Second, in rheology the principle of local action is used. This means that in the case of a simple fluid any deformation-history can be described completely in terms of the local velocity gradient. In turbulence, action is not restricted to small regions, and higher derivatives of the mean velocity are often required to describe this action. In flows with a preferred main-flow direction, a distinction can be made between transport in axial and in transverse direction, when considering a ""memory"" behaviour that determines the degree of localness of action. If, for instance, turbulence shear-stress is expressed in the transverse gradient of the mean velocity with an eddy viscosity (Boussinesq), this gradient may well vary in the transverse direction across ""memory"" distances, where the contribution to the total transport of momentum is still of importance. In some cases negative values may be obtained in a small region around a maximum of the mean velocity with an asymmetric distribution (wall jet) if the variation of this gradient is neglected. This would result in a ""negative turbulence energy production ."" These ""memory"" distances are of the order of the Lagrangian integral length-scale or of the size of the bigger eddies. The action can then no longer be considered as being local in the rheological sense. In the axial, main flow direction, the situation may be different. Because of the relatively large convection velocity in this direction the memory distance is much larger than in transverse direction, i.e. many times the size of the bigger eddies. Though often the action may be described satisfactorily as if it were local, because of the size of the eddies involved the action is not strictly local. If we consider in those flows the effect of a non-constant mean-velocity gradient on the turbulence shear stress by extending the simple Boussinesq relation with a term giving the change in axial direction of this velocity gradient. The obtained equations have been applied to a wake flow generated by a hemi-spherical cap on the wall of a constant-pressure turbulent boundary layer, and the wake-flow of a circular cylinder in a uniform free stream. The result is that the extra-memory effects are important in both the disturbed boundary-layer and the developing part of the wake at short distances from the cylinder. In the first case the eddy viscosity when corrected for the extra-memory effect and rendered dimensionless with the local wall-friction velocity and boundary-layer thickness, still follows the same distribution as for the undisturbed boundary-layer. When applying the obtained equations to actual flows, an uncertainty is presented concerning the quantitative evaluation of the length Lambda_1 and of the function G, because of lack of knowledge of the Lagrangian autocorrelation and the relaxation time or memory function. In the present paper the function G has been approximated by an exponential function, and a relation for Lambda_1. As an estimate it has proven to be useful, at least for the time being.","Turbulenz; Gedaechtniseffekte; memory effects; turbulence; Boussinesq; viscosity","de","report","TU Delft, Department of Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:c879cba8-0143-42de-b40a-ea198f54e5b2","http://resolver.tudelft.nl/uuid:c879cba8-0143-42de-b40a-ea198f54e5b2","A model of turbulent diffusion","Kranenburg, C.","","1975","Mathematical description of turbulent diffusion with constant mean velocity gradient.","turbulence; diffusion","en","report","TU Delft","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:d9491d6f-a346-43f7-992c-5e136f87502d","http://resolver.tudelft.nl/uuid:d9491d6f-a346-43f7-992c-5e136f87502d","Computational Modeling of Turbulent Transport","Lumley, J.L.; Khajeh-Nouri, B.","TU Delft","1973","A rational closure technique is presented for the first and second moment-equations in a stratified, contaminated turbulent flow, Following the application of high Reynolds/Peclet number approximations, remaining third moments are expanded about the isotropic, homogeneous state. The stratified, uncontaminated case reduces to seventeen equations in seventeen unknowns. Other authors have suggested some of the terms generated, but Some have been using the wrong terms, or the right terms for the wrong reasons. The approximation is kinetic-theoretic (turbulence/mean motion scales assumed small, and turbulence nearly in equilibrium) and results in a relaxation time, and in generalized gradient transport forms; however, gradients of one quantity can produce fluxes of another. The model relates the time scale for return to isotropy to the Lagrangian integral time scale (reducing to K-theory in a homogeneous parallel flow with orthogonal temperature gradient). Some coefficients are estimated, and preliminary computations arc presented of the unstratified 2-D turbulent wake; only component energies near the centerline are not well reproduced, probably due to the omission of a term with which temporary computational difficulties were being experienced. Stratified, contaminated 3-D calculations appear to be practical.","turbulent flow; turbulence; flow modeling","en","report","Pennsylvania State University","","","","","","","","","","","","",""
"uuid:1d74d050-23ea-4d7a-a69c-5386f58ca92a","http://resolver.tudelft.nl/uuid:1d74d050-23ea-4d7a-a69c-5386f58ca92a","Memory Effect in a Turbulent Boundary-Layer Flow Due to a Relatively Strong Axial Variation of the Mean-Velocity Gradient","Hinze, J.O.; Sonnenberg, R.E.; Builtjes, P.J.H.","TU Delft","1972","Measurements have been made of the distributions of mean velocity, turbulence intensities and turbulence shear-stress in a turbulent boundary-layer downstream of a hemi-spherical cap attached onto the plane rigid wall. The eddy-viscosity, when computed in the classical way according to Boussinesq's concept from the lateral gradient of the mean velocity and the turbulence shear-stress, showed a very strong non-uniform lateral distribution, also across the outer region of the boundary-layer. More over , the non-dimensional values of the eddy viscosity, using the wall-friction velocity and the boundary-layer thickness as the velocity scale and length scale respectively, were higher than those for the boundary-layer when not disturbed by the wake of the spherical cap. However, when account is taken of an axial memory effect of the streamwise variation of the lateral gradient of the mean-velocity, the values of the non-dimensional eddy viscosity are close to those for the undisturbed boundary-layer.","turbulence; boundary layer; fluid mechanics; memory effect","en","report","Delft University of Technology","","","","","","","","","","","","",""
"uuid:7ff6762f-d286-49e2-b4a7-df575464d8ed","http://resolver.tudelft.nl/uuid:7ff6762f-d286-49e2-b4a7-df575464d8ed","Contribution à la transition dans la couche limite","Hinze, J.O.; Leijdens, H.; Van den Brug, J.B.; Kleiweg, D.","TU Delft","1964","A l'aide de la méthode du ruban vibrant de Schubauer et Klebanoff. on a étudié expérimentalement l'évolution tridimensionnelle d'une perturbation sinusoïdale locale introduite dans une couche limite laminaire. Les distributions de la vitesse moyenne et de l'intensité de la composante axiale de la turbulence, de même que le spectre d'énergie de celle-ci, ont été mesurés par anémomètre à fil chaud. Les mesures mettent dairement en évidence le développement tridimensionnel des tourbillons perturbateurs depuis leur naissance, en passant par la forme en boude jusqu'à la forme en épingle à cheveux. Les harmoniques superieure se développent à un stade ultérieur et ne prennent de l'importance que peu avant le passage au régime turbulent, les premiers symptórnes étant l'apparition d'«épines» ou «spikes» (simple, doubles ou triples) et de bouffées de turbulence localisées.","turbulence; boundary layer transition; measurements","fr","report","Societe hydrotechnique de France","","","","","","","","","","","","",""
"uuid:e93d312b-d59d-4db7-a156-e7b690af2c55","http://resolver.tudelft.nl/uuid:e93d312b-d59d-4db7-a156-e7b690af2c55","Measurement of the Reynolds Stresses in a Circular Pipe as a Means of Testing a DISA Constant-Temperature Hot-Wire Anemometer","Patel, R.P.","TU Delft","1963","Measurements of the turbulent stresses in fully-developed pipe flow have been made as an overall test of the DISA constant-temperature hot-wire anemometer. The measurements were made with single slanting wires and a normal wire. The shearing stress was measured at two Reynolds numbers and compared with values computed from the pressure drop down the pipe: an accuracy of 10% or better was achieved. The computed longitudinal and transverse normal stresses were in good agreement with the measurements of Laufer. It was confirmed that the heat loss from the wire varied linearly with U^0.45 (Collis' law) rather than U^0.50 (King's law).","pipe flow; Reynolds stress; DISA; hot-wire anemometer; turbulence","en","report","McGill University","","","","","","","","","","","","",""
"uuid:8ff299f6-a27a-423c-b934-5db188038594","http://resolver.tudelft.nl/uuid:8ff299f6-a27a-423c-b934-5db188038594","Lecture notes on Turbulent Fluid Motion","Burgers, J.M.","","1951","Lecture notes of prof. Burgers prepared during his sabbatical at CalTech.","turbulence","en","lecture notes","CalTech, California Institute of Technology","","","","","","","","Mechanical, Maritime and Materials Engineering","","","","",""
"uuid:bd473cb6-f5c9-46fb-a826-f3d17fa055e2","http://resolver.tudelft.nl/uuid:bd473cb6-f5c9-46fb-a826-f3d17fa055e2","Application of a mathematical model to illustrate relations characteristic of turbulence (chapter VII)","Burgers, J.M.","","1951","Lecture notes on turbulence (p90 -173) (year estimated)","turbulence","en","lecture notes","TU Delft","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:65d1454c-e070-44f3-a28a-9c03cc9868ef","http://resolver.tudelft.nl/uuid:65d1454c-e070-44f3-a28a-9c03cc9868ef","Mean value and correlation problems connected with the motion of small particles suspended in a turbulent fluid","Tchen, C.M.","Burgers, J.M. (promotor)","1947","A theoretical treatment of diffusion as function of turbulence, by working out the dispersion of small particles in turbulent fluid. The method of Kolmogoroff is used to decribe the movement in a statistical ways","dispersion; diffusion; turbulence; Kolmogoroff","en","doctoral thesis","Martinus Nijhoff, The Hague","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""