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departmentresearch group programmeprojectcoordinates)uuid:ed4bd9dbf34741738ccb65634ac5fd25Dhttp://resolver.tudelft.nl/uuid:ed4bd9dbf34741738ccb65634ac5fd25BLargeeddy Simulation of Isotropic Homogeneous Decaying TurbulenceThornber, B.; Drikakis, D.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 subinertial range with the threedimensional 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.2LES; implicit; isotropic; turbulence; decay; MUSCLenconference paper)uuid:b4f624be50fa475d961515027916da4cDhttp://resolver.tudelft.nl/uuid:b4f624be50fa475d961515027916da4cERegularization modeling for largeeddy simulation of diffusion flamesGeurts, B.J.aWe analyze the evolution of a diffusion flame in a turbulent mixing layer using largeeddy simulation. The largeeddy 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 flamecenter is defined by the `stoichiometric' interface. Geometrical properties such as its surfacearea and wrinkling are characterized using an accurate numerical levelset quadrature method. This allows to quantify flameproperties as well as turbulence modulation effects due to coupling between combustion and turbulent transport. We determine the active flameregion that is responsible for the main part of the chemical conversion in the flame and compare direct and largeeddy simulation predictions.\turbulence; combustion; largeeddy simulation; regularization modeling; isosurface analysis)uuid:bc5304fadfe841af97f330015ade9b55Dhttp://resolver.tudelft.nl/uuid:bc5304fadfe841af97f330015ade9b55lDust emission modelling around a stockpile by using computational fluid dynamics and discrete element method.Derakhshani, S.M.; Schott, D.L.; Lodewijks, G.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 DE< M 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 gasparticle void fractions and momentum transfer. In addition to stockpile deformation, dust dispersion is studied and finally the accuracy of stockpile deformation results obtained by CFDDEM 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; twophase flowAmerican Institute of Physics.Mechanical, Maritime and Materials EngineeringMarine and Transport Technology)uuid:c241f58ec9484d599e9b7134beafbfdaDhttp://resolver.tudelft.nl/uuid:c241f58ec9484d599e9b7134beafbfda4Stone stability under decelerating openchannel flowAHoan, N.T.; Booij, R.; Hofland, B.; Stive, M.J.F.; Verhagen, H.J.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 nonuniform 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.Gbed protection; stone stability; decelerating flow; turbulence; ShieldsWorld Scientific!Civil Engineering and GeosciencesHydraulic Engineering)uuid:9dff055ceb6d4005a052fce8aaeea792Dhttp://resolver.tudelft.nl/uuid:9dff055ceb6d4005a052fce8aaeea792~Numerical Methods for the Optimization of Nonlinear ResidualBased SungridScale Models Using the Variational Germano IdentityMaher, G.D.; Hulshoff, S.J.The Variational Germano Identity [1, 2] is used to optimize the coefficients of residualbased subgridscale 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 subgridscale models that are nonlinear in their coefficients. Specifically, the NewtonRaphson method is employed. A leastsquares 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 BroydenFletcherGoldfarbShanno (BFGS) algorithm is used to solve the minimization problem. The developed method is applied to the onedimensional unsteady forced Burgers equation and the twodimensional steady Stokes equations. It is shown that the NewtonRaphson 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.jsubgridscale model; variational multiscale method; variational Germano identity; optimization; turbulenceCIMNEAerospace Engineering&Aerodynamics, Wind Energy & Propulsion)uuid:e055f69a4edc4f8683fd962465e71ee3Dhttp://resolver.tudelft.nl/uuid:e055f69a4edc4f8683fd962465e71ee3=Multiscale Methods in Computational Fluid and Solid MechanicsTDe Borst, R.;< Hulshoff, S.J.; Lenz, S.; Munts, E.A.; Van Brummelen, E.H.; Wall, W.A. 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, largescale 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 hpadaptive 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 spacetime VMS formulation for the compressible NavierStokes equations. The spatial discretization corresponds to a highorder 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 fluidstructure interaction problems. The basic iterative method for fluidstructure 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 coarsegrid correction. It is noted that an efficient solution strategy is made possible by virtue of the relative compactness of the displacementtopressure operator in the fluidstructure 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 fluidstructureinteraction problems.Zmultiscale methods; fluid flow; turbulence; fluidstructure interaction; multigrid methods)uuid:c2fb8e608b4e446f8535c41372a3a9daDhttp://resolver.tudelft.nl/uuid:c2fb8e608b4e446f8535c41372a3a9daiDelft University of Technology; European Community on Computational Methods in Applied Sciences (ECCOMAS))uuid:0582077531c54aa58d68cfd1103f99fcDhttp://resolver.tudelft.nl/uuid:0582077531c54aa58d68cfd1103f99fcIStereoPIV Measurement of Turbulence Shear Stress in a Stirred Flow Mixer!Shekhar, C.; Nishino, K.; Iso, Y.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 commerciallyavailable, threeblade impeller, HR100, 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 tipvelocity is equal to 59400. The turbulence statistics in the vertical measurement planes are reported before (Shekhar C, Nishino K, Yamane Y and Huang J, StereoPIV 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 tan< gential, bulk flow motion, which convects the turbulence generated at the bladewater 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.Vstirred mixer; stereo PIV; turbulence; budget equation; dissipation rate; shear stress)uuid:2878cdc3f12c4916903d7544d8746d2cDhttp://resolver.tudelft.nl/uuid:2878cdc3f12c4916903d7544d8746d2cbA 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.ETurbulence 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 tradeoff 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.Qtraffic safety; operations; turbulence; surrogate safety measures; freeway designTRBTransport & Planning)uuid:9375fecd44284a29a4e68a0a1f3cc4beDhttp://resolver.tudelft.nl/uuid:9375fecd44284a29a4e68a0a1f3cc4be=Influence of the nozzle outlet face on near flow field mixing$Khan, I.M.; Gilbert, T.; Barigou, M.SThe 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)uuid:c1b00eb366a240beab8c7417f9c2476dDhttp://resolver.tudelft.nl/uui< d:c1b00eb366a240beab8c7417f9c2476d&Stone stability under nonuniform flow>The current research is aimed at finding a dimensionless stability parameter for nonuniform 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 nonuniform flow conditions (R2=0.18). In contrast, Jongeling et al. [2], Hofland [3] and our new proposed stability parameters give better damage predictions (R2 = 0.770.81). The results confirm the strong influence of the velocity and turbulence intensity distributions on the stability of bed material.Fstone stability; bed protection; nonuniform flow; turbulence; ShieldsArizona State University)uuid:39a5176c4f4b4fa985895fe7c7279544Dhttp://resolver.tudelft.nl/uuid:39a5176c4f4b4fa985895fe7c72795445Non reflecting boundary conditions for reacting flowsProsser, R.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 twolength 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 nonreflecting boundary condition framework. The accuracy of the method is demonstrated using a curved stagnating flame, in which the reaction zone crosses the boundary.0characteristics; turbulence; boundary conditions)uuid:c7f5a261e6984d698da93c2a17be505dDhttp://resolver.tudelft.nl/uuid:c7f5a261e6984d698da93c2a17be505dcDirect numerical simulation of the interaction between unsheared turbulence and a freeslip surface5Campagne, G.; Cazalbou, J.B.; Joly, L.; Chassaing, P.^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 secondorder statistics and full Reynoldsstress budgets. It is shown that the results obtained in this configuration can help the understanding of intercomponent energytransfer mechanisms.Eturbulence; direct numerical simulation; free surface; random forcing)uuid:2b90abcc348841f4b70ef2afdd8c3a0eDhttp://resolver.tudelft.nl/uuid:2b90abcc348841f4b70ef2afdd8c3a0e6Particle Sedimentation in WallBounded Turbulent Flows^Cargnelutti, M.; Breugem, W.A.; Portela, L.M.; Mudde, R.F.; Uijttewaal, W.S.J.; Stelling, G.S.XIn this work, a comparison between the results of pointparticle direct numerical simulations and PIV/PTV experiments of a particleladen horizontal channel flow is presented. The numerical simulations were < preformed trying to mimic as much as possible the experimental conditions. The accuracy of the pointparticle 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 nearwall region some differences were found. This can be explained by the lack of resuspension present in the simulations, because we considered only the fluidparticle interaction (oneway coupling) and neglected both the particlefluid interaction (twoway coupling) and the particleparticle interaction (collisions).*particleladen flows; turbulence; DNS; PIV)uuid:c8dcce2e159d45beae23c84f7a022d65Dhttp://resolver.tudelft.nl/uuid:c8dcce2e159d45beae23c84f7a022d65DNumerical modeling for the accurate computations of archeater flowsLee, J.I.; Kim, C.; Kim, K.H.
The purpose of this paper is to develop an accurate analysis code for the flow of archeater by employing advanced numerical models. Governing equations are hyperbolictype axisymmetric NavierStokes 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 threeband model which accounts for selfabsorption and is consistent with the detailed linebyline radiation model. Turbulence effect is incorporated by twoequation 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 twoequation turbulence models combined with the threeband radiation model simulate the flow physics in archeater more accurately than any other previous models and the influence of the turbulence is as much as or bigger than radiation effect.Dplasma wind tunnel; archeater; joule heating; radiation; turbulence)uuid:e8c57b2f74d8402bb3c6e52d85344300Dhttp://resolver.tudelft.nl/uuid:e8c57b2f74d8402bb3c6e52d85344300XOn the required Reynoldsnumber dependence of variational multiscale Smagorinsky modelsMeyers, J.; Sagaut, P.OA theoretical analysis is presented on the dependence of the standard and variational multiscale 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.Rturbulence; modelling; largeeddy simulation; variational multi scale; Smagorinsky)uuid:0b8702014ae541c7972b21feb937f657Dhttp://resolver.tudelft.nl/uuid:0b8702014ae541c7972b21feb937f657MModelling Vertical Variation of Turbulent Flow Across a Surf Zone Using SWASHZijlema, M.This paper presents the application of the open source nonhydrostatic waveflow 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.Ksurf zone; wave breaking; undertow; turbulence; modelling; SWASH; ICCE 2014$Coastal Engineering Research Council)uuid:be865e1e46434239950fccbb53ded8c7Dhttp://resolver.tudelft.nl/uuid:be865e1e46434239950fccbb53ded8c7XHelical mode interactions and spectral energy transfer in magnetohydrodynamic turbulence2Linkmann, M.F.; Berera, A.; McKay, M.E.; Jger, J.VLinkmann, M.F. (author); Berera, A. (author); McKay, M.E. (author); Jger, J. (author< )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.Nspectral methods; magnetohydrodynamics; turbulence; helical mode decomposition)uuid:5e947fcfc25745c8b7500a942d5512d3Dhttp://resolver.tudelft.nl/uuid:5e947fcfc25745c8b7500a942d5512d3sLES and URANS Unsteady Boundary Layer Strategies for Pulsating and Oscillating Turbulent Channel Flows Applications%Panara, D.; Porta, M.; Schoenfeld, T.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 socalled quasisteady regime but there are discrepancies with the experimental results in the intermediate frequency range. A special attention is given to the wallshear stress prediction, and in particular on the wallshear 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 WallNormal 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.ofluid dynamics; turbulence; URANS; LES; pulsating flow; oscillating flow; near wall modeling; wall shear stress)uuid:d6379127c85d4fe5b05f63c5667f6a09Dhttp://resolver.tudelft.nl/uuid:d6379127c85d4fe5b05f63c5667f6a09Applied Sciences)uuid:15a56eea69bc46e9b24a502fc697e9ccDhttp://resolver.tudelft.nl/uuid:15a56eea69bc46e9b24a502fc697e9ccEApplication of Local Defect Correction in a 3d turbulent channel flowDe Hoogh, J.; Kuerten, J.G.M.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 lengthscales 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 pseudospectral method in two periodic directions and a Chebyshev expansion in the wallnormal 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 timedependent 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.3turbulence; local defect correction; passive scalar)uuid:fe01c539598f42dd91e9726c433bccd0Dhttp://resolver.tudelft.nl/uuid:fe01c539598f42dd91e9726c433bccd04Modelling hydrodynamics in Eelgrass (Zostera Marina)2Dijkstra, J.T.; Uittenbogaard, R.E.; Stive, M.J.F.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)uuid:9e32b3d5eae0401290207b32079b4787Dhttp://resolver.tudelft.nl/uuid:9e32b3d5eae0401290207b32079b4787_A Feature Tracking Velocimetry algorithm to determine the velocities in Negatively Buoyant Jets7Ferrari, S.; Badas, M.G.; Besalduch, L.A.; Querzoli, G.kWe 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 nonhomogeneous 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 sharpedged 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.RFeature Tracking Velocimetry (FTV); simple jet; negatively buoyant jet; turbulence)uuid:dccf9895423d4070ba7cadf4d92c8427Dhttp://resolver.tudelft.nl/uuid:dccf9895423d4070ba7cadf4d92c8427&Magnitude control of commutator errorsNonuniform filtering of the NavierStokes equations expresses itself, next to the turbulent stresses, in additional closure terms known as commutator errors. These terms require explicit subgrid modeling if the nonuniformity 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 filterwidth and the filterskewness 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.Hturbulence; largeeddy simulation; nonuniform filter; commutator errors)uuid:5e85afbce6c847c7b39c6dfe8652d04cDhttp://resolver.tudelft.nl/uuid:5e85afbce6c847c7b39c6dfe8652d04c`A new Eulerian Monte Carlo method for the joint velocityscalar PDF equations in turbulent flowsSoulard, O.; Sabel'nikov, V. 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 onepoint 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 onepoint 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 onepoint joint velocityscalar PDF. To achieve this objective, we start from existing Lagrangian stochastic models. The latter are described by SODEs, which can be considered as modeled NavierStokes 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 NavierStokes 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 velocityscalar method.}turbulence; combustion; probability density functions; stochastic partial differential equations; Eulerian Monte Carlo Method)uuid:df2adbc60c024507bf84fbb7721916daDhttp://resolver.tudelft.nl/uuid:df2adbc60c024507bf84fbb7721916daYA Reinterpretation of the NonLinear Galerkin Method as a Large Eddy Simulation TechniqueGuermond, J.L.; Prudhomme, S.UThe purpose of this paper is to show that the Fourierbased Nonlinear Galerkin Method (NLGM) constructs suitable weak solutions to the periodic NavierStokes equations in three dimensions. We reinterpret NLGM as a LargeEddy Simulation technique (LES) and we rigorously deduce a relationship between the mesh size and the largeeddy scale.iNavierStokes equations; turbulence; large eddy simulation; nonlinear Galerkin method; suitab<le solutions
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