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departmentresearch group programmeprojectcoordinates)uuid:3d6e7fe0184941108e5e5297702226beDhttp://resolver.tudelft.nl/uuid:3d6e7fe0184941108e5e5297702226beQThe stress generated by nonBrownian fibers in turbulent channel flow simulationsBGillissen, J.J.J.; Boersma, B.J.; Mortensen, P.H.; Andersson, H.I.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 secondorder 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 dragreduced 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; turbulenceenjournal articleAmerican Institute of PhysicsApplied SciencesMultiScale Physics)uuid:9cb2c3d3988b45a9a09b607f0baa6b66Dhttp://resolver.tudelft.nl/uuid:9cb2c3d3988b45a9a09b607f0baa6b66KElliptic blending model: A new nearwall Reynoldsstress turbulence closureManceau, R.; Hanjali?, K.A new approach to modeling the effects of a solid wall in onepoint secondmoment (Reynoldsstress) turbulence closures is presented. The model is based on the relaxation of an inhomogeneous (nearwall) formulation of the pressure strain tensor towards the chosen conventional homogeneous (farfromawall) form using the blending function ?, for which an elliptic equation is solved. The approach preserves the main features of Durbin s Reynoldsstress 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 reallife Reynolds number of 106, showing a good prediction of the logarithmic profile of the mean velocity.turbulence; channel flow)uuid:84cec3f1f84f4b45b23b3e58cc3020f1Dhttp://resolver.tudelft.nl/uuid:84cec3f1f84f4b45b23b3e58cc3020f10Polymer flexibility and turbulent drag reductionGillissen, J.J.J.Polymerinduced drag reduction is the phenomenon by which the friction factor of a turbulent flow is reduced by the addition of small amounts of highmolecularweight 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 gen< erated 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.kchannel flow; drag reduction; elasticity; friction; molecular configurations; polymer solutions; turbulenceAmerican Physical Society)uuid:115c96a36c4e48c1a698f831829d184cDhttp://resolver.tudelft.nl/uuid:115c96a36c4e48c1a698f831829d184cSA linear approach for the evolution of coherent structures in shallow mixing layers&Van Prooijen, B.C.; Uijttewaal, W.S.J.rThe 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 selfsimilarity of the velocity profiles. The characteristic features of the downstream 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.crivers; channel flow; mixing; turbulence; flow instability; flow visualisation; hydrology; fractals!Civil Engineering and GeosciencesHydraulic Engineering)uuid:469616a22fd94a92b21f2143e62dc721Dhttp://resolver.tudelft.nl/uuid:469616a22fd94a92b21f2143e62dc721=Numerical simulation of particleladen turbulent channel flow5Li, Y.; McLaughlin, J.B.; Kontomaris, K.; Portela, L.This paper presents results for the behavior of particleladen gases in a small Reynolds number vertical channel down flow. Results will be presented for the effects of particle feedback on the gasphase 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 t< he simulation was comparable to the smallest scales in the particlefree 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; twophase flow; fluctuations; flow simulation)uuid:e1de702f1f8b462399205252dd377dd0Dhttp://resolver.tudelft.nl/uuid:e1de702f1f8b462399205252dd377dd0aA directnumericalsimulationbased secondmoment closure for turbulent magnetohydrodynamic flows#Kenjere, S.; Hanjali?, K.; Bal, D.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 secondmoment 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 wallvicinity 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 = 2.9104,Ha = 52.5,125) and experimental data (Re = 5.05104 and Re = 9104, 0 ? Ha ? 400), show good agreement for all considered situations.pturbulence; magnetohydrodynamics; damping; internal stresses; Boltzmann equation; Poisson equation; channel flow)uuid:41f590a7a3a94d28a9f327fe6dccc209Dhttp://resolver.tudelft.nl/uuid:41f590a7a3a94d28a9f327fe6dccc209EDynamics of prolate ellipsoidal particles in a turbulent channel flowBMortensen, P.H.; Andersson, H.I.; Gillissen, J.J.J.; Boersma, B.J.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 lowspeed 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; twophase flow)uuid:a103b1f8af474f3588ca221341c91b3eDhttp://resolver.tudelft.nl/uuid:a103b1f8af474f3588ca221341c91b3evOn the performance of the moment approximation for the numerical computation of fiber stress in turbulent channel flowFiberinduced 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 NavierStoke< s 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 FokkerPlanck 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 higherorder moments, a closure relation is required to obtain as many equations as unknowns. We investigate the performance of the eigenvaluebased optimal fitted closure scheme [ J. S. Cintra and C. L. Tucker, J. Rheol. 39, 1095 (1995) ]. The closurepredicted stress and flow statistics in twoway coupled simulations are within 10% of the exact FokkerPlanck solution.turbulence; flow simulation; channel flow; drag reduction; NavierStokes equations; FokkerPlanck equation; eigenvalues and eigenfunctions)uuid:9cdda9960ab84a35a346ce912d6e6a8aDhttp://resolver.tudelft.nl/uuid:9cdda9960ab84a35a346ce912d6e6a8acSensitivity of the scale partition for variational multiscale largeeddy simulation of channel flow5Holmen, J.; Hughes, T.J.R.; Oberai, A.A.; Wells, G.N.CThe variational multiscale method has been shown to perform well for largeeddy simulation (LES) of turbulent flows. The method relies upon a partition of the resolved velocity field into large and smallscale 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 smallscale motions is examined. The error in channel flow simulations, relative to DNS results, is computed for various partitions between large and smallscale spaces, and conclusions drawn from the results.Uturbulence; channel flow; flow simulation; numerical analysis; variational techniquesDesign & Construction)uuid:c015e4798d7d41d9b9e99f6029712a4dDhttp://resolver.tudelft.nl/uuid:c015e4798d7d41d9b9e99f6029712a4dCLargeeddy simulation of a curved openchannel flow over topography1Van Balen, W.; Uijttewaal, W.S.J.; Blanckaert, K.Largeeddy simulation (LES) is performed of a curved openchannel 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 largescale bed topography had developed to a more or less stationary shape which was prescribed in the LES model as boundary conditions neglecting the smallscale dune forms by means of a straightforward immersed boundary scheme in combination with a simple wallmodeling approach. The smallscale 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 openchannel 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. Moreove<r, 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.4channel flow; flow simulation; turbulence; viscosity
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