Large eddy simulation of turbulent channel flow with transverse roughness elements on one wall

This study examines the feasibility of large eddy simulation for predicting turbulent channel flows with two-dimensional roughness elements of square, circular and triangular shapes transversely placed on the bottom wall. Results are obtained for several values of the cavity width to the roughness height ratio using various subgrid-scale turbulence models. The present large eddy simulation predictions of mean streamwise velocity, root-mean-square velocity fluctuations, and skin frictional and form drags agree reasonably well with previously published results of direct numerical simulations at a low Reynolds number. All the subgrid-scale models examined here are capable of reproducing the relevant physics associated with the effect of the rough surface on the turbulent flow, exhibiting similar performances. Moreover, the use of the turbulence models leads to an improvement in the predictions of several turbulence statistics as compared with the case when no model is considered. Large eddy simulation can be combined easily with an immersed boundary method yielding satisfactory results based on a coarser grid resolution than in direct numerical simulation and, thus, it is suitable for the investigation of turbulent channel flows with riblets of various shapes. (C) 2014 Elsevier Inc. All rights reserved

LES of particle-laden turbulent channel flow with transverse roughness elements on one wall

The effect of wall roughness on the transport of solid particles in a turbulent channel flow is numerically investigated by means of large eddy simulation coupled with a Lagrangian particle-tracking scheme. Two-dimensional transverse square elements separated by a rectangular cavity are placed on the lower wall of the channel. Results were obtained for several values of the cavity width to the roughness height ratio. It is shown that the deposition of particles, as well as the particle accumulation near the walls, and their tendency to preferentially concentrate in flow regions of low streamwise fluid velocity are significantly affected by the roughness elements. © 2009 American Institute of Physics

DNS/LES study of fluid-particle interaction in a turbulent channel flow at a low reynolds number

This paper presents results of large eddy simulations (LES) combined with Lagrangian particle tracking and a point-force approximation of particle-laden turbulent channel flows with significant momentum exchange between the two phases. The effect of particle inertia, mass fraction, gravity, and interparticle collisions on the LES predictions is addressed and analyzed. The results are compared with those predicted from the direct numerical simulation (DNS) of the same particle-laden flow cases. LES with either the standard or the dynamic Smagorinsky model is capable to reproduce qualitatively the changes in the fluid turbulence intensities due to the presence of the particles. However, LES distributions significantly deviate from the DNS results, especially for particles with large response time at high mass fractions. © 2008 American Institute of Physics

Mixed convection of a low Prandtl fluid with spatially periodic lower wall heating in the presence of a wall-normal magnetic field

Numerical simulations of the combined natural and forced convection flow in a horizontal channel have been carried out for a fluid of low Prandtl number in the presence of a uniform wall-normal magnetic field. The upper wall is maintained at a constant temperature, while a spatially periodic temperature is imposed at the lower wall. A stability diagram is created based on the results from two- and three-dimensional numerical simulations in order to examine whether the present two-dimensional flows are unstable to three-dimensional disturbances. At the range of parameters studied (5000 <= Ra <= 150,000-Rayleigh number, 5 <= Re <= 500-Reynolds number, and 0 <= Ha <= 20-Hartmann number), no stable two-dimensional unsteady flow is observed. All mixed convection flows are suppressed by the action of sufficiently strong electromagnetic forces and, in contrast to the hydrodynamic cases, the direct transition to unsteadiness is favored with an apparent lack of steady three-dimensional magnetohydrodynamic flows. The velocity and temperature features of the various flow regimes are discussed, addressing also the differences in the predictions produced by the two- and three-dimensional numerical simulations. (C) 2014 Elsevier Ltd. All rights reserved

Numerical investigation of momentum exchange between particles and coherent structures in low Re turbulent channel flow

The interaction between particles and coherent structures is studied by using discrete particle simulation combined with direct numerical simulation of gaseous flow in a vertical channel. A conditional sampling scheme is used to examine the modifications of the near-wall quasistreamwise vortices by the momentum exchange between the phases. The particle effect on the fluid flow is modeled by a point-force approximation. The particle diameters are smaller than both the smallest flow length scales and the computational grid spacing. Results are obtained for particle ensembles with four response times ranging from 10 to 200 wall units in numerical simulations with and without gravitational settling in the streamwise direction and interparticle collisions. It is found that the size of the quasistreamwise vortices is increased up to 25% in the presence of particles. The increase is larger for the smallest inertia particles studied, which is partly due to their locally nonuniform spatial distribution. The underlying organized fluid motions induced by the structures are substantially attenuated due to the momentum coupling of the phases. A reduction of 5%-55% is observed in the coherent fluid velocities and vorticities. The size of the coherent structures is additionally augmented by streamwise gravitational settling and interparticle collisions, accompanied by more obvious modifications in the surrounding fluid flow. The latter findings are explained by the stronger direct particle effect in these cases, which is also reflected on the energy redistribution between the fluid velocity components, affecting further the fluid turbulence. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553292

Effect of magnetic field on near-wall coherent structures and heat transfer in magnetohydrodynamic turbulent channel flow of low Prandtl number fluids

A numerical study is carried out of the magnetic field effects on the coherent structures and the associated heat transfer in a turbulent channel flow with constant temperature at the bottom (cold) and top (hot) walls. Results from direct numerical simulations are conditionally sampled in order to extract the dominant coherent structures in the near-wall region for flows with and without a uniform external magnetic field in the wall-normal direction. The Reynolds number based on the bulk velocity and the wall distance is 5600, while only a representative small Stuart number of 0.01 is explored. Two fluids with Prandtl numbers of 0.01 and 0.71 are studied. It is shown that the conditionally averaged quasi-streamwise vortices are modified by the magnetic field with their size being increased and their strength decreased. The underlying organized fluid motions are damped by the Lorentz force and the turbulent heat transfer related to the action of quasi-streamwise vortices is decreased by the magnetic field. For the higher Prandtl number fluid, a similarity between the coherent temperature and the coherent streamwise velocity fluctuations is observed for both types of flow. This is diminished for the lower Prandtl number fluid, especially in the magnetohydrodynamic flow, inhibiting the intrusion of cold (hot) fluid from the cold (hot) wall towards the central region. (C) 2011 Elsevier Ltd. All rights reserved

Large eddy simulation of gas-particle turbulent channel flow with momentum exchange between the phases

This paper presents results of a large eddy simulation (LES) combined with Lagrangian particle tracking and a point-force approximation for the feedback effect of particles on the downward turbulent gaseous flow in a vertical channel. The LES predictions are compared with the results obtained by direct numerical simulation (DNS) of a finer computational mesh. A parametric study is conducted for particles with two response times in simulations with and without streamwise gravitational settling and elastic, binary interparticle collisions. It is shown that the classical and the dynamic Smagorinsky turbulence models adequately predict the particle-induced changes in the mean streamwise velocity and the Reynolds stresses of the carrier phase for the range of parameters studied. However, the largest discrepancies between the LES and DNS results are found in the cases of particle-laden flows. Conditional sampling of the instantaneous resolved flow fields indicates that the mechanisms by which particles directly oppose the production of momentum and vorticity of the organized fluid motions are also observed in the LES results. However, the geometric features of the near-wall quasistreamwise vortices are overestimated by the use of both turbulence models compared to the DNS predictions. (c) 2011 Elsevier Ltd. All rights reserved

Numerical study of educed coherent structures in the near-wall region of a particle-laden channel flow

The interaction of small heavy solid particles with turbulence near the wall of a vertical downward channel flow is investigated by using direct numerical simulation (DNS) and Lagrangian particle tracking. The interest is focused on the effect of the particles on the near-wall coherent structures obtained by conditional sampling of DNS results of a particle-laden turbulent channel flow. The coherent structures are detected from instantaneous flow fields by using the vortex definition of Jeong and Hussain [J. Fluid Mech. 285, 69 (1995)]. The Reynolds number of the particle-free flow is Reγ ≈ 180 based on the friction velocity and the wall half distance. The particle response time is 200 wall units and the average mass and volume fractions φm=0.5 and φv = 6.8 × 10-5, respectively. The particle diameter is smaller than the Kolmogorov length scale and the grid spacing, the latter being small enough to adequately resolve the smaller fluid flow scales. The feedback effect of the particles on the carrier phase is taken into account by a point-force model. Purely elastic interparticle collisions are also considered. For both particle-free and particle-laden flows, the dominant coherent structures in the near-wall region are elongated quasistreamwise vortices. The addition of particles results in a weaker mean structure, with larger diameter and longer streamwise extent. The qualitative characteristics of the velocity distributions around the mean coherent structures are similar, independent of the particles. However, the coherent velocity fluctuations in the wall-normal and spanwise directions considerably decrease, and the low-speed streak is damped by the particles. The educed results show that the particles create a torque of opposite sign to the rotation of the mean vortex, which in turn reduces the streamwise vorticity of the structure. Consequently, the magnitude of fluid pressure decreases and the redistribution of turbulent kinetic energy from the streamwise to the other velocity components is significantly reduced. © 2008 American Institute of Physics

Transport and deposition of neutral particles in magnetohydrodynamic turbulent channel flows at low magnetic Reynolds numbers

The effect of Lorentz force on particle transport and deposition is studied by using direct numerical simulation of turbulent channel flow of electrically conducting fluids combined with discrete particle simulation of the trajectories of uncharged, spherical particles. The magnetohydrodynamic equations for fluid flows at low magnetic Reynolds numbers are adopted. The particle motion is determined by the drag, added mass, and pressure gradient forces. Results are obtained for flows with particle ensembles of various densities and diameters in the presence of streamwise, wall-normal or spanwise magnetic fields. It is found that the particle dispersion in the wall-normal and spanwise directions is decreased due to the changes of the underlying fluid turbulence by the Lorentz force, while it is increased in the streamwise direction. The particle accumulation in the near-wall region is diminished in the magnetohydrodynamic flows. In addition, the tendency of small inertia particles to concentrate preferentially in the low-speed streaks near the walls is strengthened with increasing Hartmann number. The particle transport by turbophoretic drift and turbulent diffusion is damped by the magnetic field and, consequently, particle deposition is reduced. (c) 2011 Elsevier Inc. All rights reserved