We study suspensions of oblate rigid particles in a viscous fluid for different values of the particle volume fractions. Direct numerical simulations have been performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle–particle and particle–wall interactions. With respect to the single-phase flow, we show that in flows laden with oblate spheroids the drag is reduced and the turbulent fluctuations attenuated. In particular, the turbulence activity decreases to lower values than those obtained by accounting only for the effective suspension viscosity. To explain the observed drag reduction, we consider the particle dynamics and the interactions of the particles with the turbulent velocity field and show that the particle–wall layer, previously observed and found to be responsible for the increased dissipation in suspensions of spheres, disappears in the case of oblate particles. These rotate significantly slower than spheres near the wall and tend to stay with their major axes parallel to the wall, which leads to a decrease of the Reynolds stresses and turbulence production and so to the overall drag reduction.@en

Abstract: The article reports on blending of the Leray-alpha regularization with the conventional Smagorinsky subgrid-scale closure as an option for large-eddy-simulation of turbulent flows at very high Reynolds number on coarse meshes. The model has been tested in the self-similar far-field region of a jet at a range of Reynolds numbers spanning over two decades (4x10(3), 4x10(4) and 4x10(5)) on two very coarse meshes of 2x10(5) and 3x10(4) mesh cells. The results are compared with the well-resolved DNS for Re-D = 4 x 10(3) on 15 million cells and experimental data for higher Re numbers. While the pure Leray-alpha can fail badly at high Re numbers on very coarse meshes, a blending of the two strategies by adding a small amount of extra-dissipation performs well even at a huge jet Reynolds number of Re-D = 4 x 10(5) on a very coarse mesh (2x10(5) cells), despite the ratio of the typical mesh spacing to the Kolmogorov length exceeding 300. It is found that the main prerequisite for successful LES, both for the classic Smagorinsky and the blended Leray-alpha/Smagorinsky model, is to resolve the shear-length L-s = root epsilon/delta(3) (where is the shear-rate modulus), defined by the constraint Delta/L-s <1, where Delta is the typical mesh-cell size. For the mixed Leray-alpha/Smagorinsky model the regularization parameter should also be related to the shear-length rather than the local mesh size or Reynolds number, for which we propose a guide criterion alpha = 0.15 divided by 0.3 L-s .@en