Orientation of inertial spheroids in turbulent Couette–Poiseuille flow with a shear-free wall

Abstract

The role of mean shear on the orientational behavior of aspherical particles is studied by Lagrangian tracking of inertial spheroids in a directly simulated turbulent Couette–Poiseuille flow. The effects of particle inertia and shape in the conventional wall-turbulence near the stationary wall closely resemble observations made earlier in turbulent channel flows. The particles’ orientational behavior in the anisotropic turbulence field near the shear-free wall is qualitatively different. Direct comparisons between the orientation statistics near the sheared and the shear-free wall provide an insight into the role played by the mean shear. We observed that disks (rods) with low inertia preferentially oriented in the directions of Lagrangian compression (stretching) of the fluid elements, similarly as already reported for tracer spheroids. Spheroids with intermediate inertia exhibited similar orientations as the low-inertia particles near the shear-free wall, although the behavior of such spheroids was greatly affected by inertia near the sheared wall. This is explained by a six-fold increase of the Kolmogorov time scale across the flow, which leads to a corresponding reduction of the local Stokes number near the shear-free wall. The most inertial spheroids, however, oriented almost randomly near the shear-free wall, a finding we ascribe to the absence of mean shear and inertial filtering of the anisotropic vorticity field. It can therefore be inferred that the presence of mean shear is essential for forming the preferential orientation of inertial spheroids in near-wall turbulence and the particle inertia alone merely randomizes particle alignment due to the inertia-filtering.