On wall-normal motions of inertial spheroids in vertical turbulent channel flows

Abstract

Dynamics of inertial non-spherical particles in upward and downward turbulent channel flows are investigated. Oblate (disk-like) and prolate (rod-like) spheroids are considered in a Lagrangian point-particle approach in which the turbulence field is obtained by direct numerical simulations. Fluid and particle statistics are conditionally sampled with the view to distinguish between particles moving toward or away from the channel walls. Outward-moving particles tend to locate more in lower-speed streamwise fluid streaks than inward-moving particles. For highly inertial spheroids, these tendencies are differently affected by gravity in upward and downward flows. The gravity force is therefore believed to influence turbophoresis. The tendency of oblate and prolate spheroids to orient parallel with and normal to the spanwise direction near the walls is stronger for outward- than for inward-moving particles. These distinctions are ascribed to the wall-normal force on the spheroids from the surrounding fluid. In the absence of gravity, heavy spheroids in the channel center align randomly with the almost isotropic fluid vorticity. Gravity is seen to have a de-isotropization effect on the particles’ orientation in the inertial frame.