MOTION OF SPHEROIDAL PARTICLES IN VERTICAL SHEAR FLOWS

The motion of non-neutrally buoyant prolate spheroidal particles in ve rtical shear flows is investigated. Using the generalized Faxen law, w e calculate the hydrodynamic forces and moments acting on such inertia l and inertialess particles, and their trajectories. The calculations are done for (i) freely rotating particles, and (ii) particles with or ientations fixed by means of an external torque exerted by a strong or ienting field. Inertial particles are found to migrate across the stre amlines, and their trajectories differ considerably from those calcula ted for inertialess particles. Neutrally buoyant spheroids, inertial o r not, which either freely rotate or have fixed orientations in shear flows, translate along the streamlines. Non-neutrally buoyant inertial ess spheroids freely moving in simple shear flow translate along perio dic trajectories with no net lateral drift. In contrast, inertial part icles under similar flow conditions drift laterally toward locations c haracterized by higher local velocities in a direction opposing gravit y. The motion of non-neutrally buoyant inertial particles with fixed o rientations may be unstable with the drift velocity growing exponentia lly with time. Conditions for the occurrence of this unstable motion a re formulated analytically in terms of particle and flow parameters. I n general the rate of drift depends on particle shape, via its aspect ratio, and its inertia. parameters. In general, the rate of drift depe nds on particle shape, via its aspect ratio, and its inertia. (C) 1998 American Institute of Physics.