PARTICLES IN A SHEAR-FLOW NEAR A SOLID WALL - EFFECT OF NONSPHERICITYON FORCES AND VELOCITIES

Hydrodynamic forces and velocities of spheroidal particles in a simple shear flow near a solid wall are calculated by a variant of the bound ary integral equation method, combined with the use of the reciprocal theorem for Stokes flow equations. It is shown that the effect of the wall decreases with increasing particle nonsphericity (decreasing aspe ct ratio). For long slender particles the effective distance where the wall effect is significant is measured by several particle shorter ax es. In the vicinity of the wall spheroids experience several interacti ons, which do not exist for spheres. These are the lift force componen t perpendicular to the wall and the corresponding rotational-translati onal coupling component of the resistance tenser. The data on particle hydrodynamic interactions are used to calculate the velocities of the inertialess spheroidal particles in a shear flow near a wall. The cal culations reveal that the effect of the wall is to create a nonzero ve locity component in the direction of the normal to the wall surface. T his Velocity is zero for spheroids in a free shear flow; near the wall it vanishes for spherical and, seemingly, for oblong particles. There fore a spheroid moving in a shear flow near the wall will perform an o scillatory motion towards and away from the wall. The wall will retard the particle motion parallel to its surface, albeit in a lesser exten t than for spheres. In addition, spheroidal particles will perform per iodic rotational motion, as they do in an unbounded shear flow, howeve r, with larger periods. For force components which act on spheres, as well as on nonspherical particles the wall effect is most pronounced f or particles whose shape is close to spherical. Several correlation fo rmulae are proposed for the forces and torques acting on spheroids, as well as for their friction tensor coefficients. Copyright (C) 1996 El sevier Science Ltd.