Computer simulation of particle aggregates during sedimentation

This work concerns with a numerical investigation of particle velocity fluctuations in a blob undergoing gravity induced sedimentation at low Reynolds number. The simulations were performed by direct computation of the hydrodynamic interactions between a large collection of spherical particles. We focus our attention on both monodisperse and slightly polydisperse spherical aggregates comprised of inertialess particles whose radii have a Gaussian distribution about the mean. At vanishing particle Stokes number the dispersed particles undergo stochastic displacements arising from the random ambient field of the fluid velocity and not from direct solid-body collisions. As the sedimentation proceeds, a monodisperse blob persists as a cohesive entity with sporadic outward particle crossings of the blob boundary. A scaling argument for the rate at which the particles leak away from the aggregate was developed. When a sufficiently high degree of polydispersity is introduced the aggregate will behave differently from a monodisperse blob depending on the solid volume fraction. The observed fluctuating motion of the particles leads to the definition of a particle pressure associated with the particulate phase of the blob if it is regarded as an effective continuum. For polydisperse blobs, the tendency to a particle spreading induced by velocity fluctuations makes clear the effect of the particle pressure on resisting the formation of solid volume fraction inhomogeneities in suspension flows.