ABSTRAKT

Flowing suspensions are limited by boundaries and to describe the flow one has two possibilities: either very complicated equations describing the whole flow and completed by simple boundary conditions, or simpler equations describing the bulk flow only and completed by more sophisticated "bulk" boundary conditions. The latter point of view is here applied to one of the simplest examples, the creeping flow of a dilute suspension of spheres. The bulk equations of motion contain forces and stresses that have been somewhat overlooked up to now. Explicit expressions are provided for the bulk boundary conditions, and one of them is shown to be extremely sensitive to the particle distribution in the boundary layers. We apply these equations and boundary conditions to the sedimentation flow between parallel vertical plates, and suggest that the assumption of a homogeneous distribution of centers is not always pertinent, even for dilute suspensions.