Boundary effects for Couette flow of granular materials: Dynamical modelling

Abstract

The influence of a boundary on granular flows is very different from that for conventional fluids. While slip at the boundary has generally been observed for granular flows, the detailed interaction mechanism between the boundary and the flowing particles is not properly understood. In this paper we examine boundary effects for simple Couette flow of granular materials using a dynamical modelling method. We find that while density profiles and slip velocities for a rough boundary are very different from those for a smooth boundary, they are strongly dependent on the speed of the boundary, so that the notion of roughness has no absolute meaning in the sense that a rough boundary becomes smooth when the boundary speed is large and a smooth boundary becomes rough when the boundary speed is small. In general, the larger the boundary speed, the larger the shear rate on the flow field. However, the exact relationship is by no means clear because of the unknown slip at the boundary. The relation between boundary speed and slip velocity is generally nonlinear, and for the same boundary conditions the slip velocity depends strongly on the particle fraction in the flow such that the larger the particle fraction the smaller the slip velocity. Further, for increasing particle fraction the boundary acts as a nuclear for a layering phenomena.