Granular Media in a State of Flow

Abstract

In Chapter 3, we established that under the influence of external stimuli, such as a vertical vibration, a granular structure can be fluidized and decompacted, and that it can be the seat of convection rolls and fragmentation processes. We came to appreciate the crucial role played by the container walls in transmitting collisions to the interior of the material and in activating friction forces. The present chapter is devoted to the dynamic properties of the free surface of a granular material when it is inclined at a sufficiently large angle. As we mentioned before (Section 2.4.2), when a granular pile is tilted at an angle greater than some threshold to be specified, a flow in sheet form is set in motion. The threshold angle constitutes a critical state of the granular structure. We will not concern ourselves here with flows through cones, nor will we treat more complex geometries such as hourglasses, all of which involve arches anchored to the walls and friction between particles and boundaries. Before diving into the topic of how a granular medium begins to flow, we will consider more specifically the equilibrium of a stack inclined at less than the threshold angle.