Internal Dynamics and Flow Properties of Dense Granular Materials

Abstract

This thesis deals with the micro-mechanics of dense granular flows and how they affect the overall flow and mixing behaviours of grains. Discrete Element Simulations of dense granular flows are performed at various flow geometries, giving us insights into the internal kinematics and dynamics of flow. This allows us to connect the micro-mechanics to the effective transport properties like self-diffusivity, viscosity and non-local rheology. The thesis is comprised of three published papers. The first paper shows how the development of granular vortices gives rise to enhanced mixing of grains in dense granular flows in plane shear flow geometries involving large widths. Rate dependent nature of the average vortex size is observed, and a general scaling law in terms of the size of granular vortices is introduced which can predict the enhanced mixing behaviour. The second paper connects the existence of granular vortices to the non-local behaviours of dense granular flows. The granular vortices are found to originate from a process of multiple orthogonal shear banding. A general non-local relation is then derived by considering the spatial redistribution of vorticity induced by these granular vortices. This relation is validated on two steady granular flow geometries involving nonlocal flow behaviours. The purely kinematic nature of this derivation suggests that non-local behaviour should be expected in flows of other materials as well that involve correlated motion of particles, like foams, pastes and gels. The final paper studies the self-diffusion behaviours in nonlocal flow geometries. The nature of grain trajectories in dense granular flow is found to depend on the stress condition. In creeping layers that have stresses below the yield stress, we observe caged dynamics of the grains and breakdown of the scaling of diffusivity with the velocity fluctuations. A scaling law is introduced that allows us to predict the rate of flow and self-diffusivity with the sole knowledge of stress condition and position.