Granular gravitational collapse and chute flow

TL;DR

This paper introduces a theory of granular eddies formed by gravitational collapse in flowing grains, explaining flow behavior and rheology in chute flows, and matching experimental flow rules.

Contribution

It proposes a novel concept of granular eddies formed by gravitational collapse, linking microscopic grain interactions to macroscopic flow behavior in chute flows.

Findings

Predicts existence of a bulk flow region with density-dependent rheology

Derives the Pouliquen flow rule from the theory

Accounts for different flow regimes in chute flows

Abstract

Inelastic grains in a flow under gravitation tend to collapse into states in which the relative normal velocities of two neighboring grains is zero. If the time scale for this gravitational collapse is shorter than inverse strain rates in the flow, we propose that this collapse will lead to the formation of ``granular eddies", large scale condensed structures of particles moving coherently with one another. The scale of these eddies is determined by the gradient of the strain rate. Applying these concepts to chute flow of granular media, (gravitationally driven flow down inclined planes) we predict the existence of a bulk flow region whose rheology is determined only by flow density. This theory yields the experimental ``Pouliquen flow rule", correlating different chute flows; it also correctly accounts for the different flow regimes observed.