Stresses in Smooth Flows of Dense Granular Media

TL;DR

This study investigates stress tensor behavior in dense granular flows within split-bottom geometries, revealing flow-dependent stress alignment and variable effective friction influenced by local flow geometry, challenging traditional models.

Contribution

It demonstrates that in dense granular flows, the stress tensor aligns with the flow field and the effective friction varies with local geometry, contrary to constant friction assumptions.

Findings

Stress tensor is co-linear with the flow field.

Effective friction varies throughout the flow zone.

Flow geometry influences stress and friction, not inertial effects.

Abstract

The form of the stress tensor is investigated in smooth, dense granular flows which are generated in split-bottom shear geometries. We find that, within a fluctuation fluidized spatial region, the form of the stress tensor is directly dictated by the flow field: The stress and strain-rate tensors are co-linear. The effective friction, defined as the ratio between shear and normal stresses acting on a shearing plane, is found not to be constant but to vary throughout the flowing zone. This variation can not be explained by inertial effects, but appears to be set by the local geometry of the flow field. This is in agreement with a recent prediction, but in contrast with most models for slow grain flows, and points to there being a subtle mechanism that selects the flow profiles.