Secondary flow in ensembles of non-convex granular particles under shear

TL;DR

This study reveals that non-convex granular particles, such as spatial crosses, exhibit a unique secondary flow under shear that is opposite to convex grains, leading to a convection cell and a reverse Weissenberg effect.

Contribution

It demonstrates that non-convex grain shapes induce qualitatively different macroscopic flow dynamics in sheared granular materials, a phenomenon not observed with convex grains.

Findings

Non-convex crosses develop a reverse secondary flow under shear.

Surface particles migrate towards the center and sink, mimicking a reverse Weissenberg effect.

Flow patterns depend on particle shape and bed height.

Abstract

Studies of granular materials, both theoretical and experimental, are often restricted to convex grain shapes. We demonstrate that a non-convex grain shape can lead to a qualitatively novel macroscopic dynamics. Spatial crosses (hexapods) are continuously sheared in a split-bottom container. Thereby, they develop a secondary flow profile that is completely opposite to that of rod-shaped or lentil-shaped convex grains in the same geometry. The crosses at the surface migrate towards the rotation center and sink there, mimicking a `reverse Weissenberg effect'. The observed surface flow field suggests the existence of a radial outward flow in the depth of the granular bed, thus forming a convection cell. This flow field is connected with a dimple formed in the rotation center. The effect is strongly dependent on the particle geometry and the height of the granular bed.