Core Precession and Global Modes in Granular Bulk Flow

TL;DR

This study investigates the transition from local to global shear modes in granular flows within a modified Couette cell, revealing how layer depth influences shear zone behavior and core precession.

Contribution

It introduces experimental evidence of a transition from local to global shear modes in granular flows, linked to layer depth and shear zone connectivity.

Findings

Shear zones reach the surface in shallow layers, leaving the core stationary.

In deep layers, shear zones meet below the surface, causing core precession.

A symmetry change in surface velocities indicates the mode transition.

Abstract

A transition from local to global shear zones is reported for granular flows in a modified Couette cell. The experimental geometry is a slowly rotating drum which has a stationary disc of radius R_s fixed at its bottom. Granular material, which fills this cell up to height H, forms a wide shear zone which emanates from the discontinuity at the stationary discs edge. For shallow layers (H/R_s < 0.55), the shear zone reaches the free surface, with the core of the material resting on the disc and remaining stationary. In contrast, for deep layers (H/R_s > 0.55), the shear zones meet below the surface and the core starts to precess. A change in the symmetry of the surface velocities reveals that this behavior is associated with a transition from a local to a global shear mode.