Unsteady flows and inhomogeneous packing in damp granular heap flows

TL;DR

This study investigates how small amounts of water induce a transition from steady to unsteady flow in granular heaps, revealing the formation of layered packing structures linked to flow dynamics.

Contribution

It experimentally demonstrates the transition mechanisms from steady to unsteady flow in damp granular heaps and links flow behavior to packing structure formation.

Findings

Unsteady flow involves avalanches and propagating jumps.

Transition occurs at a critical slope due to water cohesion.

Layered packing structures form under unsteady flow.

Abstract

We experimentally study the transition from steady flow to unsteady flow in a quasi-2D granular heap when small amounts of water are added to monodisperse glass spheres. Particles flow uniformly down both sides of the heap for low water content, but unsteady flow occurs as the water content increases. The unsteady flow mode consists of a non-depositing downslope avalanche and an upslope propagating granular jump. The transition from steady to unsteady flow occurs when the slope exceeds a critical angle as a result of water-induced cohesion. Under unsteady flow conditions, the deposited heap consists of loosely packed and densely packed layers, the formation of which is closely related to the unsteady flow dynamics.