Rapid granular flows on a rough incline: phase diagram, gas transition, and effects of air drag

TL;DR

This study investigates granular flows on inclined planes, mapping their phase diagram, examining gas-like transitions at high inclinations, and assessing air drag effects, revealing minimal influence of air except in dilute regimes.

Contribution

It provides the first detailed phase diagram of granular flows on an incline, including the effects of air drag at high velocities and dilute regimes.

Findings

Flow does not reach steady state at high inclinations.

Transition from dense to dilute (gas) flow occurs at low flow rates.

Air drag has minimal impact on flow velocities except in dilute regimes.

Abstract

We report experiments on the overall phase diagram of granular flows on an incline with emphasis on high inclination angles where the mean layer velocity approaches the terminal velocity of a single particle free falling in air. The granular flow was characterized by measurements of the surface velocity, the average layer height, and the mean density of the layer as functions of the hopper opening, the plane inclination angle and the downstream distance x of the flow. At high inclination angles the flow does not reach an x-invariant steady state over the length of the inclined plane. For low volume flow rates, a transition was detected between dense and very dilute (gas) flow regimes. We show using a vacuum flow channel that air did not qualitatively change the phase diagram and did not quantitatively modify mean flow velocities of the granular layer except for small changes in the very dilute gas-like phase.