Large Force Fluctuations in a Flowing Granular Medium

TL;DR

This study investigates force fluctuations in a flowing granular medium, revealing impulsive force characteristics, broad impulse distributions, and a power-law collision interval distribution indicating proximity to jamming.

Contribution

It provides detailed analysis of force and collision dynamics in flowing granular media, highlighting signatures of approaching jamming not previously characterized.

Findings

Impulsive forces dominate at all flow rates.

Impulse distribution decays exponentially, similar to static granular media.

Collision time intervals follow a power-law distribution near jamming.

Abstract

We report the characteristics of the temporal fluctuations in the local force delivered to the wall of a 2D hopper by a granular medium flowing through it. The forces are predominantly impulsive at all flow rates for which the flow does not permanently jam. The average impulse delivered to the wall is much larger than the momentum acquired by a single particle under gravity between collisions, reflecting the fact that momentum is transferred to the walls from the bulk of the flow by collisions. At values larger than the average impulse, the probability distribution of impulses is broad and decays exponentially on the scale of the average impulse, just as it does in static granular media. At small impulse values, the probability distribution evolves continuously with flow velocity but does not show a clear signature of the transition from purely collisional flow to intermittently jamming flows. However, the time interval between collisions tends to a power law distribution, $P(\tau)\sim \tau^{-3/2}$, thus showing a clear dynamical signature of the approach to jamming.