Pre-avalanche instabilities in a granular pile

TL;DR

This study numerically examines the transition from static to flowing states in a 2D granular pile, revealing critical contact clustering and power-law divergence near the stability limit.

Contribution

It introduces a new order parameter based on critical contact density and analyzes spatial correlations during the static/dynamic transition.

Findings

Critical contact clusters exhibit power-law size divergence near the transition.

Fluidized regions form and grow as the system approaches instability.

The results support multi-phase models of granular flow transition.

Abstract

We investigate numerically the transition between static equilibrium and dynamic surface flow of a 2D cohesionless granular system driven by a continuous gravity loading. This transition is characterized by intermittent local dynamic rearrangements and can be described by an order parameter defined as the density of critical contacts, e.g. contacts where the friction is fully mobilized. Analysis of the spatial correlations of critical contacts shows the occurence of ``fluidized'' clusters which exhibit a power-law divergence in size at the approach of the stability limit. The results are compatible with recent models that describe the granular system during the static/dynamic transition as a multi-phase system.