Granular Impact: A Grain-scale Approach

TL;DR

This paper investigates the microscopic grain-scale interactions during high-speed granular impact, revealing how force chains and collisions contribute to the macroscopic inertial drag force and its limitations at high velocities.

Contribution

It introduces a collisional model linking grain-scale dynamics to the inertial drag force in granular impact, highlighting the transition at high impact speeds.

Findings

Inertial drag arises from intermittent collisions with force chains.

A simple collisional model explains the inertial force and instabilities.

The inertial drag description fails when impact speed approaches $d/t_c$.

Abstract

This work summarizes a series of studies on two-dimensional granular impact, where an intruding object strikes a granular material at high speed. Many previous studies on granular impact have used a macroscopic force law, which is dominated by an inertial drag term proportional to the intruder velocity squared. The primary focus here is on the microscopic force response of the granular material, and how the grain-scale effects give rise to this inertial drag term. We show that the inertial drag arises from intermittent collisions with force-chain-like structures. We construct a simple collisional model to explain the inertial drag, as well as off-axis instability and rotations. Finally, we show how the granular response changes when the intruder speed approaches $d/t_c$, leading to a failure of the inertial drag description in this regime. Here, $d$ is the mean particle diameter and $t_c$ the characteristic momentum-transfer time between two grains.