Constant speed penetration into granular materials: drag forces from the quasistatic to inertial regime

TL;DR

This study investigates how the drag force on an object penetrating granular materials varies from quasistatic to inertial regimes, combining experiments and simulations to understand the physical origins of granular resistance.

Contribution

It provides new insights into the velocity-dependent behavior of granular drag forces and the transition from quasistatic to inertial regimes through combined experimental and simulation approaches.

Findings

Granular flow around the intruder influences depth-dependent drag.

Linear depth dependence correlates with the mass of flowing grains.

Inertial effects become significant at higher impact velocities.

Abstract

Predicting the force exerted on an object as it penetrates a granular medium is of interest in engineering, locomotive, and geotechnical applications. Current models of granular drag, however, vary widely in applicability and parameterization, and the physical origin of the granular resistive force itself is a subject of debate. Here we perform constant speed penetration experiments, combined with calibrated, large-scale molecular dynamics simulations, at velocities up to 2 m/s to test the effect of impact velocity on the depth dependent `hydrostatic' drag force. We discover that the evolution of the granular flow field around an intruder regulates the presence of depth dependent drag forces. In addition, we find that the observed linear depth dependence is commensurate with the mass of flowing grains. These results suggest that, as the impact speed increases beyond the quasistatic regime, the depth dependent drag term becomes intertwined with inertial effects.