The role of initial speed in projectile impacts into light granular media

TL;DR

This study combines experiments and simulations to analyze how initial impact speed influences projectile behavior and granular response in light granular media, revealing velocity-dependent cavity collapse and exponential decay of kinetic pressure.

Contribution

It provides a comprehensive analysis of impact velocity effects on granular drag and intruder dynamics, integrating experimental, numerical, and theoretical approaches.

Findings

Intruder dynamics match between experiments and simulations.

Cavity collapse behavior varies with impact velocity.

Kinetic pressure decays exponentially with a characteristic length scale.

Abstract

Projectile impact into a light granular material composed of expanded polypropylene (EPP) particles is investigated systematically with various impact velocities. Experimentally, the trajectory of an intruder moving inside the granular material is monitored with a recently developed non-invasive microwave radar system. Numerically, discrete element simulations together with coarse-graining techniques are employed to address both dynamics of the intruder and response of the granular bed. Our experimental and numerical results of the intruder dynamics agree with each other quantitatively and are in congruent with existing phenomenological model on granular drag. Stepping further, we explore the `microscopic' origin of granular drag through characterizing the response of granular bed, including density, velocity and kinetic stress fields at the mean-field level. In addition, we find that the dynamics of cavity collapse behind the intruder behaves qualitatively different with different impact velocities. Moreover, the kinetic pressure ahead of the intruder decays exponentially in the co-moving system of the intruder. Its scaling gives rise to a characteristic length scale, which is in the order of intruder size. This finding is in perfect agreement with the long-scale inertial dissipation type that we find in all cases.