Nature of granular drag in microgravity

TL;DR

This study explores how gravity affects the drag force on objects impacting granular media, revealing different behaviors in microgravity through experiments and simulations, with implications for space exploration.

Contribution

It introduces a dimensionless granular drag coefficient and analyzes its behavior in microgravity, providing new insights into granular drag physics under low-gravity conditions.

Findings

Granular drag coefficient remains ~1.2 in microgravity.

An additional velocity-dependent term appears under gravity.

Microgravity alters cavity dynamics and drag scaling laws.

Abstract

The influence of gravity on the drag force acting on a projectile impacting granular media is investigated experimentally via embedded inertial measurement unit (IMU) sensor and numerically through discrete element method (DEM) simulations. As gravity approaches zero, inertial drag dominates, yielding qualitatively different scaling laws and cavity dynamics. Analogous to fluid dynamics, we define a dimensionless granular drag coefficient $C_{\rm gd}$, which is found to stay largely at a constant $\sim 1.2$ in microgravity while an additional term inversely proportional to impact velocity arises in the presence of gravity. The constant term can be understood from momentum transfer along the penetration direction while the additional term suggests the influence of internal stress built-up due to gravity. Similar discrepancy is also found for the initial peak of the drag force. This analogy provides novel insights into the nature of granular drag in microgravity and sheds light on future space missions.