Giant drag reduction due to interstitial air in sand

TL;DR

This study reveals that ambient pressure significantly influences drag on objects impacting loose sand, with lower pressure increasing drag and the presence of interstitial air reducing it by over 90%, supported by experiments and a validated model.

Contribution

The paper demonstrates the critical role of interstitial air in sand drag reduction and provides a quantitative model explaining this phenomenon.

Findings

Drag increases with decreasing pressure in loose sand.

Interstitial air reduces drag by over 90%.

A validated model explains the pressure-dependent drag behavior.

Abstract

When an object impacts onto a bed of very loose, fine sand, the drag it experiences depends on the ambient pressure in a surprising way: Drag is found to increase significantly with decreasing pressure. We use a modified penetrometer experiment to investigate this effect and directly measure the drag on a sphere as a function of both velocity and pressure. We observe a drag reduction of over 90% and trace this effect back to the presence of air in the pores between the sand grains. Finally, we construct a model based on the modification of grain-grain interactions that is in full quantitative agreement with the experiments.