Surprising simplicity in the modeling of dynamic granular intrusion

TL;DR

This paper introduces a simple continuum model based on frictional flow and tension-free separation that effectively describes complex granular intrusion phenomena across various experiments, enabling rapid modeling of granular locomotion.

Contribution

The authors present a unified, continuum-based model that captures the physics of granular intrusions and introduces a reduced-order technique for quick predictions.

Findings

Model accurately describes wheel locomotion and legged robot intrusions.

Three primary effects govern intrusion forces: static and two dynamic.

Reduced-order Dynamic Resistive Force Theory enables rapid modeling.

Abstract

Granular intrusions, such as dynamic impact or wheel locomotion, are complex multiphase phenomena where the grains exhibit solid-like and fluid-like characteristics together with an ejected gas-like phase. Despite decades of modeling efforts, a unified description of the physics in such intrusions is as yet unknown. Here we show that a continuum model based on the simple notions of frictional flow and tension-free separation describes complex granular intrusions near free surfaces. This model captures dynamics in a variety of experiments including wheel locomotion, plate intrusions, and running legged robots. The model reveals that three effects (a static contribution and two dynamic ones) primarily give rise to intrusion forces in such scenarios. Identification of these effects enables the development of a further reduced-order technique (Dynamic Resistive Force Theory) for rapid modeling of granular locomotion of arbitrarily shaped intruders. The continuum-motivated strategy we propose for identifying physical mechanisms and corresponding reduced-order relations has potential use for a variety of other materials.