Modeling multiple animal movements through sand

The movement of solid objects through granular systems, like an animal digging through sand, is well understood, but less is known about how multiple objects affect each other while moving together through grains. Carvalho and Franklin investigate how two or three such objects, called ‘intruders,’ interact with each other while moving horizontally within a granular system.

The study reveals the existence of an optimal distance for minimum drag for a given set of intruders. The authors also show that the movements of these intruders can equilibrate to one of several specific configurations over time, suggesting this occurs because of the changes in concentration of grains in front of and behind individual intruders and the way networks of contact forces are arranged.

“Our results help us predict the behavior of solid objects moving within grains and also to design more efficient devices moving over or within grains, such as Mars rovers,” author Erick Franklin said.

To obtain the results, the authors carried out numerical computations using the discrete element method, where they modeled the objects as disks and considered both disk-disk and disk-wall contacts, driving two or three larger intruders at either constant speed or thrusting force within grains in a rectangular cell. The authors then stored the positions, velocities and forces of each object along time, and analyzed their motion and the network of contact forces.

The team hopes to investigate the influence of grain and intruder shape on the entire system and how the study’s results change with even more intruders.

Source: “Collaborative behavior of intruders moving amid grains,” by Douglas D. Carvalho and Erick M. Franklin, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0124556 .