Effects of Particle Jamming and Swimmer Inertia on Granular Locomotion

Locomotion in granular media is challenging due to the complex, yielding nature of these materials. In this study, we investigate the mechanisms enabling locomotion of a swimmer with two flapping wings using Discrete Element Method (DEM) simulations that has been validated by experiments. By reciprocally opening and closing its wings, the simulated swimmer in a frictional granular medium achieves sustained locomotion, in which the generated displacement during an opening stroke is larger than that during closing. Stagnant zones form only in front of the wings, where particles transition to a jammed state. Non-reciprocal formation of these zones is important for locomotion. In a frictionless granular medium, jamming is suppressed, and locomotion vanishes. A second locomotion mechanism was identified in a dynamic regime in which the swimmer takes a significant time to accelerate or decelerate in comparison to the cycle duration. As the swimmer’s translational direction does not immediately switch upon the change of the wing’s rotational direction, additional drag is exerted on the swimmer, which depends on the area exposed by the wings. This effect results in more displacement during closing than opening.