Propulsion in the vicinity of a granular media

In various situations, sand and sediments can be carried by wind or water, which could trigger the resuspension of the granular bed. For instance, when a helicopter lands in sandy environments, its blades trigger air recirculation which leads to the resuspension of particles, thus limiting the pilot's visibility. Here, we focus on a unique situation, in which the resuspension of particles is both sought after and well controlled. Indeed, some bottom-dwelling fish, such as the flounders and stingrays, generate a flow capable of resuspending sand to bury themselves and avoid predators. By flapping their fins with oscillating motions, they create vortices and a recirculating flow that lifts the sand particles up and deposits them on top of their backs. A simple model experiment has been developed to study this situation: a rigid or flexible foil is placed above the sand bed to mimic the fin motion.