Flounders, stingrays, and other flat, bottom-dwelling fish often hide under sand for protection. These fish move by oscillating their fins or the edge of their bodies. They use a similar mechanism to bury themselves–quickly flapping to resuspend a cloud of particles, then hitting the ground so that the sand settles down to cover them. Researchers have been investigating this process by oscillating rigid and flexible plates and observing the resulting flow. When the flapping motion exceeds a critical velocity, the vortex that forms at the plate’s edge is strong enough to pick up sand particles. Understanding and controlling how and when these vortex motions kick up particles is useful beyond the ocean floor, too. Helicopters are often unable to land safely in sandy environments because of the particles their rotors lift up, and this work could help mitigate that problem. (Image credits: TylersAquariums, source; Richmondreefer, source; A. Sauret, source; research credit: A. Sauret et al.)
