Flowfields produced by a robotic sea lion foreflipper starting from rest.

Sea lions swim using primarily their foreflippers, which is uncommon among aquatic mammals. While a significant body of literature exists which investigates the hydrodynamics of body-caudal swimming, relatively little research has looked at sea lion propulsion. In this work, particle imaging velocimetry is used to observe the flow around a robotic model sea lion flipper. The model flipper was cast in silicone from a high-resolution scan of a sample sea lion foreflipper. The model flipper was actuated at the root, and its motion was controlled by a programmable servomotor. It was observed that the thrust-producing clapping motion of the flipper entrained significant fluid momentum on the suction side of the flipper, which developed into a shed vortex and contributed to downstream momentum (and therefore thrust). Rotating the robotic flipper more quickly produced greater downstream jet velocities, but at a lower conversion of rotational velocity, suggesting that this mechanism of propulsion can be optimized based on the system needs.