Flow Structures Developed by Bio-Inspired Robotic Sea Lion Flippers of Varied Effective Flexibility

Sea lions use a combination of lift and drag-based propulsion to generate thrust with their foreflippers. Previous research analyzed the flow structures arising from a single robotic foreflipper at varied constant angular velocities and effective flexibilities through a simulated clapping motion. This revealed that the predominant flow structure is a thrust-generating vortex the strength of which is determined by the combination of effective flexibility and angular velocity. The vortex forms at the dorsal side of the flipper, grows in size until the flipper contacts a flat plate representing the sea lion’s body, and then convects in the opposite direction of the sea lion’s forward motion, producing thrust. The flow structures can be compared non-dimensionally as a function of velocity and flexibility, best characterized by an effective lag in the flow structures produced. Here, we characterize this non-dimensional relationship while additionally analyzing the effects of acceleration on the flow structures produced, closely resembling the true clap-like motion that sea lions exhibit. We also aim to understand how effective flexibility affects the flow structures during porpoising, another common swimming maneuver used by sea lions, where the foreflippers are used more passively for stabilization rather than propulsion.