The Role of foreflipper flexibility in vortex formation by the California sea lion

Sea lions inspire underwater vehicles due to their efficient and agile swimming. Their swimming stroke includes recovery, power, and paddle phases, optimizing movement, and incorporating their foreflippers in an active 'clap' motion to generate thrust. Our research shows they also use them passively by extending one or both flippers laterally to create lift and torque for agile, tight turns. This passive use for maneuvering, previously undocumented in pinnipeds, enhances their ability to navigate complex underwater environments. Thus, sea lions' foreflippers serve dual roles: producing thrust via the clap and generating lift-based forces for precise maneuverability as aquatic predators. In this talk, we present data on the thrust and flow structures produced by a sea lion flipper of varying degrees of passive flexibility. Increasing flexibility decreases vortex strength and creates a time delay in the formation of the fluid structures. We will present a scaling law based on the flipper geometry, kinematics and flexibility. Finally, this flipper optimization is applied to a full-scale, free-swimming, sea lion-based robotic platform.