Velocity Field Measurements of the California Sea Lion Propulsive Stroke Using Bubble PIV

California sea lions are among the most agile of swimming mammals. Most marine mammals swim with their hind appendages—flippers or flukes, depending on the species—whereas sea lions use their foreflippers for propulsion and maneuvering. The sea lion’s propulsive stroke generates thrust by forming a jet between the flippers and the body and by dragging a starting vortex along the suction side of the flipper. Prior experiments using robotic flippers have shown these mechanisms to be possible, but no flow measurements around live sea lions previously existed with which to compare. In this study, the flow structures around swimming sea lions were observed using an adaptation of particle imaging velocimetry. To accommodate the animals, it was necessary to use bubbles as seed particles and sunlight for illumination. Three trained adult California sea lions were guided to swim through an approximately planar sheet of bubbles in a total of 173 repetitions. The captured videos were used to calculate bubble velocities, which were processed to isolate and inspect the flow velocities caused by the swimming sea lion. The methodology will be discussed, and measured flow velocities will be presented.