Hydrodynamic function of a robotic fish caudal fin: effect of kinematics and flow speed

Fish caudal fin is a prominent example of biological propulsion, in which the caudal peduncle, fin rays and fin membrane together form a dynamic locomotory system. In this paper, we developed a bio-robotic model to mimic the caudal fin structure and kinematics of Bluegill Sunfish (Lepomis macrochirus). We coupled controlled oscillations in both heave and pitch directions to the robot to model the caudal peduncle motion of swimming fishes. Synchronized multi-axis force transducer and particle image velocimetry were then used to quantify the hydrodynamic forces and wake flow in the vertical plane. We found that changing the flow speed and the phase between the caudal fin ray and peduncle locomotion resulted in significantly different lift and thrust force. DPIV results showed that the vortex jet angle changed with flow speed and phase phi. According to current experiment, we hypothesized that the fish caudal fin may function as a "flexible vectoring propeller", and may be responsible for the three-dimensional maneuverability of fishes.