Bio-robotic model as a scientific tool for experimentally investigating hydrodynamic functions of fish caudal fin

Bio-robotic models become increasingly important for understanding biological system in field such as biomechanics. Fish caudal fin is a prominent example of biological propulsion, in which the caudal peduncle, fin ray and fin membrane together form a dynamic locomotory system. In this paper, we developed a bio-robotic model to mimic the fin ray 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. We found that the addition of three-dimensional fin kinematics significantly enhanced the lift force without deceasing thrust force compared with the no fin motion. The vortex wake directs water both axially and vertically and forms jet like structure with notable wake velocity. According to the bio-robotic model experimental data, we hypothesized that fish may actively control the caudal fin rays to achieve considerable lift force when swimming at low speed, however, negative at high speed.