Dynamic ground effects on undulatory motion in a stingray-like robotic fin.

Rays and skates have been observed with highly distinctive swimming strategies ranging from pure flapping to pure undulation. Traditional experimental studies of undulatory motions relied on an intricate series of servo motors, making it difficult to systematically investigate the wavelength and amplitude of the fin. To build on those previous studies, we designed a robotic fin that generates easily-customizable undulation with a single motor and tunable cam-shaft system. Our apparatus is capable of testing different permutations of amplitude, wavelength, and frequency of the fin undulation, and it's compatible with various planforms via interchangeable skins. To quantify hydrodynamic performance, we measured the forces produced as the fin swam in a recirculating water tunnel. Specifically, streamwise thrust and transverse lift were measured by bi-axial load cells, and energy consumption was measured by a torque sensor on the actuator. Building on our previous works, we plan to use our fin to study how planform, aspect ratio, and undulation strategy affect near-ground swimming performance. With a better understanding of the hydrodynamics that governs near-ground swimming, we hope to better explain ray fin morphology and improve bio-inspired underwater vehicle design.