Effects of cupped deformation on hydrodynamic performance of oscillating caudal fin

Fish precisely control their trunks and fins to accomplish complex swimming maneuvers such as cruising, hovering, steering, and launching, making them more flexible and efficient than conventional artificial vehicles. In Body and Caudal Fin (BCF) mode, the caudal fin can generate large thrust with high efficiency, which plays an important role in the propulsion and maneuvering process. Therefore, bionic caudal fin motion and deformation have significance for the efficient navigation of underwater vehicles. This paper conducted using the commercial CFD software Fluent, investigates the impact of cupped deformation on the hydrodynamics of caudal fin. The emphasis of the study is on discussing how the cupped phase difference affects the hydrodynamic performance of the caudal fin under different Reynolds numbers (Re) and Strouhal numbers (St). The results indicate that appropriately cupped phase difference can result in both higher caudal fin thrust and efficiency compared to a rigid caudal fin, which suggests explicitly that cupped deformation facilitates efficient swimming. Compared to a rigid caudal fin, the cupped caudal fin can simultaneously increase thrust and efficiency by 11.09% and 4.63% respectively. Moreover, the difference between the two cupped phase differences which has the most positive and negative effect on increasing caudal fin thrust, is approximately pi radians. The analysis of the caudal fin flow field reveals that vortex rings are generated alternately from the upper and lower surfaces of the caudal fin during caudal fin oscillation, and the cupped shape mainly affects the leading edge vortex generation and the caudal vortex shape. The stronger wake vortex generated by cupped deformation is advantageous for thrust generation. However, cupped deformation is not conducive to efficiency improvement under high Reynolds numbers.