Computational investigations on hydrodynamic performance of active and passive tails for carangiform swimmers

Abstract

Fish achieve efficient propulsion through their unique capabilities to undulate their bodies for locomotion, and carangiform swimmers are often associated with producing thrust from their rear through their caudal fin. The undulatory kinematics of the caudal fin closely resemble flapping-like behavior composed of heaving and pitching. In this study, we investigate the interaction between the caudal fin and the rest of the swimmer’s body through numerical simulations of fluid–structure interaction, segregated into a Two-dimensional and a Three-dimensional study. In the first part, the body and tail are modeled with NACA=0015 and NACA-0012, respectively, at two Reynolds Numbers, 500 and 5000, corresponding to a small and a large fish. The passive pitching of the tail is modeled with a linear torsional spring and a damper at its peduncle. This analysis reveals that a swimmer with a passive pitching tail does not necessarily produce higher thrust compared to its active counterpart but can achieve higher power savings. It also shows that smaller fishes can benefit from a passively pitching tail, whereas larger fishes require an actively pitching tail for optimal performance. In the second part, we extend the work to a three dimensional analysis with an actual model of a Jackfish, where we separate the caudal fin from the swimmer and examine the performance of passively pitching versus active pitching. Here, the passive pitching is modeled with a nonlinear spring to incorporate the stabilizing effect at larger amplitudes, and the study is conducted at a Reynolds Number of 3000. We find that the nonlinear spring enables larger amplitude pitching and that the passively pitching tail produces higher thrust than the actively pitching tail. Overall, the study shows that a nonlinear spring is essential for enforcing large amplitude pitching of the caudal fin which benefits the swimmer to generate higher thrust, while with a linear spring and smaller pitching amplitudes, locomotion involves a trade-off where the swimmer prioritizes thrust production or power savings.