Hydrodynamics of a Flexible Soft-Rayed Caudal Fin

The paper addresses hydrodynamic performance of a slender swimmer furnished with a flexible small-aspect-ratio soft-rayed caudal fin. The recoil of the fin is found by solving the coupled hydro-elastic problem, in which the structure of the fin is modeled by a cantilever of variable cross section and the hydrodynamic forces acting on it are modeled using the elongated body theory. It is shown that the recoil has practically no effect on the propulsion efficiency of anguilliform swimmers, but has a profound effect on the efficiency of carangiform swimmers, which can increase almost four-fold between low-speed (low-thrust) cruise and high-speed (high-thrust) burst. Whilst the magnitude of this effect furnishes a plausible argument in favor of burst-and-coast locomotion strategies, it also infers that carangiform swimmers cannot rely on elastic recoil of the caudal fin to be efficient throughout the usable speed range, and must actively flex it at low speeds.