Abstract
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.
Highlights
In order to swim efficiently using body-and-caudal-fin (BCF) propulsion, the caudal fin has to flex in coordination with its lateral motion, turning left when moving right and vice versa
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 cruise and high-speed burst
Whilst the magnitude of this effect furnishes a plausible argument in favor of burst-and-coast locomotion strategies, it 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
Summary
In order to swim efficiently using body-and-caudal-fin (BCF) propulsion, the caudal fin has to flex in coordination with its lateral motion, turning left when moving right and vice versa. The caudal muscles are indisputably active during slow swimming [4,5], whether the flex of caudal fin is active or passive at all swimming speeds is still debatable [8]–furnishing an answer to this question is one of the objectives of this study. The long-term averaged performance can be expressed in terms of locomotion efficiency:the ratio between the energy needed to drag the swimmer between the beginning and end of the course at the average swimming speed, and the energy spent. It reflects the effective propulsion efficiency of the swimmer along the course. Comparable combination has been used in Ref. [11] for the study of a flexible slender propulsor
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