Study on the thrust generated by a thin plate imitating fish’s caudal fin

Abstract

Purpose of this study is to investigate the relation between the shape of a thin plate and thrust force. The thin plate is what imitates the fish’s caudal fin. We adopt an equilateral triangular, square, regular pentagon and hexagon as the shape of the plate. There are two ways of yielding thrust of fish; 1) drag-type as seen in flatfish and cod, and 2) lift-type as seen in tuna and shark. We focus on the drag-type fin, and consider reasons of different thrust with different shape of the fin. We simulate the reciprocal motion with some frequency of the plate as the fish tail at both situations in water being at rest and under a uniform flow. We measure the force acting on the plate directly, and consider the generating mechanism of the thrust due to the motion of the plate. To estimate force acting on the plate, we introduce the momentum consideration and the circulation of vortices due to the impulsive pressure of an edge of the plate starting suddenly. We discuss the relation between the shape of the plate and force due to the accelerating and decelerating motion of the plate in this paper. The most powerful shape of the fin is the triangular for an impulsive start. The spin parameter has to exceed 0.84 to produce the thrust force in the flow.

Keywords

• thrust
• plate
• shape
• fish’s fin
• vortex
• circulation