The mechanism of rapid starting of slender fish 1

Abstract

The mechanism of rapid acceleration from rest (lunging) in fishes is studied by means of a slender-body theory adapted to deal with unsteady, curvilinear large amplitude movements in water at high Reynolds numbers. The vortex wake is represented here by the circulation shed from the fins and body, calculated by means of indicial aerodynamic theory.

The caudal fin is shown to play a dominant role in the production of the thrust force required for the tremendous accelerations measured in various species of fish (50 msec⁻² for pike and 40 msec⁻² for trout). The theoretical analysis also shows that an optimal value of the ratio of thrust produced to energy expenditure is obtained when the caudal fin moves perpendicularly to the direction of movement of the fish’s centre of gravity, while being at a small angle of incidence.

Filmed sequences of accelerating fish were examined. showing that for species with good lunging ability, the starting process is made up of three stages. First the fish, which was initially stretched straight curls up into an L-shaped curve. At the end of this, the tail swings around and moves in the manner predicted by the theory. This second stage is repeated in some cases and finally the fish straightens out moving at an angle to its original orientation, roughly proportional to the acceleration.

The forces and moments on the fish calculated by the present method are in good agreement with these experimental observations.