The effects of caudal fin deformation on the hydrodynamics of thunniform swimming under self-propulsion

Abstract

To investigate the effects of the caudal fin deformation on the hydrodynamic performance of the self-propelled thunniform swimming, we perform fluid-body interaction simulations for a tuna-like swimmer with thunniform kinematics. The 3-D vortices are visualized to reveal the role of the leading-edge vortex (LEV) in the thrust generation. By comparing the swimming velocity of the swimmer with different caudal fin flexure amplitudes fa, it is shown that the acceleration in the starting stage of the swimmer increases with the increase of fa, but its cruising velocity decreases. The results indicate that the caudal fin deformation is beneficial to the fast start but not to the fast cruising of the swimmer. During the entire swimming process, the undulation amplitudes of the lateral velocity and the yawing angular velocity decrease as fa increases. It is found that the formation of an attached LEV on the caudal fin is responsible for generating the low-pressure region on the surface of the caudal fin, which contributes to the thrust. Furthermore, the caudal fin deformation can delay the LEV shedding from the caudal fin, extending the duration of the low pressure on the caudal fin, which will cause the caudal fin to generate a drag-type force over a time period in one swimming cycle and reduce the cruising speed of the swimmer.