Abstract
The high-speed and efficient swimming characteristics of tuna are valuable for designing bio-inspired underwater vehicles. Tuna use their highly deformable caudal fins as propulsors during swimming. Caudal fin deformation is categorized into skeletal-controlled active deformation and fluid-induced flexible passive deformation. To investigate how flexible passive deformation affects propulsion performance, simulations of four caudal fins with varying flexibilities under two St numbers in a uniform flow are conducted using the finite volume method. This study finds that the medium-flexibility caudal fin achieves a higher time-averaged thrust coefficient without sacrificing efficiency under both high and low St numbers. At a high St number, the medium-flexibility caudal fin enhances thrust by reducing detrimental secondary flows. At a low St number, the medium-flexibility caudal fin increases thrust by strengthening vortex rings, which induces a stronger backward jet.