Numerical investigation on evolutionary characteristics of the leading-edge vortex induced by flapping caudal fin

The caudal fin of a fish is one of the main determinants of various maneuvering motions. In this paper, the evolutionary characteristics of the leading-edge vortex (LEV) induced by three kinds of forked caudal fins with different chord lengths are studied. Numerical results show that the emergence and development of the LEV are directly related to the distribution law of the angle of attack (AoA) caused by the leading-edge configuration. However, when adopting a fixed motion mode, any temporal evolution in the AoA of the leading-edge location is determined by the distance between the leading-edge location and the pitching axis, the combined effects of heaving and pitching motions, and the Strouhal number. An increase in the chord length enhances the strength of the LEV, leading to more vortex-augmented thrust. Nonetheless, the chord length of a forked caudal fin cannot be extended indefinitely as this will alter the temporal evolution of the AoA and seriously delay the generation of the LEV. Our research is helpful in understanding how the locomotor force is derived from the motion of the caudal fin and provides a reference for biomimetic roboticists to choose appropriate propellers for underwater vehicles.