Vortex dynamics and hydrodynamic performance enhancement mechanism in batoid fish oscillatory swimming

The effects of chordwise deformation and the half-amplitude asymmetry on the hydrodynamic performance and vortex dynamics of batoid fish have been numerically investigated, in which the two parameters were represented by the wavenumber ( $W$ ) and the ratio of the half-amplitude above the longitudinal axis to that below ( $HAR$ ). Fin kinematics were prescribed based on biological data. Simulations were conducted using the immersed boundary method. It was found that moderate chordwise deformation enhances the thrust, saves the power and increases the efficiency. A large $HAR$ can also increase thrust performance. By using the derivative-moment transformation theory at several subdomains to capture the local vortical structures and a force decomposition, it was shown that, at high Strouhal numbers ( $St$ ), the tip vortex is the main source of thrust, whereas the leading-edge vortex (LEV) and trailing-edge vortex weaken the thrust generation. However, at lower $St$ , the LEV would enhance the thrust. The least deformation ( $W=0$ ) leads to the largest effective angle of attack, and thus the strongest vortices. However, moderate deformation ( $W=0.4$ ) has an optimal balance between the performance enhancement and the opposite effect of different local structures. The performance enhancement of $HAR$ was also due to the increase of the vortical contributions. This work provides a new insight into the role of vortices and the force enhancement mechanism in aquatic swimming.