Propulsion characteristics of a bionic manta ray considering flexible pectoral fin-free surface interaction

Manta ray has become the focus of bionic fish research because of its excellent motion performance in the complex ocean environment. Most studies focus on the influence of the deep-water environment on its motor performance. However, the free surface nonlinear effect has a great influence on the motion performance of the manta ray, which is rarely mentioned in previous studies. In this study, the Reynolds-averaged Navier–Stokes method, volume of fluid method, overset mesh, and deformable mesh technology were used to study the bionic manta ray at different conditions. The characteristics of propulsive performance, vortex structure, and wave structure under the influence of the free surface were compared emphatically. The results show that the main range of free surface effect is that the immersion depth d is less than the motion amplitude A. Within this range, as the pectoral fins flutter, a unique wave-making is generated, which involves interference by three groups of waves: two sets generated by the tips of the pectoral fins and one trailing wave. With a large amount of air involved, the wave causes a large amount of energy dissipation, resulting in a net thrust attenuation of 35.1% and a propulsion efficiency reduction of 11.3%, especially at high Fr, leading to reduced speed for the manta rays.