A study on the hydrodynamic performance of manta rays swimming in staggered arranged group

Abstract

To investigate the hydrodynamic performance of manta rays swimming in staggered arranged group, a morphological and kinematic model of manta rays are developed based on biological observations, and then establishes a numerical calculation method for group swimming of manta rays based on the Immersed Boundary Method and the Sphere function-based Gas Kinetic Scheme (IB-SGKS), and systematically researches the group swimming of two manta rays with a fixed vertical spacing of 0.1 times the body thickness, and a flow direction spacing of 0-1.5 times the body length. The average thrust/efficiency of the group system and each individual in the group are analyzed by combining the global 3D vortex structure and the characteristic cross-section 2D vortex structure. The numerical results show that: when the streamwise spacing between individuals is small, the propulsive performance decreases sharply compared to swimming alone; as the streamwise spacing increases, the propulsive performance of the leader manta ray is consistently better than that of swimming alone, with the maximum thrust enhancement up to 11.24% when <i>Dx</i>=0.4<i>BL</i>, and the maximum efficiency enhancement up to 3.58% when <i>Dx</i>=0.3<i>BL</i>; with the increase of the streamwise spacing, the thrust/efficiency curves of the follower manta ray appears volatility, with the maximum thrust enhancement of 48.14% when <i>Dx</i>=0.4<i>BL</i> and the maximum efficiency enhancement of 12.39% when <i>Dx</i>=0.5<i>BL</i>; the system average thrust and efficiency enhancement are both maximum when <i>Dx</i>=0.4<i>BL</i>, which are 29.69% and 6.77%, respectively, which is because the tail vortex of the leader manta ray happens to cross over the leading edge of the follower manta ray and acts directly on the initial shedding fin tip vortex of follower manta rays, substantially increasing their vortex energy.