Hydrodynamics of morphology for thunniform swimmers: Effects of the posterior body shape

Morphology of posterior body for thunniform swimmers plays an important role in determining the propulsion performance. Recent literature mainly focused on the effects of thunniform swimmers specialized features, e.g., finlets, crescent caudal fin, etc. However, it is unknown to what extent posterior body morphological change is optimized for hydrodynamic performance. In this paper, by varying the height and width of posterior body, numerical simulations of thunniform swimming at cruising state have been carried out. To account for the influence of fluid–structure interaction on the hydrodynamic performance, a sharp-interface immersed boundary method (IBM) is utilized to solve the incompressible viscous flow. As the width or height increases, stronger PBVs (posterior body vortices) are generated alongside the posterior body. Then the constructive interaction between PBVs and LEVs (vortex generated on the leading edge of caudal fin) could enhance the caudal fin thrust significantly. This thrust enhancement mechanism is more evident with increasing height of the posterior body. The pressure difference between two sides of posterior body resulting from PBVs helps diminish the trunk drag, which is more evident as width is decreased. Subsequently, a systematic study of the Reynolds number (Re) and Strouhal number (St) effects has been conducted to quantify and evaluate the hydrodynamic performance with various posterior body shapes. Our work can help understand tuna fish propulsion mechanism due to posterior body morphology, based on which engineered bionic swimmers can reach excellent swimming performance by optimizing the trunk shape.