Numerical analysis of hydrodynamics characteristics for autonomous swimming of bionic tunas

A two-dimensional computational model for the bionic tuna is established according the structure and swimming characteristics of tuna fishes and a UDF program for controlling the swing of the fish body is coded. The low speed cruise state and C shape rapid start behavior of the bionic fish in the process of autonomous swimming are numerically simulated by using the computational fluid dynamics software Fluent with the dynamic mesh technique. The effects of the swing frequency and swing mode of bionic fishes on the distributions of pressure, velocity and vorticity around their bodies are investigated, and the stress state of their bodies is also analyzed. The corresponding hydrodynamics characteristics of their autonomous swimming are revealed. The results show that, the bionic fish swings its body to produce an high pressure area and a low pressure area on its both sides and to obtain the propulsive force from the positive pressure gradient in the flow field in cruise stage and that it produces the propulsive force and accomplishes a rapid start through fast and large backswings in the C shape rapid start stage. The study presented in this paper will provide an important reference for the conceptual design and performance optimization of bionic underwater propulsions.