Hydrodynamics of the self-diving function of thunniform swimmer relying on switching the caudal fin shape

This article introduces a novel self-diving movement that relies on the morphological variation of caudal fin. We use the method of decomposing the computational domain and solving Navier–Stokes equations in parallel to simulate the unsteady flow field effectively. The overlapping grid technology in ANSYS Fluent is applied to manipulate the bionic tuna’s overall translation and rotation based on the fin shape variation. The entire self-diving behavior can be divided into three phases: deflection, transition, and cruise. The results show that the tuna with spanwise flexible caudal fin can generate a deflection moment, then rely on the transition phase to complete the switch of the tail shape, and finally simulate a stable cruise in the forward-downward direction. The number of spanwise swings D and the spanwise phase difference δ in the deflection phase are essential parameters affecting the self-diving performance. We can combine these two variables to obtain different hydrodynamic conditions and judge the effect of diving through relevant evaluation parameters. The corresponding pressure clouds and vortex profiles of the tuna surface can indirectly reveal the evolution process of fin morphological transformation and the dynamic performance of self-diving.