Numerical study on the hydrodynamics of tuna morphing median fins during C-turn behaviors

Abstract

Tuna median fins, including the first dorsal, second dorsal and anal fins, are capable of changing their area and shape. Because morphing fins can adjust the hydrodynamic forces, they have the additional ability to affect the yaw and turning maneuvers. Based on the kinematic equations of C-turn behavior and the numerical method of self-propelled swimming, the hydrodynamic analysis of the two tunas with erected and depressed median fins are carried out in this paper. The results show that the morphology of the median fins has a great influence on hydrodynamics and flow field structure during C-turn behaviors. When the median fins are erected, the hydrodynamic forces acting on the tuna are increased, and the first dorsal fin contributes the most to the hydrodynamic increments. The overall vortex strength in the erected morphology is also greater due to the participation of the first dorsal fin. Compared with the depressed morphology, the erection of median fins can increase the yaw angle and reduce the turning radius to a certain extent, which means that the tuna with erected fins has higher turning maneuverability. Moreover, as the initial speed increases, the erection of median fins is more helpful to turning maneuvers.