The three-dimensional hydrodynamics of thunniform swimming under self-propulsion

This paper presents the hydrodynamics of thunniform swimming under self-propulsion as the body and caudal fin kinematic behaviors are systematically varied. The swimmer is a three-dimensional (3D) tuna-like flexible model with prescribed kinematics of thunniform mode. Simulations are carried out for various body behaviors by varying the wave frequency f and the tail amplitude Ap and for various caudal fin behaviors by varying the maximum angle of attack αmax and the phase difference φ. The results show that the swimming velocity as well as the propulsive efficiency is an increasing function of both f and Ap, but their trend with αmax and φ is found to be not monotonic. Specially, for peak swimming velocity generation, αmax occurs at 20-30°, and φ ranges from 60° to 75°, while for peak efficiency, αmax is 20-30° and φ nears 90°. The wake structure formed by the swimmer is single row wake consisting of a series of disconnected vortex rings that are resembled by sickle-like shape vortices. As φ increases, the tail of the sickle-shape vortices is shortening gradually while the head is stretching. As αmax increases, the aligned sickle-like vortices have almost identical shape with each other, but the wake strength is reducing gradually.