Fluid Mechanics Analysis of the Sweep Motion of Tuna Median Fins

Study has shown that tuna can improve hydrodynamic performance and turning maneuverability by rapidly and accurately changing the shape of its median fins. In this paper, the shape change of tuna median fins is simplified as the sweep motion. The fluid mechanics analysis of the sweep motion is carried out from the aspects of rectilinear cruise and turning maneuver. The results show that the sweep motion of the median fins plays a different role in rectilinear cruise and turning maneuver. When in a rectilinear cruise, tuna should increase the sweep angle to reduce the drag force. For turning maneuver, a larger sweep angle of the median fins can provide more positive turning torque to assist tuna to steer efficiently in the early and middle stage of turning process, while a smaller sweep angle should be taken in the later turning period.