Role of the near-tip region of a fin in fish propulsion

Abstract

Fish combine a wide variety of passive and active control mechanisms to generate thrust. Muscle actuation is used by fish in order to dynamically alter the shape or the stiffness of different parts of the body, to ultimately control propulsion. We uncover the effects of locally altering the tip region of a flapping fin system, by using a low-order robotic model. We show how the actuation of this region can largely modify propulsion. Detailed forces and torque measurements and a low-order eigenmode reconstruction of the flow around the flapping fin show the effects of varying the phase difference between the tip region and the body of the fin on the propulsive performance. Kinematics that force the trailing edge to move behind the main body of the fin are beneficial in terms of efficiency and thrust generation. For a very small range of phase differences, large thrusts can be combined with large side forces that could be used by fish to increase maneuverability.

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