Flow-induced dorso-ventral deformation enhances propulsive efficiency in flexible caudal fins

Fish swim with flexible fins that stand in stark contrast to the rigid propulsors of engineered vehicles. Using numerical simulations of the dynamics of flow-structure interaction, we have found that dorso-ventral deformation in flexible caudal fins results in a 70% increase in efficiency of caudal fin swimmers compared to a rigid fin generating the same amount of thrust. By correlating fin deformation to the flow physics, we find that the greater power requirements of rigid fins can be largely attributed to their propensity to generate high-magnitude lateral forces. In contrast, flexible fins achieve high efficiency local-redirection of force where deformations orient pressure forces on the fin in fore-aft and dorso-ventral directions to reduce the power demand of generating thrust forces. These deformations occur at phases in the tail-beat cycle where the fin experiences large lateral velocities and pressure differentials and this reduces the net power expended by the flexible fins. In this way, the flexibility of a caudal fin offers a simple and elegant solution for efficient locomotion which does not require sensing, computation and control that might otherwise be provided by the nervous system of a fish or a computer within a underwater vehicle. These flow-induced dorso-ventral fin deformations therefore imbue a mechanical intelligence in these fins that provides propulsive advantages to caudal fin swimmers and they also offer solutions for efficient propulsion in engineered systems.