Inhibited swimming capacity of fish entrained in wake vortices behind a semi-cylinder

The loss of fish swimming capacity induced by those energetic vortical structures is detrimental, and limited research on the mechanism of fish entrainment in the vortex has been reported. The present study investigated the entrainment of a fish in vortices generated in wake of a semi-cylinder. The swimming fish was modeled using an undulatory NACA0012 airfoil behind semi-cylinders with a diameter D from 0.1 to 2.4 times the fish body length (L). It was found that the fish couldn’t get higher propulsion from the low momentum flow for D > 0.8L. The wake of the fish was distorted by the shedding vortices and trailing-edge vortices (TEVs) arose at fishtail. The length- and time-scale of TEVs were positive correlated with these of oncoming co-directional vortices. The uncontrollable hydrodynamics induced by TEVs could elucidate the disruption of fish stability by surrounding vortices, manifested as a loss of propulsion, the generation of a unidirectional torque, and increased power consumption. The fish head gradually got energy from the turbulent flow of oncoming vortices, but fails to compensate adverse effects of the TEVs. The recovery of swimming capacity occurred only in the less disturbed flow between the wake vortices.