The roles of fish median fins on the hydrodynamics and muscle actuation in carangiform swimming

Compared with swimmers with only caudal fins, swimmers with additional median fins, i.e. dorsal and anal fins can improve fish locomotive performance in cruising swimming. However, the effects of these fins on hydrodynamics, particularly on muscle actuation, remain poorly understood. In this study, we investigated four models (model i: with dorsal and anal fins; ii: without dorsal and anal fins; iii: with dorsal fin and without anal fin; iv: without dorsal fin and with anal fin) of cruising crucian carp using three-dimensional computational fluid dynamics (CFD) modeling. This study shows that both speed and thrust are improved in the presence of dorsal and anal fins, as the wake shed from these fins is intercepted by the caudal fin. Dorsal fin is discovered to increase the swimming efficiency, internal torque and the speed of the torque wave, whereas anal fin decreases these parameters. The adverse effects of these two fins are due to the difference in the phase of the forces generated on them and consequently affect the muscle actuation in the midbody and energy transmission around the peduncle. Our results provide new insight into the roles of the median fins in swimming performance and muscle actuation, which might benefit the design of fish-like robots.