Numerical study on the turn maneuvering of a biomimetic robotic fish driven by pectoral fins in labriform mode under self-propulsion

The median and/or paired fin (MPF) swimming mode of fish has extremely strong maneuverability, which is urgently needed for unmanned underwater vehicles. Therefore, determining the mechanism of greater maneuverability of fish in the MPF swimming mode is particularly important. To fill the research gap in the entire turn maneuvering process in MPF swimming mode under self-propulsion, a numerical solution method for three degree-of-freedoms self-propelled swimming of biomimetic robotic fish (BRF) coupled with fluid dynamics and body dynamics were established. Our results revealed that the turning radius of the BRF increases with the increase in the pectoral fin rotation amplitude in both drag-based and lift-based modes. Interestingly, owing to the special streamlined shape of the fish body, it can passively generate thrust during turn maneuvering. According to vortex dynamics, the trailing-edge vortex (TEV) and tip vortex (TV) generated in the power stroke form a vortex ring together with the TEV generated in the recovery stroke during one cycle in drag-based mode. The TEV and TV generated in every half cycle in lift-based mode form a vortex ring, resulting in two vortex rings in one cycle. The vortex ring generation mechanism is the mechanism by which the pectoral fins cannot generate continuous thrust in drag-based mode but can generate continuous thrust in the lift-based mode. The results reveal the BRF labriform mode turning characteristics as well as the relation mechanism between vortex dynamics and thrust, which lays a theoretical foundation for highly maneuverable BRF development.