Numerical simulation of the self-propelled swimming performances and mechanisms of a biomimetic robotic fish with undulating fins under different fin waveforms

To study the self-propelled swimming performances and mechanisms of biomimetic robotic fish with undulating fins (BRFUF) under different waveforms, a numerical simulation system coupled with body dynamics and fluid dynamics was established to study the starting, accelerating, and cruising processes of a biomimetic robotic fish in a median/paired fin swimming mode. A systematic parametric study was carried out on the swimming performance of a BRFUF under the cooperative propulsion of two fins, and the mechanism of thrust generation and the influence mechanisms of waveform and kinematic parameters of fins on swimming performance were analyzed based on the hydrodynamic performance, surface pressure distribution, vortex dynamics, and longitudinal velocity iso-surface of the flow field. The results showed that a larger fin ray oscillation angle amplitude increased the acceleration and cruising velocity of the BRFUF from the static state to the cruising stage. A highly concentrated vortex generated at the trough of the fin creates a jet mass that generates a reactive (added-mass) force perpendicular to the propulsive element, which is the mechanism by which the high pressure always covers the trough of the fin. Driven by the flexible fluctuations of the fins, the high-pressure region continuously moves toward the trailing edge along with the vortex. Along with the generation and shedding of the vortex, the high-pressure region is constantly generated, moving and disappearing on the surface of the fin and providing continuous thrust for BRFUF self-propulsion.