Hydrodynamic performance study of a self-propelled manta-like vehicle employing non-sinusoidal flapping

To fill the gap in the research on the self-propelled swimming process of underwater bionic vehicles with non-sinusoidal flapping, this study established non-sinusoidal flapping equations for the vehicle, and a deformation-kinematic coupling code was developed based on a user-defined function in the computational fluid dynamics software FLUENT. A two-degree-of-freedom self-propelled numerical strategy has been developed, and the effects of non-sinusoidal flapping on the kinematic characteristics, hydrodynamic performance, and vortex evolution mechanisms of bionic vehicles, along with their underlying mechanisms, have been revealed. The following conclusions are drawn: Compared to sinusoidal flapping, square-wave flapping significantly increases peak thrust, enhancing the vehicle's maneuverability. When the non-sinusoidal parameter K=1.3, the peak thrust increases by up to 99.3%. The sawtooth wave flapping enhances the vehicle's cruise efficiency. When K=−0.9, the cruising efficiency increases by 9.3%. The flapping waveform dominates thrust discrepancy by influencing the intensity and distribution of the shear layers. As swimming velocity increases, this mechanism gradually shifts to a combined effect of the flapping waveform and swimming velocity acting on the evolution of the leading-edge vortex, thereby affecting thrust. The discrepancy in energy dissipation power generated by flapping waveforms is also influenced by swimming velocity, shifting from a combined influence of the fin-tip vortex (TV) and trailing-edge vortex to the dominance of the TV. The above findings reveal the fluid mechanisms underlying non-sinusoidal flapping in vehicles and provide a reference for developing vehicle control strategies.