Numerical study on the hydrodynamic performance of an unconstrained carangiform swimmer

Undulations are ubiquitous in natural swimmer propulsion, propelling in omni-direction. In the present work, the hydrodynamic performance of an unconstrained carangiform swimmer in the absence of a free stream is numerically investigated at different Reynolds numbers. Propulsive speed is found to increase with an increase in undulatory frequency, wavelength, and Reynolds number. The passive lateral oscillating amplitude is closely related to the wavelength and kinematic viscosity, but insensitive to the undulatory frequency. The propulsive mechanisms for the variation of the propulsive speed are revealed by the vortical structures and the time-averaged velocity field. Scaling relationships are investigated, and we found that the effects of the wavelength on the propulsive performance cannot be neglected at high Reynolds number, i.e., Reref≥1000, and the scaling relationship between the flapping Reynolds number and the propulsive Reynolds number is refined with the wavelength adopted as the characteristic length, which generalizes the previous scaling law proposed by Gazzola et al. [“Scaling macroscopic aquatic locomotion,” Nat. Phys. 10, 758–761 (2014).] In addition, the scaling relationships related to the power consumption, the cost of transport, the Strouhal number, and the passive lateral oscillating amplitude are revealed. These results are crucial in furthering our understanding of carangiform's self-propulsion and will aid the development of advanced bio-inspired propulsors.