Ground effects on oscillatory and undulatory batoid-inspired fins

Experiments and simulations were employed to study the propulsion of batoid-inspired fins near the ground, focusing on the effect of wavenumber. Three wavenumbers were tested, ranging from oscillation (wavenumber<1) to undulation (wavenumber>1). Unlike 2D hydrofoils, which produce both suction and repulsive forces in the lateral direction, the 3D fins here produced only suction forces. These suction forces were most prominent at low wavenumbers, low ground distances, and high Strouhal numbers. Using inviscid simulations, we determined that the suction force resulted from the dominance of negative quasi-steady lift over positive wake-induced lift, with the added-mass lift staying equal to zero. Thrust generation and power consumption also increased with Strouhal number and decreased with wavenumber, but they were not as susceptible to ground effects as lift. Three-dimensional flow measurements demonstrated that fins employing oscillatory motion generated stronger trailing-edge vortices and exhibited wider wakes due to higher wave speeds, making them more sensitive to ground presence as compared to their undulatory counterparts. Finally, we analyzed the efficiency, instantaneous lift, and angle of attack of the fins to provide insights into batoid-like propulsion of biological and engineering systems.