Shape matters in near-ground oscillatory and undulatory swimming*

Previously we presented a robotic platform for studying the thrust and efficiency of a flexible fin while varying ground distance, frequency, and fin wavenumber (oscillatory vs. undulatory). We found that the fin experiences a lateral suction force that increases with Strouhal number, but decreases with wavenumber and ground proximity. Thrust generation and power consumption follow the same trend and are not susceptible to ground effects. In this study, we test how different fin shape contributes to performance, when coupled with ground distance, frequency, and fin wavenumber. We measure the force response and power consumption on a rectangular (control), triangular (manta ray), and elliptical (bluespotted ribbontail ray) fin. Consistent with previous studies, we observe that ground effect has strong influence on the lateral suction force; it is most prominent when operating with the rectangular fin at low wavenumber, low ground distance, and high Strouhal number. Thrust generation and power consumption are not as heavily influenced by the ground than lift. Finally, we measure the three-dimensional wake structures using multi-layer stereo PIV to explain the observed performance differences and simulate 3D undulatory fins to better understand the lateral suction force. This study advances our understanding of fin shapes in oscillatory and undulatory swimming, offering valuable insights into stingray locomotion and the design of bio-inspired robotic systems.