Oscillatory and undulatory swimming of a stingray-inspired platform*

Different stingray species have evolved different swimming modes depending on how closely they live to the sea floor, i.e., substrate. Oscillatory rays (fin wavenumber < 1) tend to live far from the substrate (pelagic), while undulatory rays (fin wavenumber > 1) tend to live near the substrate (benthic). The hydrodynamic differences between oscillatory and undulatory stingray locomotion have not previously been studied using a single platform. Here, we study the hydrodynamic performance of both swimming modes using a stingray-inspired robotic platform that prescribes wavenumber using a modular rotary cam-train system. Near-ground swimming tests reveal that both swimming types produce a frequency-dependent lateral suction force that is inversely related to the wavenumber. Therefore, the oscillatory mode requires a higher positive angle-of-attack to maintain level swimming comparing with the undulatory mode. At matched frequencies, oscillatory swimming outperforms undulatory swimming in both thrust generation and efficiency, regardless of substrate proximity. Finally, we measure the three-dimensional wake structures using multi-layer stereo PIV to explain the observed performance differences. This study advances our understanding of oscillatory and undulatory swimming, offering valuable insights into stingray locomotion and the design of bio-inspired robotic systems.