Hydrodynamics of a swimming batoid fish at Reynolds numbers up to 148 000

Abstract

Flow around a tethered model of a swimming batoid fish is studied by using the wall-modelled large-eddy simulation in conjunction with the immersed boundary method. A Reynolds number ( $Re$ ) up to 148 000 is chosen, and it is comparable to that of a medium-sized aquatic animal in cruising swimming state. At such a high $Re$ , we provide, to the best of our knowledge, the first evidence of hairpin vortical (HV) structures near the body surface using three-dimensional high-fidelity flow field data. It is observed that such small-scale vortical structures are mainly formed through two mechanisms: the leading-edge vortex (LEV)–secondary filament–HV and LEV–HV transformations in different regions. The HVs create strong fluctuations in the pressure distribution and frequency spectrum. Simulations are also conducted at $Re=1480$ and 14 800 to reveal the effect of Reynolds number. Variations of the flow separation behaviour and local pressure with $Re$ are presented. Our results indicate that low- $Re$ simulations are meaningful when the focus is on the force variation tendency, whereas high- $Re$ simulations are needed when concerning flow fluctuations and turbulence mechanisms.

Propulsive performance of a heaving and pitching foil with large amplitudes in unsteady ground effect

“…However, many flows are accompanied by larger foil fluctuations. For example, the tip-tip excursion of Rhinoptera bonasus (cownose ray), a typical benthic zone feeder, is approximately 1.2 times the average pectoral fin width (Rosenberger 2001, Zhang andHuang 2023). Therefore, in this work, we mainly aim to explore the ground effect on the propulsive performance of heaving and pitching foils with large amplitudes.…”