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

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.

Automated summary

The flow around a tethered model of a swimming batoid fish is studied using wall-modelled large-eddy simulation and the immersed boundary method. High Reynolds numbers up to 148,000 are chosen and hairpin vortical (HV) structures near the body surface are observed for the first time. The HVs are mainly formed through two mechanisms and create strong fluctuations in the pressure distribution and frequency spectrum.