Numerical study of leading-edge vortices on the flapping wing of a penguin at high Reynolds number

Previous studies on flapping-wing flight of animals and small unmanned aircraft at low Re (Reynolds number) revealed that a large lift can be generated at a large angle of attack due to the stable attachment of a leading-edge vortex (LEV) on the wing, that is so-called delayed stall. At high Re under water, however, vortex structures around the flapping wings are not fully understood yet. In this study, we perform high-fidelity simulations on a multi-GPU supercomputer to investigate LEV dynamics around the flapping wing of a penguin at high Re. Our computational model combines the cumulant lattice Boltzmann method (LBM) with the direct-forcing immersed boundary method (IBM), utilizing adaptive mesh refinement (AMR) method to optimize memory usage and ensure computational efficiency. The analysis focuses specifically on how the feathering angle influences vortex topology and the resulting hydrodynamic forces. Results indicate that even at high Re, a stable LEV forms at high angles of attack, inducing delayed stall. However, this vortex is observed to burst after the wing's midstroke. We demonstrate that variations in the feathering angle significantly alter flow separation, which in turn affects the drag forces acting on the wing. These findings provide a deeper understanding of high-Re flapping wing hydrodynamics, offering insights for the design of bio-inspired underwater and aerial vehicles.