Flapping wings and aerodynamic lift: The role of leading-edge vortices

The role of leading-edge vortex (LEV) in enhancing aerodynamic lift during flapping flight is discussed. The LEV is generated from the balance between the pressure gradient, the centrifugal force, and the Coriolis force in the momentum equation. The LEV generates a lower pressure area, which results in a large suction on the upper surface. The LEV's main characteristics and the implications on lift generation change as the Reynolds number varies. The LEV can enhance lift by attaching a bounded vortex core to the upper leading edge during wing translation. The LEV needs to maintain a high axial flow velocity in the core and remains stable along the spanwise direction to be effective in enhancing lift. The higher Reynolds number yields much more pronounced axial flow at the core of the LEV, which together with the LEV forms a helical flow structure near the leading edge.