Ground effect aerodynamics of a flapping airfoil in hover

Aerodynamics and flow physics of a 2D flapping airfoil hovering in ground effect is investigated numerically by solving the unsteady incompressible laminar Navier-Stokes equations using the finite volume method. The pressure coefficient distribution on the airfoil, the aerodynamic forces, and the detailed vortex structures in the flow field for a flapping airfoil hovering in ground effect are provided in this paper for the first time in the literature. It is shown that the ground effect of a 2D flapping airfoil in hover can be divided into two regimes: the transition regime for H / C ≥ 2.5 where the lift decreases, and the ground effect regime for H / C < 2.5 where the lift increases; H is the height above the ground and C is the chord length. In the transition regime, the ground restricts the descending reverse vortex, which first affects the newly shed vortex and then the vortex near the airfoil. In the ground effect regime, the ground directly affects the vortex near the airfoil.