Ground effect on tandem flapping wings hovering

The ground effect on tandem wings hovering in an inclined stroke plane is studied using immersed boundary projection method by solving two-dimensional incompressible Navier-Stokes equations. A virtual tandem flapping wing model performing idealized sinusoidal kinematics is employed in the study. The computations are carried out at Re=100 in a quiescent fluid at different heights from the ground. The phase relationship between the forewing and hindwing and its effect on the aerodynamic coefficients at various inter-wing distances are studied. Flow induced by the vortices rebound from the ground changes the effective angle of attack of the wings and influences the force generation. Complex wing-wing and wing-vortex interactions are observed. In-phase stroking generates the maximum vertical force, but produces a wake with a swirl. Counter-stroking generates a wake with predominant vertical velocity and increases the stability of the body in hovering.