Experimental and Numerical Investigation of Flapping Wing Propulsion and its Application for Boundary Layer Control

A thrust generating flapping airfoil adds energy to the flow and therefore can be used as a means for active boundary layer control if positioned close to a profile’s surface. This effect is investigated with a small flapping airfoil placed in the flow at varying distances above a flat plate. The experimental data are based on flow visualization using dye injection and Laser Doppler Velocimetry data from a water tunnel. Vortical structures and time averaged velocity profiles are compared with numerical computations from an existing two-dimensional Navier Stokes solver and show good agreement. Even asymmetric wake structures which develop above a critical reduced frequency are obtained numerically. It is found that the boundary layer can be eccelerated both upstream and downstream of the airfoil and that there exists a beneficial ground interference effect if the airfoil is placed close to the surface. Furthermore it is shown that the wake pattern along the wall is dominated by clockwise vortices at low reduced frequencies and counterclockwise vortices at high reduced frequencies. Finally a stationary profile with trailing edge separation is used in the water tunnel and it is demonstrated that flow reattachment can be obtained applying a small flapping airfoil downstream. Parametric studies are performed to determine the influence of plunge frequency, amplitude, airfoil size and airfoil location and considerations to find the optimum parameters are presented.