Drag Reduction on Gurney Flaps by Three-Dimensional Modifications

Miniflaps at the trailing edge of airfoils, that is, Gurney flaps, change the Kutta condition and thereby produce higher lift. Unfortunately, because of the flow separation downstream of such trailing edges, the drag also increases. Investigations are described with the aim to stabilize the wake flow to achieve drag reduction. When hot-wire anemometry is used, a tonal component in the spectrum of the velocity fluctuations downstream of the Gurney flap is shown. This points to the existence of a von K´arm´an vortex street. Modifications of the Gurney flap can reduce this flow instability, which results in a drag reduction. Trailing-edge modifications, such as slits or holes in Gurney flaps and vortex generators, were tested in experiments. The experiments were carried out using straight wings and a swept wing at a Re = 1 × 10^6 At lower angles of attack of the airfoils with geometrical modifications a drag reduction was observed. This drag reduction was determined through force measurements. The flowfield behind the Gurney flaps was also investigated numerically, using methods based on Reynolds averaged Navier–Stokes and detached eddy simulation. The results confirm the existence of a two-dimensional vortex street as well as drag reduction by introducing slits in the Gurney flaps.