Splitter-plate (c)

The first concept for damping the flow structures is to prohibit motion between the top and bottom end of the flap by introducing a splitter-plate. The plate is mounted perpendicular to the center of the Gurney-flap and its length is similar to the flap height $h/c=1\%$. A similar concept has already been successfully applied to cylinder flows [9]. Such kinds of flows are characterized by vortex shedding with a single dominant frequency which is able to transfer a large amount of energy from the mean flow and feed it into dissipation processes giving rise to increased drag. The splitter-plate however, should separate the upper and lower shear layer and avoid the momentum flux between them. Numerical simulations at $Re=50,000$ could clearly reproduce this effect for the flow around a cylinder. A plate with a length of one cylinder diameter caused a significant change in the flow-structures of the near wake and the drag could be decreased by $37\%$ [20].

In the present computations of an airfoil with Gurney-flap and splitter-plate the mean lift coefficient slightly decreases compared to the airfoil with a standard Gurney. At the same time the induced drag could be reduced by $36\%$.

Lift fluctuations and the flow structures remain almost undamped, but occur slightly downstream. The main difference between a cylinder flow and the present application is that in the case of a Gurney-flap the separation points on both ends of the flap are fixed whereas in the cylinder flow separation strongly oscillates. Consequently, one effect of the splitter-plate is to damp the fluctuations due to movements of the separation point.