Comparison of experimental and numerical work on three dimensional trailing edge modifications on airfoils

Miniflaps at the trailing edge of airfoils (i.e., Gurney flaps) change the Kutta condition and thus produce higher lift. Unfortunately, however, the drag also increases due to the flow separation downstream of such modified trailing edges. The present work describes investigations aimed at the stabilization of the wake flow in order to achieve drag reduction and a decrease of sound generation. Using hot wire velocimetry, the existence of a tonal component could be observed in the broadband spectrum of the velocity fluctuations downstream of the Gurney flap. This points to the occurrence of a von Kárman vortex street (i.e., an absolute instability). This phenomenon persists even for turbulent boundary layers on the airfoil. Geometrical modifications of the Gurney flaps can reduce or eliminate the instability of the flow and can thus reduce or eliminate the existence of the tonal component of the velocity fluctuations of the wake. The effectiveness of trailing edge modifications like slits in Gurney flaps was investigated experimentally. These experiments were carried out with two-dimensional airfoils. The chord Reynolds number of these experiments was Re = 1×10. At lower angles of attack where the instabilities did occur, the geometrical modifications produced an appreciable drag reduction. This drag reduction was determined directly by force measurements using a wind tunnel balance. For a better understanding of the underlying flow physics, the unsteady flow structures in the wake of an airfoil with Gurney flap were additionally investigated by numerical simulations. These simulations were based on Reynolds-averaged Navier-Stokes methods (URANS) and Detached Eddy Simulation (DES) methods. The results from the simulation confirmed the existence of a two-dimensional vortex street. The application of Gurney-flaps with slits, however, led to more complex three-dimensional flow structures coupled with lower lift fluctuations and reduced drag.