Experimental Study of a Naca 4412 Airfoil With Movable Gurney Flap

Abstract

A NACA 4412 airfoil was tested, in a boundary layer wind tunnel, with the aim to study the effect of a Gurney mini-flap, as an active and passive flow control device submitted to a turbulent flow field. The main objective was the experimental determination of flow pattern characteristics downstream the airfoil in the near wake. The untwisted wing model used for the experiments had 80cm wingspan and 50cm chord, with airfoil NACA 4412. Three different movable Gurney mini-flap mechanisms were tested, as active devices. The miniflap, in all cases, was located on the lower surface at a distance, from the trailing edge, of 8%c (c airfoil chord). The Reynolds number, based upon the wing chord and the mean free stream velocity was 326,000 and 489,000. The turbulence intensity was 1.8%. The models were located into the wing tunnel between two panels, in order to assure a close approximation to two-dimensional flow over the model. One was a movable up-down miniflap geared by an electromagnetic mechanism; from the analysis of the obtained results one could appreciate an increment of Cl when the excitation frequency increases, in comparison with the clean airfoil. Also is observed that the Cl values of the model with the Gurney miniflap fixed are something greater than the corresponding values for the movable Gurney condition. Regarding the Cd behavior, it diminishes when the frequency increases, but its minimum value is something greater than the case for the clean airfoil. The others were rotating mini-flaps, geared by an electro-mechanical system, one rotate to a 90° (vertical) position, and the other rotate to a 30° position. In these cases the wake pattern and pressure values near the trailing edge were measured. The results obtained, for these mechanism, show us that the oscillating mini-flap change the wake flow pattern, alleviating the near wake turbulence and enhancing the vortex pair near the trailing edge at the mini-flap level and below that level, magnifying the effect described first by Liebeck. That effect is more evident as the oscillating frequency grows. Additionally, the wake alleviation probably affects also the far wake. All of these facts suggest us to continue with the experiments, trying to measure the pressure distribution around the airfoil in all the cases, obtaining the lift and drag characteristics.