COMBINED NUMERICAL AND EXPERIMENTAL STUDY ON THE USE OF GURNEY FLAPS FOR THE PERFORMANCE ENHANCEMENT OF NACA0021 AIRFOIL IN STATIC AND DYNAMIC CONDITIONS

The use of power augmentation devices in wind energy applications has been receiving increasing interest from both the scientific and the industrial community. In particular, Gurney Flaps (GFs) showed a great potential thanks to the passive functioning, the simple construction and the possibility to add them as a retrofit to existing rotors. The possibility of having a high quality set of airfoil data for a wide range of both angle of attack (AoA) and Reynolds number is pivotal in the design phase of newly developed machines. Such data are usually available in the technical literature for smooth airfoils, while there is a lack of generalized results in case of airfoils with GFs. Within this context, the authors have performed an extended investigation on the lift increase capabilities of GFs for the well-known NACA 0021 airfoil, which has been used in several wind energy applications up to now. The present paper shows the results of a combined experimental and numerical analysis considering different geometrical configurations of the flaps under both static and dynamic conditions. Experimental data were first obtained for the AoA range of 180 degrees at a low value of Reynolds number (i.e. Re=180 k) to analyze the impact of three different geometrical configurations of the GF on the aerodynamic behavior for the full range of incidence angles. The geometrical configurations were defined by varying the length of the flap (i.e. 1.4% and 2.5% of the chord length) and its inclination angle with respect to the blade chord (i.e. 90 degrees and 45 degrees). The experimental investigation involved also dynamic sinusoidal pitching movements at multiple reduced frequencies to evaluate the stall hysteresis cycle. Subsequently, an unsteady CFD numerical model was first calibrated against wind tunnel data at low values of the Reynolds number. Then, the virtual model was exploited to extend the investigation to a wider range of Reynolds number for dynamic AoA rates of change typical for example of vertical-axis wind turbines, i.e. characterized by higher reduced frequencies with a non-sinusoidal motion law.