Abstract. This wind tunnel study investigates the aerodynamic effects of mini Gurney flaps (MGFs) and their combination with vortex generators (VGs) on the performance of airfoils and wind turbine rotor blades. VGs are installed on the suction side aiming at stall delay and increased maximum lift. MGFs are thin angle profiles that are attached at the trailing edge in order to increase lift at pre-stall operation. The implementation of both these passive flow control devices is accompanied by a certain drag penalty. The wind tunnel tests are conducted at the Hermann-Föttinger Institut of the Technische Universität Berlin based on two airfoils that are characteristic of different sections of large rotor blades. Lift and drag are determined using a force balance and a wake rake, respectively, for static angles of attack between −5 and 17∘ at a Reynolds number of 1.5 million. The impact of different MGF heights including 0.25 %, 0.5 % and 1.0 % and a VG height of 1.1 % of the chord length is tested and evaluated. Furthermore, the clean and the tripped baseline cases are considered. In the latter, leading-edge transition is forced with Zig Zag (ZZ) turbulator tape. The preferred configurations are the smallest MGF on the NACA63(3)618 and the medium-sized MGF combined with VGs on the DU97W300. Next, the experimental lift and drag polar data are imported into the software QBlade in order to design a generic rotor blade. The blade performance is simulated with and without the add-ons by means of two case studies. In the first case, the retrofit application on an existing blade mitigates the adverse effects of the ZZ tape. Stall is delayed and the aerodynamic efficiency is partly recovered leading to an improvement of the power curve. In the second case, the new design application allows for the design of a more slender blade while maintaining the rotor power. This alternative blade appears to be more resistant against the adverse effects of forced leading-edge transition.

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Power augmentation devices in wind energy applications have 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. 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 Reynolds number of 180 k to analyze the impact of three different geometrical configurations of the GF on the aerodynamic behavior. The geometrical configurations were defined by varying the length of the flap (1.4% and 2.5% of the chord) and its inclination angle with respect to the blade chord (90 degrees and 45 degrees). The experimental investigation involved also dynamic sinusoidal pitching movements at multiple reduced frequencies to evaluate the stall hysteresis cycle. An unsteady CFD numerical model was calibrated against wind tunnel data and then exploited to extend the investigation to a wider range of Reynolds numbers for dynamic AoA rates of change typical of vertical-axis wind turbines, i.e. characterized by higher reduced frequencies with a non-sinusoidal motion law.

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