The Gurney Flap (GF) consists of a flat profile, which is attached perpendicular to the pressure side of the airfoil trailing edge. The crucial geometrical characteristic is the GF-height relative to the chord-length (%c). Compared to other passive flow control devices, such as vortex generators or trailing edge serrations, GF change the Kutta-condition and increase the circulation of a given airfoil [1]. For typical GFs in the range of 1%c ≤ GF-height ≤ 2%c, the lift-increase is accompanied by a detrimental drag penalty. That is why previous research of the authors suggests the use of rather small GF-heights of 0.5%c or less, in order to achieve the lift-enhancing effect while maintaining, or even improving, the airfoil L/D-ratio for Re-numbers between one and two million [2].

This research-project investigates the aerodynamic effect of so-called Mini-GFs with a height lesser than 1%c on three different airfoils which are relevant to the blade design of Horizontal Axis Wind Turbines (HAWT) [3]: the DU97W300, the AH93W174 and the NACA 633618. All experiments were conducted in the 2.0m x 1.4m test-section of the large, closed-loop wind tunnel of the Technical University of Berlin, reaching airfoil-based Re-numbers of up to 1.5 million. Lift and drag forces were measured by means of a six-component force-balance at a static AoA-range of -5° to +20°, i.e. including stall onset. Furthermore, both free and forced transition cases were considered. In the latter case Zig Zag tape with a height of 0.4mm was applied close to the leading edge of both suction and pressure side, hence emulating the effects of surface erosion on rotor blades. Subsequently, three different Mini-GFs were attached at heights of 1%c, 0.5%c and 0.3%c.

The experimental results confirm that the aerodynamic effects depend on the relation between GF-height and boundary layer thickness at the pressure side of the trailing edge. Hence, the optimal GF-height of the ZZ-configuration is found to be slightly higher due to the expanding boundary layer in case of early transition. In general, it could be shown that the overall L/D-ratio improves with decreasing GF-heights (≤ 0.5%c) and that the early stall-behavior remains similar. In addition, XFOIL simulations of both the clean and the ZZ-configuration indicate the optimum GF-height to be close to the displacement thickness, i.e. in the range of ⅓ to ½ of the calculated boundary layer height.

Next, the measured airfoil data was fed into a standard BEM performance simulation-tool, in order to investigate the impact of Mini-GFs on the blade-design of the NREL 5MW Reference Turbine [4]. Results show that using Mini-GF as a simple add-on (retrofit application), they evoke an increase in both axial and tangential rotor-induction, essentially lowering the angles-of-attack (AoA) at the blade. This effect is promising for the root region, i.e. up to a blade-length of approximately 50%, where high AoA persist due to structural constraints, as such mitigating the adverse effects of early separation and dynamic loads. Furthermore, considering Mini-GFs already in the blade-design process (design application), they allow for chord-length reductions throughout most of the blade length due to increased lift-coefficients, hence contributing to the development of more slender blades.

Conclusively, a complete blade-optimization strategy of the NREL reference blade is presented by combining the beneficial effects of the retrofit application in the first half, with the design application throughout the second half of the blade span.

[1] Liebeck, AIAA 1976-406, p.547-561 (1978)

[2] Alber et al., ASME -Turbo Expo, GT2017-64475 (2017)

[3] Pechlivanoglou, Ph.D thesis, p.45, Technical University of Berlin, Germany (2013)

[4] Jonkman et al., Technical Report NREL/TP-500-38060 (2009)