On the use of Gurney Flaps for the aerodynamic performance augmentation of Darrieus wind turbines

Abstract

Gurney Flaps (GFs) can enhance the aerodynamic performance of airfoils, making them generate more lift and delaying the onset of stall. Since their potential was discovered in the early ‘70 s, GFs have been applied in several fields, including wind turbines. Here, the research has been focused mostly on the use of GFs in Horizontal Axis Wind Turbines (HAWTs), whereas a lack of studies involving the application of these devices on Darrieus Vertical-Axis Wind Turbines (VAWTs) is apparent in the literature. The benefits induced by GFs could actually be particularly interesting for this type of wind turbines, which are presently receiving a renewed attention from the industry.In the present work, an extended numerical analysis using Computational Fluid Dynamics (CFD) was carried out with the aim of evaluating the potential of using Gurney Flaps for the power augmentation of Darrieus wind turbines.After a validation of the numerical approach using wind tunnel experimental data on a static airfoil, the simulations have assessed the impact of different GF mounting and height on board airfoils moving in the cycloidal motion typical of Darrieus wind turbines. The results on a single rotating airfoil allowed the analysis to highlight the physical phenomena taking place past the rotating blades, including the delay of stall and the modifications induced on the surrounding flow field; power enhancements higher than 20% were shown for some configurations. Then, impact of GFs on a real three-blade turbine was analyzed. The best configuration resulted in a 2%c GF installed in the inner side of the airfoil, so to have a better torque extraction in the downwind half of the revolution. The GF benefits were apparent especially at lower tip-speed ratios, suggesting its use both for newly-designed turbines and even as a retrofitting solution in existing rotors.