Performance enhancement of a straight-bladed vertical axis wind turbine utilizing a wedge flap

Abstract

Urban areas are major contributors to carbon emissions. Vertical axis wind turbines, such as Darrieus turbines, can be effectively used in urban conditions to provide renewable energy, thereby reducing dependence on fossil fuels. However, the low power coefficient (CP) of this type of turbine has been identified as a main drawback that limits its application. Many researchers have proposed various airfoil modifications aimed at enhancing aerodynamic characteristics and limiting unsteady interactions with the atmospheric boundary layer. The current study evaluated the potential of adding a wedge flap (WF) at the trailing edge (TE) of the NACA 0021 airfoil numerically and experimentally using a wind tunnel. The effect of different WF configurations—on one side and two sides, as well as variations in height, angle, and optimal length-to-height (L/H)—on the aerodynamic performance and flow over the airfoil was studied systematically using two-dimensional computational fluid dynamics (CFD) simulation. Subsequently, the rotor blades were manufactured using a three-dimensional printer, considering the optimal WF dimensions derived from CFD simulation. The effect of adding the WF was then tested experimentally in a small wind tunnel at different wind speeds. The results indicate that the addition of the WF can effectively suppress flow separation and enhance aerodynamic efficiency across all studied cases compared to the clean airfoil. The aerodynamic performance is significantly influenced by the height of the WF, while the length at L/H < 1 has a relatively minor effect. The inclined WF achieved the highest lift and lift-to-drag ratios, with total maximum increments of 71.67% and 45.79%, respectively, at optimal height and length of 6%c and 1%c, respectively, compared to the clean airfoil case. The experimental results show that adding the WF at the TE of the blades can substantially improve the CP at different wind speeds. A WF appears to be an effective passive flow control device that can be used in new wind turbines or retrofitted to existing ones, provided its dimensions are properly selected.