Aerodynamic techniques to mitigate the 3D loss in the power coefficient of vertical axis wind turbines

The three-dimensional tip loss effect is a critical phenomenon affecting the performance of vertical -axis wind turbines. The vortices on the blades become more complex with the turbine rotation, most notable at the tips, leading to three-dimensional aerodynamic losses and decreasing the turbine power coefficient. Therefore, mitigating the aerodynamic losses at the tips could improve the technological development of these devices. However, the numerical methods to analyze the tip vortices in VAWTs have a high computational cost, and experimental data to verify them is limited. In this study, three passive flow control techniques that mitigate the three-dimensional effects of a vertical axis wind turbine Darrieus H -type have been investigated experimentally in a wind tunnel to quantify their impact on the power coefficient and validate the theoretical results found in the literature. The tested passive flow control aerodynamic designs of the blade tips consist of leadingedge serration, anhedral, and endplate techniques. The results of this work include the experimental power coefficient curves as a function of the tip speed ratio for a two -blade VAWT model without any modification and with the three passive control techniques. The experimental process characterizes the VAWT techniques in a range of Tip Speed Ratios between 0.37 and 3.1. Our results indicate that the leading -edge serration, anhedral, and endplate techniques effectively mitigate the threedimensional losses, increasing the power coefficient by 7.1%, 14.3%, and 25%, respectively, compared to the reference model. The endplate model exhibited the best Cp improvement. However, it could cause the highest bending on the blade tips of the three tested techniques.