Boundary Layer Control for a Vertical Axis Wind Turbine Using a Secondary-Flow Path System

Vertical axis wind turbines (VAWTs) have typically lower efficiency compared to their horizontal counterparts (HAWTs), but are attractive for places where taller structures are prohibited, as well as for regions where available wind speeds are lower. For HAWTs, the blades are always perpendicular to the incoming wind, providing a continuous thrust throughout the rotation. Contrary to HAWTs, VAWTs have advancing blades and retreating blades, where blades backtrack against the wind, causing lower efficiency. Hence, any modifications that can be made to improve the efficiency of VAWTs can be beneficial to the wind industry. Passive flow control permits the airfoil geometry to be modified by means of grooves or slots without requiring heavy mechanisms or actuators. Hence, this form of boundary layer control seems advantageous for wind turbines, so that minimal amount of maintenance is required, while complexity of the turbine is not significantly increased. Such modification changes the boundary layer over an airfoil reducing flow separation and reversed flow. This study introduces a new form of passive flow control: Secondary-flow control system, which works on the principle of mass removal, eliminating flow separation at different apparent angles of attack in a VAWT. CFD analysis is used to investigate passive flow control for the airfoils NACA8H12 and LS0417 in a three-bladed VAWT configuration. A secondary flow path is initially designed and optimized in a single airfoil configuration, and then used to adjust the wind turbine blade design. The effects of secondary-flow control system in a VAWT design configuration are investigated by comparison with the non-modified airfoil design. The CFD results indicate that secondary-flow path system can be used to modify and control the boundary layer for a wind turbine. It is believed that secondary-flow control system incorporated in VAWT design has potential for improving turbine efficiency. Further research should be conducted to optimize the secondary-flow path system according to the shape of the airfoil in a 3D VAWT configuration, so that blades interference can be captured.