The effect of blade geometry on the structure of vertical axis wind turbine wakes

Vertical axis wind turbines (VAWT) may offer operational and design advantages over their horizontal axis counterparts, but the occurrence of dynamic stall and the resulting cyclical stresses present engineering and operational challenges. We present the results of an experimental study of the effects of blade sweep on the wake characteristics of VAWTs performed using high resolution particle image velocimetry. Several sweep configurations were tested while holding the turbine solidity constant. As the sweep angle was increased dynamic stall was reduced, which could yield a potential reduction in the amplitude of cyclical torque variations. Increased sweep also resulted in a more uniform wake exhibiting a faster wake recovery, and an increase in the planform energy flux in the turbine wake when compared to the straight-bladed turbine. The results support previous suggestions that the power generation of VAWT farms is bounded by the planform energy flux from above.