Visualization and Image Analysis of Dynamic Stall Phenomenon for a Darrieus Wind Turbine.

The flow field around a Darrieus wind turbine is studied by flow visualization and PIV measurement in a rotating frame of reference to understand the unsteady nature of dynamic stall appearing at low tip-speed ratios. The qualitative nature of the dynamic stall observed by the flow visualization using dye injection technique is quantitatively reproduced in the instantaneous velocity distributions around the blade measured by PIV technique. These results indicate that two pairs of stall vortices are generated in one cycle of the turbine rotation and they grow in size as the tip-speed ratio decreases. The mechanism of the dynamic stall is found to be due to the flow separation over the suction side of the blade followed by the generation of in-flow motion from the pressure side to the suction side of the blade through the trailing edge.