Flow visualization and PIV measurement of flow field around a darrieus rotor in dynamic stall

The flow field around a Darrieus rotor in dynamic stall is studied by flow visualization and PIV measurements. The visualization is carried out by dye injection technique while the phase averaged velocity distributions around the blade are measured by PIV combined with a conditional imaging technique. The results indicate the appearance of dynamic stall phenomenon due to the shedding of two pairs of vortices from the blade during one rotation of the rotor. These stall vortices are produced by the separation of flow over the inner surface of the blade and the formation of roll-up vortices from the outer surface. The second stall vortices develop along the blade and strongly interact with the flow field near the blade, affecting the aerodynamic performance of the rotor.