Abstract
Flow near and around a Darrieus wind turbine operating at low tip-speed ratios is studied by flow visualization with dye injection technique and also by measuring the velocity field with a particle imaging and conditional sampling technique. The instantaneous flow patterns observed in the visualization pictures are well reproduced in the measured velocity distributions, vorticity con-tours and maps of velocity magnitude. It is found that two pairs of mashroom-type vortices are shed from the blade during a rotation of the rotor, which is due to the dynamic stall phenomena. These stall vortices are produced by the flow separation over the suction side of the blade and the formation of roll-up vortices from the pressure side.
