Observations of dynamic stall on Darrieus wind turbine blades

Flow field around a Darrieus wind turbine blade in dynamic stall is studied by flow visualization and particle image velocimetry (PIV) measurement in stationary and rotating frames of reference. The experiment is carried out using the small-scale Darrieus wind turbine in a water tunnel. The unsteady nature of the dynamic stall observed by the flow visualization is quantita tively reproduced in the instantaneous velocity distributions by PIV measur ement, which describes the successive shedding of two pairs of stall vortic es from the blade moving upstream. The mechanism of dynamic stall is due to the successive generation of separation on the inner surface of the blade followed by the formation of roll-up vortices from the outer surface. Altho ugh the qualitative nature of the dynamic stall is independent of the tip-s peed ratios, the blade angle for stall appearance and the growth rate of th e stall vortices are influenced by the change in tip-speed ratios. (C) 2001 Elsevier Science Ltd. All rights reserved.