Investigation of the unsteady flow velocity field above an airfoil pitching under deep dynamic stall conditions

The unsteady flow field above a NACA 0012 airfoil pitching under deep dynamic stall conditions has been investigated in a low-speed wind tunnel by means of particle image velocimetry. The measurements of the instantaneous flow velocity field show the characteristic features of the dynamic stall process: formation and development of an organized vortex structure for increasing incidences and the subsequent separation. Vorticity and divergence estimated from the measured data give a good insight into the complex flow behaviour during the downstroke motion. Furthermore, small-scale structures could be observed in the separated flow field and even within the dynamic stall vortex.The authors would like to thank Dr. Schäfer (ISL) for his support in organizing the cooperative measurements, Mr. Seyb (DLR) for his help during the recording of PIV images, Dr. Bretthauer (DLR) and Mr. Vollmers (DLR) for his assistance during the phase of evaluation and post processing of the PIV recordings and Dr. Geißler (DLR) for helpful discussions on the dynamic stall problem.