DEMONSTRATION BY PIV OF THE NON-REPRODUCIBILITY OF THE FLOW-FIELD AROUND AN AIRFOIL PITCHING UNDER DEEP DYNAMIC STALL CONDITIONS AND CONSEQUENCES THEREOF

The unsteady flow field above a NACA 0012 airfoil pitching under deep dynamic stall conditions has been investigated in a low-speed wind tun nel by means of Particle Image Velocimetry (PIV). The comparison of th e velocity vector fields obtained for the same instantaneous airfoil i ncidence in successive oscillation cycles has shown a strong effect of non-reproducibility when the flow field is fully separated from a poi nt close to the maximal incidence and during the phase of decreasing i ncidences. The consequences on the validity of the ''phase averaging'' method widely used for pointwise measurements (e.g. Laser Doppler Ane mometry (LDA), pressure, hot wires) in unsteady flow fields assumed to be periodic are analysed using LDA results obtained on an OA 312 pitc hing airfoil. The meaning which should be given to the mean and fluctu ating quantities derived from this method are then discussed.