Effect of Amplitude and Mean Angle of Attack on the Unsteady Surface Pressure of a Pitching Airfoil

Details of pressure distributions on a two dimensional airfoil oscillating inpitch through the sta ll in a 0.8 x 0.8 m2 lout-speed wind tunnel are presented.Pitching occurred around the airfoils quarter-chord axis. Pitch rate, Reynolds number, and oscillation amplitudes were uaried to determine th eeffects on pressure and lift distributions . It was found that the mean angle of attack and pitching amplitude had stronq effects on the flow field hence pressure distrilnition in the immediate vicinity of the airfoil leading edge , x/c _0.4 For pressure ports located at x/c 0.4 , the aforemention ed effects were not sironq . It seems that during the oscillatory motions th e flow was mostly separated. This investigation shows weak unsteady effects when the maximum dynamic angle ofattack was below that of the static stall; i.e. alpha(max). dynamic - alpha (max).static. for higher angles of attack, strong unsteady effects appear, which depend on the mean angle of attack, frequency and amplitude of the oscillation. Dynamic stall and dynamic reattachment contribute to a favorable effect of unsteadiness on thesurface pressure signature and hence the mean lift coefficient which increases as compared to the steady state lift and pressure data.