Visualization of Unsteady Flow Over Oscillating Airfoils

This study is concerned primarily with the complex nature of leading edge flow separation occurring on airfoils oscillating in a uniform flow field at low Reynolds numbers. The flow field past an oscillating airfoil and a fixed airfoil with an oscillating flap were investigated using various visualization techniques in water. The role of mean flow velocity, instantaneous angle of attack, mean angle of attack and amplitude and frequency of oscillation as well as the location of the support point are examined. The results which were obtained over a range of parameters substantially beyond previous studies include new information regarding the effect of these parameters on the nature and onset of separation. Two basic forms of leading edge separation have been observed. At low values of reduced frequency (ω* \u3e 0.5) the separation resembles leading edge separation on stationary airfoils with the separated flow remaining detached from the upper surface. At higher values, beyond ω*cr, a strong vortex (roller) is formed at the leading edge with the flow reattaching downstream from it. In addition to these two flow regimes, a third regime made its appearance at extremely high values of frequency and amplitude where the flow around the airfoil has collapsed. The frequency of oscillation is found to govern the angle, αcr, at which leading edge separation occurs at low reduced frequencies (ω* \u3c 0.2); at higher frequencies αcr occurs at the portion of the cycle where d2 α/dt2 is near its maximum