An experimental investigation was carried out to study the unsteady, near wakes behind an oscillating airfoil. The airfoil was given the harmonic pitching motion about the one-quarter chord axis. To assess the effect of reduced frequency and mean incidence, the reduced frequency was varied from 0.09 to 0.20 and the mean incidence of oscillation from zero to four degrees. The amplitude of oscillation was 7.4 degrees. When the mean incidence was four degrees, the airfoil was found to undergo a deep stall. Streamwise velocity and turbulence intensity profiles were measured by hot-wire anemometry, and are presented. The near wake measurements made it possible to detect and characterize the emergence of unsteady separation. It was found that the unsteady boundary-layer separation occurred at a greater phase angle (i.e., separation was delayed) when the reduced frequency was greater. When the mean incidence of oscillation was larger, the unsteady separation occurred at a smaller phase angle (i.e., separation was promoted). The linear relationship between the phase angle and the reduced frequency obtained in the present range of experiment assured us that the characteristic time scale of unsteady separation was the flow time scale given by the ratio of chord length to free stream velocity. Smoke-wire flow visualization was also effected to elucidate the link between some of the measured wake data and the various flow states around the airfoil. The phase lag of wake velocity profile relative to the motion of the trailing edge was estimated. The phase lag was found to increase linearly with downstream distance except for the region very near the trailing edge.
