Unsteady Flow Field in the Wake of an Airfoil

An experimental investigation was carried out to study the unsteady flow field behind a sinusoidally pitching airfoil at reduced frequencies 0.091, 0.182 and 0.273. Streamwise instantaneous velocity and streamwise mean velocity in the wake were measured by hot-wire anemometer at downstream distance from trailing edge of 0.25 chord. To assess the effect of mean incidence, data were taken at mean incidence angles of 0, 2.5 and 5 degrees. In all cases, the oscillation amplitude was set at 8 degree, and the Reynolds number, based on the chord length, was 50000. It is found that the mean incidence angle and reduced frequency have important influences on the instantaneous velocity profiles. As reduced frequency increases, more uniformity and stability in the wake are observed. When mean incidence angle is larger, more variations on thickness of profiles and velocity defect are found. Furthermore, mean velocity profiles and estimations of the momentum deficit coefficients are obtained. The results show that with increasing mean incidence, the velocity defect and momentum deficit increase. Instead, the higher reduced frequency decreases the momentum deficit and wake thickness.