Freestream disturbance effects on an airfoil pitching at constant rate

Effects of freestream disturbances on the dynamic stall process of an NACA 0012 airfoil undergoing ramp-type pitching motion from 0-30 deg were studied using measurements of unsteady surface pressures. A thin circular cylinder was located upstream of the test airfoil and was offset vertically between -0.133 and 0.133 chord lengths with respect to the airfoil pitching axis to provide the freestream disturbances. The measurements were carried out at a constant chord Reynolds number of 8 x 10(4) for a relatively low reduced pitch-rate range k = 0.01-0.04. As compared with the undisturbed flow case, the imposed freestream disturbances were able to produce a significant increase in absolute magnitude of the peak suction pressure near the leading edge, resulting in a noticeable phase delay in stall process. The growth of the leading-edge suction peak caused by insertion of the upstream rod was found to be sensitive to its vertical offset position. Nevertheless, the pressure variation caused by the freestream disturbances was appreciable only in the forward 20% chord from the leading edge, bringing a lower effect on the airfoil lift and momentum coefficients