Unsteady Aerodynamic Loads on an Airfoil at High Angle of Attack in a Randomly Surging Flow

An unsteady surging flow with a von Karman turbulence spectrum is used to simulate atmospheric turbulence in a wind tunnel. The aerodynamic forces and moments produced by an NACA-0009 airfoil at α = 12° and α = 15° in the unsteady freestream flow are documented and compared to the steady flow performance. Lift, drag, and pitching moment are recorded, as well as, surface pressures on the suction surface of the airfoil. High-speed particle image velocimetry is used to measure the velocity field on the suction surface of the airfoil under steady and unsteady flow conditions. Correlations between the overall flow field and reference signals of freestream velocity and surface pressure are computed as functions of frequency using the cross power spectral density functions. Spatial maps of the correlations show the receptivity of the leading edge shear layer and the wake to specific frequencies in the freestream disturbances. The suction parameter is computed from the flow field velocity data and correlated with the leading edge flow separation. The critical suction parameter for separation is found to be larger than the suction parameter value that occurs when the flow is fully separated.