Aerodynamic Coefficients of a Symmetrical Airfoil in an Oscillating Flow

A symmetrical airfoil at fixed angle of attack from 0 to 35 degrees was placed in an air flow with controlled velocity oscillations at 10 Hz. The instantaneous pressure distribution and flow velocity were measured simultaneously. Pressure information was integrated numerically to determine instantaneous values for lift, drag and moment about the quarter chord. The results were plotted as three-dimensional surface plots in angle of attack, cycle time and performance coefficient axes, or with chordwise position, cycle time and pressure coefficient axes, as appropriate. The pressure distribution over the airfoil was shown to be primarily a function of freestream static pressure. Aerodynamic coefficients as a function of angle of attack and time exhibited seemingly random, erratic behavior, whereas the mean values of the same coefficients were similar in nature to those obtained in steady flow. The data indicate that the instantaneous performance of an airfoil in an oscillating flow is not merely a continuum of many quasi-steady states.