Enhanced performance of airfoils at moderate mach numbers using zero-mass flux pulsed blowing

Oscillatory wall-jets were introduced through spanwise slots along a flapped NACA 0015 airfoil to establish lift augmentation and drag reduction by the unsteady forcing of the separated flow. Pressure coefficient distributions, lift coefficients, and wake velocity profiles, to determine the drag coefficient, were measured over the test-section speed range of 25m/s < U_∞ < 140m/s in the NDF. The present results demonstrated for the first time (Kites; 1997) the effectiveness of the oscillatory blowing technique as a separation control scheme at moderate Mach numbers, which exhibit compressibility effects. It is encouraging that lift-enhancement was observed over the entire range 0.1 < M < 0.4, even with the small amount of unsteady blowing applied in these experiments. As a result of the pulsed blowing, the lift coefficient increased by as much as 80%. Maximum pressure coefficients of nearly -5.0 for A/ = 0.4 experiments indicated the flow was supercritical near the leading edge of the airfoil, whereas it was not before the application of oscillatory blowing. The improvement in lift coefficient was found to be sensitive to the forcing frequency, even at the higher Mach numbers. Measurements at low angles of attack with a 20° flap showed that low amplitude pulsed blowing (0.02%) from the flap provided a 27% increasing in lift while steady blowing from the flap was detrimental to lift even at blowing coefficients as high as 3.5%. Oscillatory blowing with coefficients <C_μ> between 0.01% and 0.02%, based on RMS velocity, was shown to yield substantially better performance than steady blowing with C_μ in the range 0.5% to 3.5%. In is estimated that steady blowing of at least 10% would be required to reach the same levels of lift coefficient seen with the oscillatory blowing.