Mean Loading and Turbulence Scale Effects on the Surface Pressure Fluctuations Occurring on a NACA 0015 Airfoil Immersed in Grid Generated Turbulence

Abstract

Detailed surface pressure measurements have been made on a NACA 0015 immersed in two grid generated homogenous flows at Re = 1.17 x 10⁶ for a = 0°, 4°, 8°, 12°, 16°, and 20°. The goal of this measurement was to reveal and highlight mean loading and turbulence scale effects on surface pressure fluctuations resulting from turbulence/airfoil interaction. Also, measurements are compared with the theory of Amiet (1976a,b). The surface pressure response shows a dependance on angle of attack, the nature of which is related to the relative chord/turbulence scale. The dependance on turbulence scale appears to be non-monotonic at low reduced frequencies, wr = Pi*f*c/U with both an increase and decrease in unsteady pressure magnitude occurring with increasing mean load. A reduced frequency overlap region exists at wr > 10 where the two different scale flows begin to produce similar effects on the surface pressure with increasing angle of attack manifesting as a rise in unsteady surface pressure magnitude. Also, the interaction of the full 3-dimensional wavenumber spectrum affects the distance over which pressure fluctuations correlate and the extent of correlation is affected by angle of attack as demonstrated in the chordwise and spanwise pressure correlation. Amiet's theory is shown to agree favorably with measurements in the leading edge region although demonstrates insufficiencies in predicting unsteady pressure phasing.