Space-time correlations of airfoil noise

Abstract

Space-time correlation measurements of surface pressure fluctuations on airfoils, immersed in a cold air jet, with their farfield sound were made over the exit velocity range of 68–190 m/sec (Mach 0.2–0.6). Measurements of surface pressure fluctuations were taken both at the one-third chord and the trailing edge of a 1-in. chord airfoil. The position of the farfield microphone was varied throughout the hemispherical region downstream of the jet exit. Analysis of the cross-correlation and delay time measurements on a 1-in. chord airfoil indicate that the dominant source region for the radiated sound is near the trailing edge. However, for the probe microphone mounted 1 in. from the leading edge of a second, 3-in. chord airfoil, two correlation peaks were recorded. The first corresponds to direct radiation from the probe location, and the second is due to acoustic transmission upstream from the trailing edge. The normalized cross-correlation coefficients were in a ratio of 1:5, respectively. This indicates that the sound from midchord is down 14 dB from the trailing edge sound; theory would indicate even less midchord sound. [Work partially supported by NASA and the TRW Foundation.]