Acoustic intensity measurements in the presence of low Mach number turbulent flow

Abstract

Acoustic intensity measurements using the two‐sensor cross‐spectral density technique are susceptible to errors when made in the presence of mean flow. These errors are predominantly due to turbulent pressure fluctuations acting on and near the sensors. To quantify these errors, intensity measurements were performed in a sound field that contained both an independent acoustic source and a mean, low Mach number (M<0.1), turbulent boundary‐layer flow. The microphones were flushed mounted in the wall supporting the turbulent boundary layer and the acoustic source was positioned a large distance downstream. The streamwise component of intensity was measured with and without the flow keeping the sound output of the source constant. Associated bias errors were calculated in accordance with a theoretically established bias error [G. C. Lauchle, Noise Control Eng. J. 23, 52–59 (1984)]. As predicted theoretically, the calculated bias error decreases rapidly as the sensor separation distance increases relative to the correlation length associated with the turbulent boundary‐layer pressure fluctuations. [Work supported by various U.S. Navy projects at NSWC and ARL Penn State.]