Measurement of boundary-layer pressure fluctuations at low wave number on a smooth wall

Abstract

Measurements of the spectral density of the wall-pressure fluctuations at low wave number on a smooth wall are presented. The data were obtained with a linear array of six microphones mounted flush in the wall of a wind tunnel specifically designed to have very low background noise levels. The outputs from 4,5, and 6 microphones in the array were combined in a manner suggested by Maidanik and Jorgensen [“Boundary Wave-vector Filters for the study of the Pressure Field in a Turbulent Boundary Layer,” J. Acoust. Soc. Am. 42, 494–501 (1967)] to provide a direct measure of the spectral density as a function of wave number and frequency. Data were obtained at two test locations for a number of flow speeds ranging from 30 ft/sec (9.1 m/sec) to 160 ft/sec (48.8 m/sec) and for frequencies up to 20 kHz. The two test locations allowed the boundary layer displacement thickness to be varied independent of flow speed. A dynamic range greater than 70 dB was obtained at each speed for the magnitude of the spectral density. The magnitude of the spectral density at low wave number is presented in nondimensional form for a range of Strouhal numbers.