This experimental study was carried out at a free-stream Mach number of 0.6 and a Reynolds number per foot of 3.45 x 106. The magnitudes of the wall-pressure fluctuations agree with the Lilley-Hodgson theoretical results. Space-time correlations of the wall-pressure fluctuations generally agree with Willmarth's results for longitudinal separation distances. The convection velocity of the fluctuations is found to increase with increasing separation distances, and its significance is explained. Measurements with the longitudinal component of the velocity fluctuations indicate that the contributions to the wall-pressure fluctuations are from two regions, an inner region near the wall and an outer region linked with the intermittency.

When a turbulent boundary layer is produced by air flow past a solid surface, the turbulence in the boundary layer can generate a sound field in the free stream and will also induce fluctuating loads on the solid surface. If the surface is flexible, this motion will generate an additional sound field on both sides of the surface. In an initial investigation of the latter form of sound generation, suitable equipment has been developed to measure the fluctuating wall pressure in the turbulent boundary layer. The equipment includes a specially designed low-noise and low-turbulence-level wind tunnel and a small barium titanate transducer and preamplifier combination for frequencies up to 50 kilocycles. The transducer and preamplifier may be useful for other applications. Using this equipment, some of the properties of the wall pressure fluctuations in a turbulent boundary layer have been measured. It was found that the spectrum of the wall pressure fluctuations extended to 50 kilocycles and that the root-mean-square wall pressure was a constant part (0.0035) of the free stream dynamic pressure for 0.2

The turbulent boundary layer (TBL) noise generated inside an aircraft can have a detrimental impact on humans. In an effort to better understand TBL physics and measurement techniques, measurements of the fluctuating wall-pressure beneath a TBL have been performed in an anechoic wind tunnel. The measurements were made using an array of pressure microphones provided by Bombardier Aerospace. Analysis of frequency spectra indicated several shortcomings of the array, namely a low saturation limit and electrical issues, however the low speed spectra agree well with the Goody prediction model. Furthermore, an investigation of the relationship between wall-pressure and velocity fluctuations was performed using a pinhole mounted microphone and simultaneous hot-wire boundary layer measurements. This revealed flow interference from the hot-wire apparatus, obscuring the expected relationships and trends. Several areas of improvement have been identified, including higher saturation limit microphones, LDA for velocity measurements, and increasing rigidity of the experimental setup.

Measurements of the turbulent pressure field on the outer surface of a 3 inch diameter cylinder were made at a point 24 feet downstream of the origin of the turbulent boundary layer. The root-mean square wall pressure was 2.42 times the wall shear stress. The normalized power spectrum at high frequencies contained twice the energy density of the spectrum beneath a plane boundary layer. The convection speed was the same as that in a plane boundary layer but the eddy size was smaller by a factor of two. The smaller eddy size and unchanged convection speed account for the greater energy in the spectrum at high frequencies. (Author).