The report deals with some aspects of the correlations and the spectra of the fluctuating wall pressure. Some of the available experimental data on the pressure fluctuations is reviewed and different correlation models for the fluctuating pressure are investigated chiefly from the point of view of estimation of transducer corrections and of power input to an infinite plate. It was demonstrated that both these aspects are rather sensitive to the kind of correlation model assumed.

The attenuation of turbulence induced wall pressure fluctuations through elastomer layers was studied experimentally and analytically. Wall pressure statistics were measured downstream from a backward facing step, with no elastomer present and beneath 2, 3 and 4 mm thick elastomers in a water tunnel facility. The step height, h, was 0.635 cm and the wall pressures were measured at non-dimensional distances of x/h=10, 24, 36 and 54 downstream from the step. The backward facing step was employed to increase the turbulent boundary layer wall pressure spectral levels above those of the water tunnel facility noise. Velocity statistics were measured at locations corresponding to the wall pressure measurements to aid in the interpretation of the wall pressure data. The attenuation of the wall pressure spectra beneath the elastomer layers that was measured experimentally was then compared to analytical model predictions.In the absence of an elastomer layer, the wall pressure spectra, cross-spectra and velocity statistics measured at the various locations downstream from the backward facing step were in excellent agreement with those reported in the archival literature. With the elastomer layers employed at the x/h=10 location, the measured wall pressure spectral levels were the same as those measured in the absence of an elastomer for frequencies at and below the spectral peak. At higher frequencies, the elastomer layers attenuated the wall pressure spectral levels; an effect that increased with increasing elastomer thickness. The streamwise coherence measured beneath the elastomer layers was higher than that measured in the absence of an elastomer layer, an effect which increased with increasing elastomer thickness. It is speculated that this increase in coherence level is due to the ability of the elastomer to support shear stresses, which effectively increases the area over which an eddy influences the stresses measured by the pressure sensors. The high wavenumber filtering of the elastomers was also observed in the coherence at the smallest streamwise separation of /=2.27.An analytical elastomer transfer function, which models the transfer of turbulent boundary layer wall pressures on the surface of an elastomer to the normal stresses through the elastomer, was applied to the turbulent boundary layer wall pressure measurements in the absence of an elastomer layer and compared to measurements beneath the 2, 3, and 4 mm thick elastomer. The attenuation of the turbulent boundary layer wall pressure fluctuations through the elastomer layer using the analytical elastomer transfer function were in excellent agreement with the attenuation measured experimentally through all thicknesses of elastomer and all free stream velocities at which the experiments were performed.

The state of the art in measurement and interpretation of dynamic wall pressure beneath a turbulent boundary layer is reviewed. The mean pressure increase for shear flow over an orifice in a wall is explained, using triple deck theory, to stem from streamline contraction resulting from removal of the no-slip boundary condition. The effect in viscous hole diameters is too small to suggest that the dynamic pressure increase reported by Bull and Thomas (1976) for flow over a 'pinhole' microphone stems from this mechanism. It appears that any failure of high frequency spectra to collapse when made non-dimensional on inner wall variables is more likely due to transducer proudness or to error in the measurement of mean wall shear stress. The Corcos model is shown to be inadequate to describe cross-spectrum measurements. Both amplitude and phase depend also on the ratio of transducer separation to displacement boundary layer thickness. (JES).

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.

Turbulent boundary layer wall pressure measurements were made with 'pinhole' microphones three times smaller (relative to boundary layer thickness) than microphones used in earlier work. The improved high frequency resolution permitted examination of the influence of high frequency eddies on smooth wall pressure statistics. It was found that the space-time decay rate is considerably higher than previously reported. Measurements of cross-spectral density made with 5 Hz band width filters disclosed low phase speeds at low frequency and small separation. Measurements were repeated on rough walls and parallels were drawn from knowledge of a smooth wall boundary layer structure to propose a structure for a rough wall boundary layer. The effect of independently varying roughness height and separation on the large and small scale turbulence structure was deduced from the measurements. It was found that roughness separation affected the very large scale structure, whereas the roughness height influenced the medium and very small scale turbulence. (Author).