Measurement Of Turbulent Boundary Layer Unsteady Wall Pressures Beneath Elastomer Layers Of Various Thicknesses On A Plate PDF Download

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.