Direct Measurement of the Mode O Turbulent Boundary Layer Wall Pressure and Wall Shear Stress Spectra Using Air-Backed and Oil-Filled Multichannel Wavenumber Filters

摘要

This report describes research that resulted in the first direct multichannel measurements of the wall pressure and wall shear stress spectra beneath a turbulent boundary layer formed over a long, thin cylinder in an axial flow field. These measurements, processed as wavenumber-frequency spectra, were made with a multichannel array composed of an air-backed cylinder structure and a 32-channel aperture of PVDF film sensors having an axisymmetric response with respect to the array circumference. The measurements resolve the wavenumber spectrum in the +/- 131-rad/m range. Unexpected results were observed at a 5- knot freestream flow speed, where the magnitude of the wall shear stress convective ridge level exceeded the magnitude of the wall pressure convective ridge level. From 10 to 20 knots, the traditional relationship was observed, in which the wall pressure magnitude exceeds the wall shear stress magnitude in the vicinity of the convective ridge. Additionally, accurate resolution of the shape of the convective ridge as a function of wavenumber was accomplished with the air-backed cylinder array. An experimental calibration of the 'hose transfer function' for pressure was obtained by using data from an array of ceramic hydrophones mounted interior to an oil-filled cylinder in unison with data from the air-backed cylinder array. Theoretical dynamic elasticity simulations were required to remove the effects of the surface-generated pressure fields, which were different for each structure. Previous to this research, a mathematical model of the hose transfer function was used without the aid of experimental data or verification of any kind. The oil-filled array measurements spanned the wavenumber spectrum in the +/- 206-rad/m range and included measurements at kappa = 0 that were free from acoustic contamination.