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This paper introduces a new method for measuring the spectral character of wall pressure fluctuations under turbulent boundary layers by measuring the sound that they radiate in the presence of hydrodynamically smooth sinusoidal ridges in the surface. The theoretical basis for the method and experimental tests demonstrating its viability are described. The sound spectrum radiated by the sinusoidal surface reveals a cut through the full three-dimensional wavenumber frequency spectrum of the wall pressure at the wavenumber of the surface. Since sinusoidal ridges can be made with very small wavelengths, this technique can be used to probe the structure of the wall pressure spectrum on scales far smaller than those that can be reached using conventional wall-mounted transducers. Furthermore, the method reveals the wavenumber frequency spectrum directly, without the need for multi-point measurements. The wall pressure wavenumber frequency spectra measured using this technique bear a close qualitative and quantitative similarity to Chase’s (1980, 1987) model forms, with the exception that they show a high frequency decay that occurs at different rates than assumed in the models. I. Introduction This paper introduces a new method for measuring the spectral character of wall pressure fluctuations under turbulent boundary layers by measuring the sound that they radiate in the presence of hydrodynamically smooth sinusoidal ridges in the surface. Following the analysis of Glegg and Devenport (2009), consider a textured surface in the y1,y3 plane with a surface elevation y2=[(y1,y3), subjected to a homogeneous fluctuating hydrodynamic pressure field, ps. As can be established directly from Lighthill’s equation, with an appropriate selection of the Green’s function, the pressure fluctuations will scatter off the surface generating sound. The pressure field associated with the sound p heard at position x and frequency Z is |
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