Willmarth, William W (California Institute of Technology)
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. Usilg 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 < M < 0.8 and 1.5 < Re < 20 x 10 to the sixth. A few typical spectra are given for different values of Reynolds number and Mach number.
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