An Assessment of Acoustically Transparent Wind Tunnel Walls

Abstract

One of the main limitations of microphone measurements in an open-jet wind tunnel is spectral broadening. The phenomenon of spectral broadening is observed when a single frequency sound wave passes through a free shear layer. Inside the turbulent shear layer the wave interacts with the turbulence, which leads to broadening of the tone in the frequency domain. The spectrum of a single frequency tone measured with a microphone will not only include the peak but also two shoulders containing the broadened frequencies, one on each side of the peak. Spectral broadening does not only a_ect single microphone measurements but also has a detrimental e_ect on acoustic array measurements. The distortion caused by the shear layer will reduce the coherence between the microphones in the array. This limits the size and thus resolution of the array. This spectral broadening is particularly problematic when examining objects which produce a tonal noise, e.g. open rotors, sharp edges.