Array Measurements in Wind Tunnels with open Test Sections

Abstract

Acoustically optimized wind tunnels usually have an anechoic chamber around an open test section. For array measurements in this type of tunnels the acoustic array is typically placed outside the flow field - beside or above the test section. The sound emitted from sources in the flow has to propagate through a turbulent shear layer to reach the microphones. During their passage through the shear layer the sound waves interact with turbulent structures. The higher the frequency of the wave, the more significant the effect of the shear layer is. In the present paper these interaction of sound waves with a turbulent shear layer is investigated. It is analyzed which effects can occur when sound waves propagate through a turbulent shear layer and how these effects can influence the results of array measurements. Experiments with a test source in a wind tunnel with open test section are performed. It is shown that turbulent structures in the shear layer lead to systematic phase fluctuations. These phase fluctuations have a certain spectrum and a distinct spatial correlation. Beside these effects the spectral broadening of tones due to the propagation trough the shear layer is investigated.