Systematic Comparison of Microphone Array Measurements in Open and Closed Wind Tunnels

The phased microphone array has become a widely used tool in aeroacoustic testing for the localisation and quantification of sound sources. The measurements are often carried out in open or closed wind tunnels using scaled models. Test sections in both types of tunnels have advantages and drawbacks when used for aerodynamic and aeroacoustic testing. The closed-test section wind tunnel is normally preferred for measuring aerodynamic properties due to the reduced uncertainties compared with the open section. Conducting aerodynamic and aeroacoustic measurements simultaneously or at least back-to-back helps to reduce wind tunnel access time and costs. Furthermore, in many cases a better microphone array resolution can be achieved because of the shorter distances between array and model. In-flow measurements in closed test sections using flush mounted microphones in the wind tunnel wall are typically subjected to strong hydrodynamic pressure fluctuations induced by the turbulent boundary layer, which can result in a dramatic decrease of the signal-to-noise ratio (SNR). The reverberations in the closed test section are also of concern, since the beamforming accuracy may be affected. In contrast, the open jet acoustic wind tunnel is not plagued by these particular drawbacks. Nevertheless, other problems will occur and can influence the quality of the measurement results. The sound waves have to pass through the wind tunnel shear layer before reaching the microphone. That results in a spatial coherence loss due to turbu- lent scattering and refraction and potentially leads to degraded source maps and under-estimated source levels. In comparison with the closed wind tunnel, far-field measurements can be normally performed