ADVANCES IN MICROPHONE ARRAY MEASUREMENTS IN A CRYOGENIC WIND TUNNEL

The prediction of full-scale airframe noise based on small-scale model measurements via the phased microphone array technique is well known and in common use in closed test sections. Since conventional wind tunnels cannot generally achieve full-scale Reynolds numbers, measurements are often performed in cryogenic and pressurized wind tunnels which are capable of higher Reynolds number flows. Thus, the characteristics of the moving fluid are adapted to the scale of the model. At the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. To this end, a microphone array consisting of 144 microphones was designed and constructed. In order to use a microphone array in a cryogenic environment, coming to grips with cold hardiness and ensuring long term stability of the array fairing and the electronic devices, especially the microphones, are the primary challenge. Measurements of the radiated noise from a single rod configuration have been conducted. The results showed very good agreement between theory and measured sound radiation. A Reynolds number dependency of the measured and predicted sound power can be shown.

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