Advanced acoustic wind tunnel measurement technologies

Abstract

Measurements in hard-walled, closed section wind tunnels are desirable for the development of quiet aircraft and to validate computational results and whilst Openjet anechoic facilities are a better measuring environment acoustically; closed-section wind tunnels offer high confidence in the aerodynamic characteristics of the testing conditions. Aeroacoustic noise from aircraft continues to be a major issue to government and industry and the accuracy and validity of acoustic measurements in Closed Section Wind Tunnels is of paramount importance.

This project began by building on existing concepts; augmenting and modifying technology to fit various wind tunnel facilities. After successful implementation of the microphone array in an industrial setting further research into improving the physical technology was started. One of the restrictions of such testing is the poor signal-tonoise ratio (SNR) when using arrays of microphones mounted on the wind tunnel wall. This can limit the ability to discern acoustic sources which are near, or below, the background noise level of the facility.

The second part of this study looked to investigate how sensor mounting details can help to improve SNR. Within this report a systematic study of microphone mounting strategies is presented. Results showed that recessing individual microphones by the depth of the microphone diameter (d) up to 2d can provide up to 3dB improvement. Increasing the recess depth beyond 2d provided up to 10dB improvement, with recessing to 10d depth providing up to 20dB improvement. The greatest improvements occurred below 25 kHz, although there is improvement across the 0 to 48 kHz range. The effect of countersunk recessing was either no improvement, or an increase in the background noise level of up to 20dB, possibly due to cavity mode oscillations within the recess aperture. Significant differences in SNR were observed between Kevlar cloths of different densities, and with a silk covering. A reduction in background noise level of 5 to 10dB was observed when acoustic foam lining was added to the floor of the recessed array. Overall this study concludes that the use of recessed arrays with acoustic foam lining may significantly improve microphone array SNR in hard-walled wind tunnel testing.

The final part of the study aimed to find ways of improving the microphone array for a given number of sensors, looking at directivity from noise sources from test models in the wind tunnel. The primary concern was to find the range at which the array is a viable tool for source location and to determine the error in sources at the extremes of the range of the array to improve the measurement technologies for the future.

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