Abstract
Aeroacoustic measurements performed by flush-mounted microphone arrays on the walls of closed-section wind tunnels are contaminated by the hydrodynamic pressure fluctuations of the wall’s boundary layer. This study evaluates three different microphone cavity geometries for mitigating this issue. Their improvement to the signal-to-noise ratio (SNR) and the accuracy of their acoustic imaging results are compared to a flush-mounted microphone array. The four geometries include: (1) an array of flush-mounted microphones as the baseline, (2) a cylindrical hard-plastic cavity with a countersink, (3) a conical cavity made of melamine acoustic absorbing foam, and (4) a conical cavity with star-shaped protrusions, also made of melamine. The three arrays with cavities were covered with a steel-wire cloth to reduce the boundary layer fluctuations at the microphone while the baseline array was uncovered. Two sound sources were tested in an aeroacoustic wind tunnel for assessing the performance of the different cavities: a speaker placed outside the flow and a distributed sound source generated by a flat plate inside of the flow. When using conventional frequency domain beamforming, both cavities made of melamine offer up to a 30 dB increase in SNR with respect to the flush-mounted case, followed by the hard-walled cavity with up to a 20 dB increase. This is a 20 dB improvement when compared to the single microphone cases. The melamine cavities also provide cleaner acoustic source maps and accurate spectral estimations for a wider frequency range. The effect of cavity placement and geometry on the coherence, which affects the beamforming analysis of the acoustic signal was negligible for all cases. Distributed sound source measurements using the three arrays agreed with predictions using the Brooks, Pope, and Marcolini (BPM) model, showing that the cavities could detect vortex shedding that was undetectable by the flush array.
Highlights
Aeroacoustic experiments in wind tunnels are often performed in open-jet facilities as they allow for placing the microphones outside of the flow [1]
This work quantifies the impact of cavity geometry on the signal-to-noise ratio (SNR), and on the accuracy of acoustic imaging results for microphone arrays
One with a hard walled countersink and two conical cavities with melamine walls, all covered with a high thread count stainless steel cloth, are compared with a baseline flush-mounted microphone array
Summary
Aeroacoustic experiments in wind tunnels are often performed in open-jet facilities as they allow for placing the microphones outside of the flow [1]. The tunnel’s walls, the tunnel’s machinery, and reflections that propagate within the tunnel’s closed test section For this application, microphones are typically mounted flush and, the measurements are contaminated with TBL noise. The signal-to-noise ratio (SNR) of the microphone array when measuring the far-field emissions of an acoustic source can be increased by attenuating the level of TBL noise at the microphone. This can be achieved in two ways. Employing acoustic beamforming and applying techniques that average out the incoherent noise, which includes TBL noise [4]
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