Simulating the acoustic response of cavities to improve microphone array measurements in closed test section wind tunnels.

Colin Vandercreek,Daniele Ragni,Francesco Avallone,Mirjam Snellen

doi:10.1121/10.0009274

Colin Vandercreek, Daniele Ragni + Show 2 more

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https://doi.org/10.1121/10.0009274

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Abstract

Cavities placed along wind tunnel walls can attenuate the turbulent boundary layer (TBL) fluctuations as they propagate into the cavity. Placing microphones within the cavities can thus improve the signal-to-noise ratio of acoustic data. However, standing waves form within these cavities distorting the acoustic measurements. This work uses a finite element (FE) solver to evaluate how cavity geometry (depth, diameter, and wall angle) and wall material (hard-walled and melamine foam) affect the amplitude and eigenfrequency of standing waves when excited by an incident acoustic plane wave. Good agreement between predicted and measured acoustic transfer functions is shown. Compared to cylindrical cavities, countersunk and conical cavities improve the overall response, i.e., reducing the quality factor quantifying the resonance and damping characteristics. Stainless steel coverings also reduce the quality factor. A finding is that the shape of the external foam holder rather than the cavity shape drives the standing wave characteristics for the melamine foam cavities. The optimization problem of minimizing the acoustic response while also attenuating the TBL is thus decoupled by using the melamine foam. Consequently, these considerations can be addressed independently by optimizing the outer cavity shape for acoustics and the melamine foam insert for TBL attenuation.

Highlights

Cavities placed along wind tunnel walls can attenuate the turbulent boundary layer (TBL) fluctuations as they propagate into the cavity
This work uses a finite element (FE) solver to evaluate how cavity geometry and wall material affect the amplitude and eigenfrequency of standing waves when excited by an incident acoustic plane wave
The optimization problem of minimizing the acoustic response while attenuating the TBL is decoupled by using the melamine foam

Summary

Introduction

Cavities placed along wind tunnel walls can attenuate the turbulent boundary layer (TBL) fluctuations as they propagate into the cavity. A typical covered cavity attenuates the TBL noise, illustrated by the dashed line, with increasing frequency, improving the overall SNR.[14] the same cavity distorts the acoustic signal by introducing a standing wave centered at a specific frequency, as shown by the solid line. This standing wave amplifies the measured sound level, introducing errors into the acoustic measurements, and must be accounted for when processing data. These responses can be addressed independently through the use of sound-absorbing melamine

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