Acoustic wall treatments for wind tunnel aeroacoustic measurements

Bento, Hugo F.M.; Ragni, Daniele; Avallone, Francesco; Simons, Dick G.; Snellen, Mirjam

doi:10.1016/j.apacoust.2022.108989

Cited by 7 publications

(11 citation statements)

References 23 publications

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“…Beamforming results can be improved further by removing the main diagonal of the cross-spectral matrix [19]. Typical aeroacoustic measurements being performed in closed test-section wind tunnels will employ the combination of recessed microphone arrays and processing techniques like beamforming that are able to suppress the influence of turbulent boundary layer pressure fluctuations [15][16][19][20][21][22]26]. This has become a standard approach for aeroacoustic measurements, but it is not without its drawbacks.…”

Section: Standard Practicesmentioning

confidence: 99%

“…Although, porous materials will affect boundary layer profiles requiring correction terms. Materials such as melamine and wool have been found to be successful in lowering the reflection coefficient [20][21]. An example of a wind tunnel section with acoustically treated walls is shown below in Figure 1.1 which depicts the test section inside NASA Glenn's 9'×15' low-speed wind tunnel [21].…”

Section: Introductionmentioning

confidence: 99%

“…The other problem facing closed test section wind tunnel measurements is the turbulent boundary layer that submerges the section walls imparts intense pressure fluctuations on surface mounted microphone arrays [16,19]. The noise produced by the turbulent boundary layer is a significant source of background noise [20]. Not only are these pressure fluctuations acting on surface mounted microphones, but their amplitudes can be larger than those trying to be measured [16].…”

Section: Introductionmentioning

confidence: 99%

“…As the name suggests, open-jet wind tunnels do not confine the jet flow with any walls, but instead let it flow into an acoustically treated chamber. Open-jet wind tunnels tend to have better acoustic performance and lesser aerodynamic performance than closed section wind tunnels [20]. In order to reproduce desired aerodynamic conditions and measure acoustics accurately, researchers have developed ways of mitigating the effects of the turbulent boundary layer on acoustic measurements.…”

Section: Introductionmentioning

confidence: 99%

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Directionally Sensitive Sensor Based on Acoustic Metamaterials

Braaten

Damani

Alexander

et al. 2023

AIAA AVIATION 2023 Forum

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Phased microphone arrays are valuable tools for aeroacoustic measurements that can measure the directivity of multiple acoustic sources. However, when deployed in closed test-section wind tunnels, the acoustics suffer due to intense pressure fluctuations contained in the wall-bound turbulent boundary layer. Furthermore, phased microphone arrays require many sensors distributed over a large aperture to ensure good spatial resolution over a wide frequency range. Microphone arrays of such large count are not always feasible due to constraints in space and cost. This thesis describes an alternative approach for measuring single broadband acoustic sources that uses an acoustic metasurface. The metasurface is comprised of a meandering channel of quarter-wave cavities and an array of equally spaced half-wave open throughcavities. A series of tests were conducted in Virginia Tech's Anechoic Wall-Jet Tunnel where combinations of a wall-bound turbulent jet-flow and a single broadband acoustic source were used to excite the metasurface and produce acoustic surface waves. Measurements of the acoustic surface waves were performed using two methods: a pair of traversing microphones scanning the pressure field along the length of the metasurface 0.25 mm beneath its bottom face, and an array of unequally spaced microphones embedded inside the metasurface. Spectral analysis on the measurements revealed that the inclusion of multiple through-cavities leads to constructive reinforcement of select acoustic surface waves as a function of the acoustic source location. In the case of the embedded microphones, acoustic beamforming was applied in order to extract spatial information. This reinforcement was observed during measurements made with both flow and acoustic excitation, up to Wall-Jet Tunnel nozzle exit speeds of 40 𝑚/𝑠 beyond which it was no longer seen. A series of quiescent measurements made with a range of speaker locations constituted a calibration for the metasurface which was used to locate an unknown broadband acoustic source within an The Root-Mean-Square (RMS) error of 1.06 °.

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Section: Standard Practicesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Directionally Sensitive Sensor Based on Acoustic Metamaterials

Braaten

Damani

Alexander

et al. 2023

AIAA AVIATION 2023 Forum

View full text Add to dashboard Cite

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“…The noise scattering from wind tunnel wall porous liners has been previously investigated. Literature [17] indicates that a good closed wind tunnel liner has a low surface roughness and inertial resistivity, and a high viscous resistivity, which leads to high acoustic absorption. Closed wind tunnel designs can benefit from research which assesses the improvement obtained from lining individual segments of the circuit, e.g.…”

Section: Introductionmentioning

confidence: 99%

Wall treatments for aeroacoustic measurements in closed wind tunnel test sections

Bento

VanDercreek

Avallone

et al. 2023

AIAA AVIATION 2023 Forum

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Aeroacoustic tests in closed wind tunnels are affected by reflections in the tunnel circuit and background noise. Reflections can be mitigated by lining the tunnel circuit. The present study investigates if lining exclusively the most accessible segment of a closed wind tunnel circuit, in particular the test section, is an approach which improves acoustic measurements. Literature shows that a wind tunnel lining material should have high acoustic absorption, low inertial resistivity and low surface roughness. Therefore, the test section of TU Delft's closed Low Turbulence Tunnel is lined with melamine foam wall liners. A total of 4 test section configurations were tested: baseline; test section with lining on the floor and ceiling; test section with lined side-panels; and test section lined at all surfaces (floor, ceiling and side-panels). An omnidirectional speaker is used for evaluating the wind tunnel's acoustic performance. A geometric modelling algorithm, based on the mirror-source method, is used to predict the effect of lining on primary reflections in the test section. In addition, reflections in the test section and in the tunnel circuit are characterized experimentally. The results show that the closed loop of the tunnel circuit is responsible for a long reverberation time in the test section. However, reflections inside the test section itself are the dominant source of acoustic interference at the microphone array location. The low fidelity geometric modelling algorithm is shown to be a valuable approach for an initial estimation of the acoustic benefit of lining, for both flow-off and -on conditions. Lining of the test section walls significantly reduces reflections from the reference source, as well as the aerodynamic background noise that reaches the array.

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