American Institute of Aeronautics and Astronautics 1 Noise Produced by Fabric and Wire Mesh Covered Panels in Low-Speed Anechoic Wind Tunnels W. Nathan Alexander 1 and William Devenport 2 Center for Renewable Energy and Aerodynamic Testing, Virginia Tech, Blacksburg, VA 24061 The use of fabric surfaces to shield microphone instrumentation and acoustic absorbers has become standard practice, but little is known about the self-noise produced by these surface treatments. In this study, the self-noise produced by 15 candidate cloth covered perforate surface treatments designed for use on acoustic absorbers was considered. The self- noise of the surface treatments have distinct high and low frequency regions controlled by parameters of the cloth and underlying perforate geometry. Low frequency noise is primarily influenced by the diameter and open-area ratio of the perforate while high frequency noise is a function of the open-area ratio of the perforate and cloth weave. To minimize low frequency noise, high open-area ratio, small diameter perforate should be used. In contrast, to reduce high frequency noise, low open-area ratio perforate with larger diameter should be used. In either case, increasing the thread count of the surface cloth reduces the self-noise. Two mechanisms were investigated as the source of the low frequency noise: fluctuating mass flow through the perforate aperture and diffraction of the unsteady wall pressure by the perforate edges. The diffraction mechanism tends to estimate the spectral shape more accurately although absolute predictions were not possible due to lack of data regarding the attenuation of the wall pressure fluctuations by the surface cloth. I. Introduction Fabric coverings are increasingly used in aeroacoustic testing to shield microphone instrumentation and acoustic treatment, such as foam wedges or acoustic blankets which line the walls to create an anechoic environment. Virginia Tech’s Stability Wind Tunnel was the first tunnel to employ an anechoic test section with large Kevlar windows spanning the entire section backed by anechoic chambers in which instrumentation can be placed (Devenport et al., 2013). Both walls and the floor and ceiling are treated to limit acoustic reflections through the test section. This treatment relies on the acoustic transparency of the Kevlar as well as its ability to contain the flow. The floor and ceiling also have structural support created by adhering the fabric to 610 mm x 610 mm perforated panels. This provides solid footing for the engineers to walk through the test section. The design of this facility was impart derived from the study of Jaeger et al. (2000). In this, they show a clear advantage of recessing a phased microphone array behind a screen of Kevlar® 120 cloth in order to reduce measurement of turbulent pressure fluctuations while retaining the acoustic signal. By recessing their array 12.7 mm, they showed a background noise reduction of up to 20 dB for frequencies below 3500 Hz. They did notice a slight increase in background noise levels at higher frequencies speculated to be from flow interaction with the covering material. Through the work at Virginia Tech and by Jaeger et al. (2000), these fabric wall treatments have become standard for aeroacoustic applications. This method is being emulated at other facilities such as the Anechoic Flow Facility at NSWCCD and JAXA’s 2 m x 2 m wind tunnel but a thorough study of the impact of large-scale surface treatments on background noise levels has yet to be completed. Alexander and Devenport (2014) presented a small experimental acoustic dataset addressing the noise produced by these fabric treatments. They presented far field noise from the boundary layer flow over fabric covered perforate surfaces measured in the Anechoic Wall-Jet Facility at Virginia Tech. The surfaces in this study were placed directly on top of the flat plate of the wall-jet. Noise spectra had a distinctive double hump shape with low and high frequency regions that varied independently with local maximum velocity. The spectral magnitude of the low frequency region varied as the local velocity to the sixth power and the high frequency region varied as the local velocity to the eleventh power. Using the diffraction noise theory of Glegg and Devenport (2009), they hypothesized that the observed double 1 Research Assistant Professor, Department of Aerospace and Ocean Engineering, AIAA Member. 2 Full Professor, Department of Aerospace and Ocean Engineering, AIAA Associate Fellow. Downloaded by BEIHANG UNIVERSITY on March 2, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2015-3261 21st AIAA/CEAS Aeroacoustics Conference 22-26 June 2015, Dallas, TX 10.2514/6.2015-3261 Copyright © 2015 by William Nathan Alexander. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. AIAA AVIATION Forum