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The aeroacoustic properties of porous fabrics are investigated experimentally with the goal of finding a fabric that serves as an improved interface between wind tunnel flow and quiescent conditions. A total number of eight porous fabrics were investigated, namely, four glass fiber fabrics, two plain-weave Kevlar fabrics, and two modified plain Kevlar fabrics with their pores irregularly clogged. Two custom-made rigs were used to quantify the transmission loss (TL) and self-noise of all fabrics. The pores were found to serve as a low-resistance gateway for sound to pass through, hence enabling a low TL. The TL was found to increase with decreasing open area ratio (OAR), whereas other fabric properties had a minor impact on TL. The thread density was found to be a primary factor in determining the frequency range of porous fabrics’ self-noise, with the OAR potentially playing a secondary role in the self-noise levels. Fabrics with irregular pore distribution showed a more broadband self-noise signature associated with lower frequencies compared to fabrics with periodic pore patterns. Overall, fabrics with an irregular pore distribution or fabrics with increased thread density were identified as two potential ways to obtain superior aeroacoustic behavior compared to commonly used Kevlar fabrics. 

Previous studies have demonstrated that structures such as a canopy or finlets placed within a boundary layer over an aerodynamic surface can attenuate pressure fluctuations on the surface without compromising aerodynamic performance. This paper describes research into the fundamental mechanisms of this pressure shielding. Experiments and analysis are performed on elemental canopy configurations which are arrays of streamwise rods placed parallel to the wall in order to eliminate the confounding effects of a leading edge support structure. Experiments show that such a canopy produces attenuation in three distinct frequency ranges. At low frequencies, where convective scales are much greater than the canopy height, attenuation spectra scale on the canopy height Strouhal number, but at high frequencies, a dissipation type frequency scaling appears more appropriate. There is mid-freqeuncy region which shows reduction in attenuation and is observed for all canopy structures tested. Attenuation in this region appears to scale with Strouhal number based on canopy spacing. 

A new Anechoic Wall Jet Wind Tunnel was built at Virginia Tech. A detailed design based on the old wall jet tunnel was done to improve the quality of the resultant flow. Aerodynamic and acoustic calibrations were performed in order to understand properties and characteristics of the flow generated by this new facility which can be used for various aeroacoustic studies. Far-field acoustics were measured using half-inch B&K microphones in a streamwise array to characterize and reduce the background noise. Sound pressure levels were lower by 10 dB for frequencies up to 700Hz in comparison to the old facility. The turbulent surface pressure fluctuations of the wall-jet flow were studied using Sennheiser microphones placed along streamwise and spanwise locations to record surface pressure fluctuations. Comparison of the autocorrelation plotted for microphones along the same span indicate uniform flow features. A decay in the turbulence levels is observed along the downstream direction as expected. Aerodynamic calibrations included mean velocity measurements along different spanwise locations, wall-jet boundary layer profiles and streamwise cross-sections. Spanwise and cross-sectional velocity profiles show good uniformity of the flow. Detailed boundary layer analyses were performed with the parameters obtained from the experiments.