Wall Features of Wing-Body Junctions: Towards Noise Reduction
Owens, David Elliot
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Much research and experiments have gone into studying idealized wing-body junction flows and their impact on horseshoe vortex and wake formation. The vortices have been found to generate regions of high surface pressure fluctuations and turbulence that are detrimental to structural components and acoustics. With the focus in the military and commercial industry on reducing the acoustical impact of aircraft and their engines, very little research has been done to examine the potential impact wing-body junctions may have on acoustics, especially for high lifting bodies such as propellers. Two similar tests were conducted in the Virginia Tech Open Jet Wind Tunnel where boundary layer measurements, oil flow visualizations, acoustic linear array and surface pressure fluctuation measurements of a baseline Rood airfoil model and two novel junction fairing designs were all taken. Boundary layer measurements were taken at four locations along the front half of the flat plate and the profiles were shown to be all turbulent despite the low Reynolds number of the flow, (test 1: Re_"<1400, test 2: Re_"<550). Oil flow visualizations were taken and compared to those of previous researchers and the location of separation and line of low shear along with the maximum width of the wake and width of wake at the trailing edge all scaled relatively well with the Momentum Deficit Factor, defined for wing-body junction flows [Fleming, J. L., Simpson, R. L., Cowling, J. E. & Devenport, W. J., 1993. An Experimental Study of a Turbulent Wing-Body Junction and Wake Flow. Experiments in Fluids, Volume 14, pp. 366-378. ]. A linear microphone array was used to estimate the directivity of the facility acoustic background noise to be used to improve background subtraction methods for surface pressure fluctuation measurements. Surface pressure fluctuation spectra were taken ahead of the leading edge of the plate and along the surface of the models. These showed that the fairings reduced pressure fluctuations along the plate upstream of the leading edge, with fairing 1 reducing them to clean tunnel flow levels. On the surface of the models, the fairings tended to reduce low frequency (<1000Hz) pressure fluctuation peaks when compared to the baseline model and increase the pressure fluctuations in the high frequency range. Simple scaling arguments indicate that this spectral change may be more beneficial than detrimental as low frequency acoustics especially those between 800 Hz and 1200 Hz are the frequencies that humans perceive as the loudest noise levels. Scaling the frequencies measured to those of full scale applications using Strouhal numbers show that frequencies below 1000 Hz in this experiment result in frequencies at the upper limit of the human hearing frequency range. Low frequency acoustic waves also tend to travel farther and high frequency acoustic waves are more apt to be absorbed by the surrounding atmosphere.
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