Aeroacoustics of Turbulent Jets : Flow Structure, Noise Sources, and Control(Keynote Lecture)

抄録

This paper reviews research performed to advance the understanding of state-of-the-art technologies capable of reducing coaxial jet noise simulating the exhaust flow of turbo fan engines. The review focuses on an emerging jet noise passive control technology known as chevron nozzles. The fundamental physical mechanisms responsible for the acoustic benefits provided by these nozzles are discussed. Additionally, the relationship between these physical mechanisms and some of the primary chevron geometric parameters are highlighted. Far-field acoustic measurements over a wide range of nozzle operating conditions illustrated the ability of the chevron nozzles to provide acoustic benefits. Reductions in EPNL as high as 2.6 EPNdB were documented. Spectral and directivity results showed the chevrons to be most effective at angles close to the exhaust axis and at lower frequencies in the range of the peak jet noise frequency. The acoustic benefit diminished at more forward angles and at higher frequencies. The far-field measurements successfully identified trends in the acoustic benefit with respect to the chevron geometry and the nozzle operating condition. Chevron design with a higher level of penetration into the flow provided greater low frequency reduction but provided an inferior high frequency benefit as compared to a lower penetration design. Each of the chevron nozzles tested also showed a strong dependence on the nozzle shear velocity, which is defined as the difference in the velocity of the core stream and that of the surrounding air. Detailed mappings of the acoustic near-field provided more insight into the chevron source mechanisms by successfully identifying two primary chevron effects consistent with the results of the far-field measurements. First, the chevrons provided very effective suppression of low frequency noise, in the range of the peak jet noise frequency. Reductions as high as 10dB were observed at axial distance in excess of 7 equivalent nozzle diameters. This low frequency effectiveness is consistent with the low frequency, aft angle benefits seen in the far-field results. The second effect was an increase in mid and high frequency noise at axial distances between 1 and 4 equivalent diameters. This increase was shown to be directly dependent on the chevron geometry and the nozzle operating condition, with higher levels of chevron penetration and nozzle shear velocity independently producing more mid and high frequency noise. These noise increases did not produce corresponding increases in the farfield due to the effects of atmospheric absorption and attenuation. However, this increased mid to high frequency noise is consistent with the diminished acoustic benefit seen at high frequencies in the far-field. The overall impact of these effects on the acoustic near-field was to draw the peak noise generating region closer to the nozzle exit plane and reduce its spatial extent. Mean and turbulence data identified the physical flow mechanisms responsible for the effects documented in the far- and near-field studies. These measurements were successful in linking the two effects identified by the near-field mappings with two fundamental flow mechanisms. Mean flow results showed the chevron to create a radial lobe structure which distributes energy outward from the high velocity core jet to the lower velocity fan stream. The result of this redistribution of energy is a more rapid decay of the peak velocities in the jet plume and a consequent shortening of the jet potential core. Under certain operating conditions, the chevrons reduced the peak jet velocity by nearly 10% at an axial distance of 5 equivalent diameters. This effect is consistent with the reduced low frequency jet noise seen in the far- and near-field acoustics results. However, the increased radial velocity associated with the lobe structure produces increases in turbulent kinetic energy (TKE) as

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