Potential factors responsible for discrepancies in jet noise measurements of different studies

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Karon, Aharon Z.
Ahuja, Krishan K.
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Jet noise measurements have been acquired at many anechoic jet-facilities around the world. These measurements have been used to form the basis for prediction schemes and to understand the generation and radiation of jet noise. Often, when jet noise measurements from different jet-facilities that are thought to be acquired at or corrected to similar conditions are compared, differences are observed in the spectra. These comparisons are typically performed on the basis of the same nozzle-exit diameter, jet velocity, microphone distance from the jet flow, and ambient conditions. This phenomenon has spurred much discussion in the aeroacoustics community, with some even claiming that some of the measurements are contaminated with rig-noise. This study investigates following four factors that can be responsible for the differences in jet noise measurements: (1) rig noise contamination, (2) the jet’s Reynolds number, (3) the nozzle-exit boundary layer of the jet, and (4) reflections and shielding from surfaces inside the anechoic jet-facility. First, the Doubling-Diameter Method, a scheme used to detect rig-noise contamination in jet-noise measurements, is verified and used on jet noise measurements acquired in the Georgia Tech Research Institute (GTRI) Anechoic Jet-Facility to verify the cleanliness of the jet noise measurements. Second, the effect of Reynolds number of the jet is investigated qualitatively and quantitatively for its effect on jet noise measurements. Third, the effect of the nozzle-exit boundary layer on jet noise measurements is categorized qualitatively and quantitatively. In addition, a potential correction is developed that can be used to account for the differences in jet noise measurements from jets with different nozzle-exit boundary layer states. Finally, surfaces inside the anechoic chamber itself, such as, the plenum chamber, and a secondary nozzle are investigated as potential reflectors and shields that can cause waviness and modifications in the jet noise spectra, respectively.
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