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non-uniform azimuthal noise reductions, experiments64 with six hard-wall corrugations show a much more uniform azimuthal behavior. Current experiments are focused on the effect of forward flight as well as the effects of increased scale to further test this promising noise reduction technology.
Conclusion
Our goal has been to introduce the reader to some of the recent research regarding jet noise generation and propaga- tion from high-performance military aircraft and comple- mentary research into noise source characterization and reduction using numerical simulations and laboratory-scale models. The intense sound levels radiated near the jet gener- ally appear to be represented by large and fine-scale turbu- lence models of jet mixing noise, with the large-scale struc- tures accounting for the dominant directional radiation. Furthermore, a significant step forward in array processing of jet noise has been achieved through an implementation of near-field acoustical holography that can be compared against wave packet and other equivalent source modeling. Finally, the full-scale military jet data show that nonlinear propagation is present in the near and far fields as source- generated skewed waveforms progressively steepen and acoustic shocks form, principally in the maximum radiation direction dominated by the large-scale radiation. The on- going efforts to reduce large-scale turbulence noise are prom- ising in that they have shown reductions in level in the peak radiation direction.
The improved physical understanding of heated super- sonic jets through detailed experiments, numerical simula- tions, and development of noise reduction methodologies provides additional paths forward toward mitigation of the noise impact of tactical aircraft for both military personnel and nearby communities. Although significant work remains to “solve” the jet noise problem, it is likely that some of the findings thus far could be used to guide study of other heat- ed jets, such as solid rocket motors or volcanoes. Ultimately, in light of the collective advances by aeroacousticians con- cerning high-speed jet noise generation and propagation, we conclude with a final thought – that perhaps Professor Lamb would find cause for additional optimism when considering present-day understanding of the sound generated by a supersonic, turbulent jet!
Acknowledgments
This article was prepared under a grant from the U.S. Office of Naval Research, “Detailed Characterization of the Near-Field Noise Environment from a Full-Scale Heated, Supersonic Jet,” monitored by Brenda Henderson and Joseph Doychak. The measurements were collected with funding from the Air Force Research Laboratory through the SBIR program, monitored by Richard McKinley, and a Cooperative Research and Development Agreement (CRDA) between Blue Ridge Research and Consulting, Brigham Young University, and the United States Air Force. Joseph Doychak, Brenda Henderson, Philip Morris, Tim Colonius, Guillaume Brès, Joseph Nichols, Sanjiva Lele, Nathan Murray, Mike Rudy, and John Spyropoulos are thanked for
their contributions and discussions related to this article. Distribution A – Approved for Public Release; Distribution is Unlimited – JSF12-1991, JSF12-2322, JSF13-0450, 88ABW- 2012-0525, 88ABW-2012-4298, 88ABW-2012-3123, 88ABW-2013-2231.AT
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