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  • 1
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    Some aspects of the aeroacoustics of high-speed jets (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Some of the background to contemporary jet aeroacoustics is addressed. Then scaling laws for noise generation by low-Mach-number airflows and by turbulence convected at 'not so low' Mach number is reviewed. These laws take into account the influence of Doppler effects associated with the convection of aeroacoustic sources. Next, a uniformly valid Doppler-effect approximation exhibits the transition, with increasing Mach number of convection, from compact-source radiation at low Mach numbers to a statistical assemblage of conical shock waves radiated by eddies convected at supersonic speed. In jets, for example, supersonic eddy convection is typically found for jet exit speeds exceeding twice the atmospheric speed of sound. The Lecture continues by describing a new dynamical theory of the nonlinear propagation of such statistically random assemblages of conical shock waves. It is shown, both by a general theoretical analysis and by an illustrative computational study, how their propagation is dominated by a characteristic 'bunching' process. That process associated with a tendency for shock waves that have already formed unions with other shock waves to acquire an increased proneness to form further unions - acts so as to enhance the high-frequency part of the spectrum of noise emission from jets at these high exit speeds.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-CR-191458 , NAS 1.26:191458 , ICASE-93-20 , AD-A267027
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    A general introduction to aeroacoustics and atmospheric sound (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-08-27
    Description: A single unifying principle (based upon the nonlinear 'momentum-flux' effects produced when different components of a motion transport different components of its momentum) is used to give a broad scientific background to several aspects of the interaction between airflows and atmospheric sound. First, it treats the generation of sound by airflows of many different types. These include, for example, jet-like flows involving convected turbulent motions (with the resulting aeroacoustic radiation sensitively dependent on the Mach number of convection) and they include, as an extreme case, the supersonic 'boom' (shock waves generated by a supersonically convected flow pattern). Next, an analysis is given of sound propagation through nonuniformly moving airflows, and the exchange is quantified of energy between flow and sound; while, finally, problems are examined of how sound waves 'on their own' may generate the airflows known as acoustic streaming.
    Keywords: ACOUSTICS
    Type: In: International Congress on Recent Developments in Air- and Structure-Borne Sound and Vibration, 2nd, Auburn Univ., AL, Mar. 4-6, 1992, Proceedings. Vol. 1 (A93-55851 24-31); p. 5-34.
    Format: text
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  • 3
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    Report on the final panel discussion on computational aeroacoustics (1992)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Some important conclusions about future prospects for aeroacoustics in general, and for computational aeroacoustics in particular, that were reached in the course of the Final Panel Discussion of the Workshop on Computational Aeroacoustics held from 6 to 9 April 1992 by ICASE and NASA Langley Research Center are summarized by the panel chairman. Aeroacoustics must now be involved in interactions with computational fluid dynamics (as applied not only to deterministic flows but also to the statistical characteristics of turbulence), while additionally incorporating rigorous comparisons with experiment. The new Computational Aeroacoustics will press forward in two parallel ways. In one of them, CFD will be used to determine aeroacoustic source strengths, the associated radiation being derived by the Acoustic Analogy approach in one of its forms. In the other, a direct Computational Aeroacoustics will apply CFD techniques over a region extending beyond the flow field so as to include at least the beginnings of the acoustic far field. There are some particularly important areas of study, including rotor noise, boundary-layer noise, and the noise of supersonic jets, where it is strongly recommended that use of both methods is continued. On the other hand, important problems of the diffraction of radiation from aeroacoustic sources around complicated aircraft shapes will require the use of comprehensively Computational Aeroacoustics, while Acoustic Analogy methods seem better suited to estimating subsonic jet noise. The study of model problems to allow comparisons with experiment will be valuable in both lines of attack.
    Keywords: ACOUSTICS
    Type: NASA-CR-189718 , NAS 1.26:189718 , ICASE-92-53 , AD-A257106 , ICASE/NASA Workshop on Computational Aeroacoustics; Apr 06, 1992 - Apr 09, 1992; Hampton, VA; United States
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  • 4
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    A general introduction to aeroacoustics and atmospheric sound (1992)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A single unifying principle (based upon the nonlinear 'momentum-flux' effects produced when different components of a motion transport different components of its momentum) is used to give a broad scientific background to several aspects of the interaction between airflows and atmospheric sound. First, it treats the generation of sound by airflows of many different types. These include, for example, jet-like flows involving convected turbulent motions (with the resulting aeroacoustic radiation sensitively dependent on the Mach number of convection) and they include, as an extreme case, the supersonic 'boom' (shock waves generated by a supersonically convected flow pattern). Next, an analysis is given of sound propagation through nonuniformly moving airflows, and the exchange is quantified of energy between flow and sound; while, finally, problems are examined of how sound waves 'on their own' may generate the airflows known as acoustic streaming.
    Keywords: ACOUSTICS
    Type: NASA-CR-189717 , NAS 1.26:189717 , ICASE-92-52 , ICASE/NASA Langley Research Center Workshop on Aeroacoustics; Apr 06, 1992 - Apr 09, 1992; Hampton, VA; United States
    Format: application/pdf
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