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  • 1
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    Large-scale coherent structures in free turbulent flows and their aerodynamic sound (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: After interpreting the observed physical features of large-scale coherent structures in free shear flows on the basis of conservation principles, the role of such structures in sources of turbulent jet sound is discussed. It is found that the lower-frequency sound, which comes from lower-frequency coherent structures peaking further downstream, radiates preferentially nearer the jet axis; the peak radiation moves away from the jet axis as the frequency increases.
    Keywords: ACOUSTICS
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  • 2
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    The generation of sound by vorticity waves in swirling duct flows (1977)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Swirling flow in an axisymmetric duct can support vorticity waves propagating parallel to the axis of the duct. When the cross-sectional area of the duct changes a portion of the wave energy is scattered into secondary vorticity and sound waves. Thus the swirling flow in the jet pipe of an aeroengine provides a mechanism whereby disturbances produced by unsteady combustion or turbine blading can be propagated along the pipe and subsequently scattered into aerodynamic sound. In this paper a linearized model of this process is examined for low Mach number swirling flow in a duct of infinite extent. It is shown that the amplitude of the scattered acoustic pressure waves is proportional to the product of the characteristic swirl velocity and the perturbation velocity of the vorticity wave. The sound produced in this way may therefore be of more significance than that generated by vorticity fluctuations in the absence of swirl, for which the acoustic pressure is proportional to the square of the perturbation velocity. The results of the analysis are discussed in relation to the problem of excess jet noise.
    Keywords: ACOUSTICS
    Type: Journal of Fluid Mechanics; 81; June 24
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  • 3
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    The large-scale organized structure in free turbulent shear flow and its radiation properties (1978)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Modeling of the large-scale coherent structures in a turbulent free shear flow is described. The objective is to gain insight into the interaction between the large-scale structure and the fine-grained turbulence. Some aerodynamic sound radiation properties of the large-scale structure are examined. It is found that the dominant contributions to the radiation come from shear noise rather than self-noise. The fact that a pure tone excitation leads to broadband amplification is most probably due to the enhancement of the fine-grained turbulence by the coherent structure. Numerical modeling of the coherent structure is also discussed.
    Keywords: ACOUSTICS
    Type: In: Structure and mechanisms of turbulence II; Proceedings of the Symposium on Turbulence; Aug 01, 1977 - Aug 05, 1977; Berlin
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  • 4
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    Sound generated aerodynamically revisited - Large-scale structures in a turbulent jet as a source of sound (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The theoretical capability of identifying the source of turbulent jet noise is assessed in comparison with experimental data. Account is taken of axisymmetric and spiral turbulence modes in Lighthill's (1952) formulation of turbulent noise. Coherent structures interacting with the mean flow and the fine-grained turbulence are the primary noise sources, modeled as an oscillating streamwise distribution. Low-frequencies arise farther downstream while high-frequencies congregate close to the nozzle lip. Previous measurements at various exit velocities, angles with respect to the nozzle axis, the Strouhal number and downstream distance are discussed. The model successfully predicted the angular distribution of noise frequency due to coherent structures. Further work is indicated on compressibility effects.
    Keywords: ACOUSTICS
    Type: Philosophical Transactions, Series A (ISSN 0080-4614); 311; 1516
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  • 5
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    Aerodynamic sound in a relaxing medium (1977)
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    Publication Date: 2019-06-27
    Description: A theory of aerodynamic sound propagation, when inhomogeneities characterized by a relaxation process are present in both the source and propagation region, is formulated. The details of the relaxation process need not be specified at the outset, although the relaxation process is characterized by a relaxation time and by an equilibrium and a frozen sound speed in a propagation region which is otherwise in equilibrium. Propagation is described in terms of a D'Alembertian characterized by the frozen sound speed relaxing toward one characterized by the equilibrium sound speed, while the source is interpreted in terms of a frozen Lighthill stress tensor relaxing toward the equilibrium stress tensor. An appropriate Green's function for the three-dimensional relaxing wave propagation operator is used to construct an exact integral for the aerodynamic sound. The sound generated far from the source is then estimated in terms of the aerodynamic sound source.
    Keywords: ACOUSTICS
    Type: Journal of Fluid Mechanics; 83; Dec. 21
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