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  • 1
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    Non-linear propagation in near sonic flows (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A nonlinear analysis is developed for sound propagation in a variable-area duct in which the mean flow approaches choking conditions. A quasi-one-dimensional model is used and the nonlinear analysis represents the acoustic disturbance as a sum of interacting harmonics. The numerical procedure is stable for cases of strong interaction and is able to integrate through the throat region without any numerical instability.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration; 75; Apr. 8
    Format: text
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  • 2
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    Intensification and refraction of acoustical signals in partially choked converging ducts (1980)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A computer code based on the wave-envelope technique is used to perform detailed numerical calculations for the intensification and refraction of sound in converging hard walled and lined circular ducts carrying high mean Mach number flows. The results show that converging ducts produce substantial refractions toward the duct center for waves propagating against near choked flows. As expected, the magnitude of the refraction decreases as the real part of the admittance increases. The pressure wave pattern is that of interference among the different modes, and hence the variation of the magnitude of pressure refraction with frequency is not monotonic.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration; 73; Dec. 22
    Format: text
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  • 3
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    A comparison of experiment and theory for sound propagation in variable area ducts (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: An experimental and analytical program has been carried out to evaluate sound suppression techniques in ducts that produce refraction effects due to axial velocity gradients. The analytical program employs a computer code based on the method of multiple scales to calculate the influence of axial variations due to slow changes in the cross-sectional area as well as transverse gradients due to the wall boundary layers. Detailed comparisons between the analytical predictions and the experimental measurements have been made. The circumferential variations of pressure amplitudes and phases at several axial positions have been examined in straight and variable area ducts, with hard walls and lined sections, and with and without a mean flow. Reasonable agreement between the theoretical and experimental results has been found.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration; 71; July 22
    Format: text
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  • 4
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    Acoustic propagation in partially choked converging-diverging ducts (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A computer model based on the wave-envelope technique is used to study acoustic propagation in converging-diverging hard walled and lined circular ducts carrying near sonic mean flows. The influences of the liner admittance, boundary layer thickness, spinning mode number, and mean Mach number are considered. The numerical results indicate that the diverging portion of the duct can have a strong reflective effect for partially choked flows.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration; 81; Apr. 22
    Format: text
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  • 5
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    Propagation of spinning acoustic modes in partially choked converging ducts (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A computer model based on the wave-envelope technique is used to study the propagation of spinning acoustic modes in converging hard-walled and lined circular ducts carrying near sonic mean flows. The results show that with increasing spinning mode number the intensification of the acoustic signal at the throat decreases for upstream propagation. The influence of the throat Mach number, frequency, boundary-layer thickness, and liner admittance on the propagation of spinning modes is considered.
    Keywords: ACOUSTICS
    Type: Acoustical Society of America; vol. 71
    Format: text
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  • 6
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    Transmission of sound through nonuniform circular ducts with compressible mean flows (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: An acoustic theory is developed to determine the sound transmission and attenuation through an infinite hard-walled or lined circular duct carrying compressible, sheared mean flows and having a variable cross section. The theory is applicable to large as well as small axial variations, as long as the mean flow does not separate. The technique is based on solving for the envelopes of the quasi-parallel acoustic modes that exist in the duct instead of solving for the actual wave, thereby reducing the computation time and the round-off error encountered in purely numerical techniques. A number of test cases that demonstrate the flexibility of the program are included. Convergence of the transmission coefficients and the acoustic pressure profiles with an increasing number of modes is illustrated.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 79-0622
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  • 7
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    Nonlinear propagation of a wave packet in a hard-walled circular duct (1975)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The method of multiple scales is used to derive a nonlinear Schroedinger equation for the temporal and spatial modulation of the amplitudes and the phases of waves propagating in a hard-walled circular duct. This equation is used to show that monochromatic waves are stable and to determine the amplitude dependance of the cutoff frequencies.
    Keywords: ACOUSTICS
    Type: Acoustical Society of America; vol. 57
    Format: text
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  • 8
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    Sound propagation through nonuniform ducts (1976)
    In:  CASI
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    Publication Date: 2019-06-27
    Description: Methods of determining the transmission and attenuation of sound propagating in nonuniform ducts with and without mean flows are discussed. The approaches reviewed include purely numerical techniques, quasi-one-dimensional approximations, solutions for slowly varying cross sections, solutions for weak wall undulations, approximation of the duct by a series of stepped uniform cross sections, variational methods and solutions for the mode envelopes.
    Keywords: ACOUSTICS
    Type: NASA. Langley Res. Center Advan. in Eng. Sci., Vol. 3; p 821-833
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  • 9
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    A wave-envelope technique for wave propagation in non-uniform ducts (1976)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The method of variation of parameters is used to analyze wave propagation in variable area, plane ducts with no mean flow. The method has an explicit representation of the fast axial variation of the acoustic modes, and numerical integration is required only for the slower axial variations of the mode amplitudes and phases. Results are presented which demonstrate the numerical advantages of the method. Comparison of the results with those of a small perturbation theory are given. The relationship between this method and the method of weighted residuals is discussed.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 76-496 , Aero-Acoustics Conference; Jul 20, 1976 - Jul 23, 1976; Palo Alto, CA
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  • 10
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    An analytical and experimental study of sound propagation and attenuation in variable-area ducts (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The performance of sound suppression techniques in ducts that produce refraction effects due to axial velocity gradients was evaluated. A computer code based on the method of multiple scales was used to calculate the influence of axial variations due to slow changes in the cross-sectional area as well as transverse gradients due to the wall boundary layers. An attempt was made to verify the analytical model through direct comparison of experimental and computational results and the analytical determination of the influence of axial gradients on optimum liner properties. However, the analytical studies were unable to examine the influence of non-parallel ducts on the optimum linear conditions. For liner properties not close to optimum, the analytical predictions and the experimental measurements were compared. The circumferential variations of pressure amplitudes and phases at several axial positions were examined in straight and variable-area ducts, hard-wall and lined sections with and without a mean flow. Reasonable agreement between the theoretical and experimental results was obtained.
    Keywords: ACOUSTICS
    Type: NASA-CR-135392
    Format: application/pdf
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