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  • 1
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    On broadband shock associated noise of supersonic jets (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The characteristics and generation mechanisms of noise associated with the interactions of turbulence with the quasi-periodic broadband shock cells of supersonic jet engines are reviewed. The noise possesses broadband spectra and directionality that are completely different from noise caused by turbulence. Experimental data have shown that broadband noise is most prominent in the forward arc, with peak frequencies being a function of the observation angle and the pressure mismatch in the engine. The noise originates in the engine as turbulence-shock interactions occur during downstream movement. Features of the phased point-source array model and the large turbulence structures-shock cells interaction model are defined and model predictions are compared with experimental data on noise sources. Only a scaling of the noise component is found to be currently possible. More complete characterization depends on consideration of the jet temperature and analysis of turbulence-shock interactions, broadband shock and screech tones and shock noise in several flow configurations.
    Keywords: ACOUSTICS
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  • 2
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    The near and far acoustic fields of broadband shock associated noise (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A stochastic model theory is presented which suggests that the broadband shock associated noise of supersonic jets is generated by the weak interaction between downstream propagating large turbulence structures and the quasiperiodic shock cells in the jet plume. The large turbulence structures are modelled by a superposition of the intrinsic instability waves of the mean flow of the jet, and the quasiperiodic shock cells are broken down into time-independent waveguide modes of the jet flow using multiple scales expansion. The observed broadband shock associated noise consists of a superposition of the spectra generated by the different waveguide modes. Starting from the equations of motion of a compressible fluid, formulas for calculating the far field and near field noise are developed. Good general agreement is found with previous results.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 86-1943
    Format: text
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  • 3
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    On the screech tones of supersonic rectangular jets (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The shock cell structure and screech tone frequencies of nonaxisymmetric supersonic jets are examined. A linear shock cell model that utilizes a vortex sheet as the mixing layer of the jet is developed in order to estimate the characteristics of the shock structures and tone frequencies. Eigenvalue problems for rectangular and elliptical jets are solved, and shock cell structure solutions for the jets are derived. The shock cell spacings calculated with the model are compared to the data of Powell (1953) and Hammitt (1961); good correlation is obtained over a wide range of Mach numbers. The shock cell spacing formula is applied to the computation of screech tone frequencies of rectangular jets. It is observed that the calculated frequencies agree with the measurements of Powell and Krothapalli et. al. (1986) for the Mach number range 1.15-1.80.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 86-1866
    Format: text
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  • 4
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    Shock associated noise reduction from inverted-velocity-profile coannular jets (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Acoustic measurements show that the shock noise from the outer stream is virtually eliminated when the inner stream is operated at a Mach number just above unity, regardless of all the other jet operating conditions. At this optimum condition, the coannular jet provides the maximum noise reduction relative to the equivalent single jet. The shock noise reduction can be achieved at inverted-as well as normal-velocity-profile conditions, provided the coannular jet is operated with the inner stream just slightly supersonic. Analytical models for the shock structure and shock noise are developed indicate that a drastic change in the outer stream shock cell structure occurs when the inner stream increases its velocity from subsonic to supersonic. At this point, the almost periodic shock cell structure of the outer stream nearly completely disappears the noise radiated is minimum. Theoretically derive formulae for the peak frequencies and intensity scaling of shock associated noise are compared with the measured results, and good agreement is found for both subsonic and supersonic inner jet flows.
    Keywords: ACOUSTICS
    Type: NASA-CR-3454 , LG81ER0162
    Format: application/pdf
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  • 5
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    The radiation of sound by the instability waves of a compressible plane turbulent shear layer (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The problem of acoustic radiation generated by instability waves of a compressible plane turbulent shear layer is solved. The solution provided is valid up to the acoustic far-field region. It represents a significant improvement over the solution obtained by classical hydrodynamic-stability theory which is essentially a local solution with the acoustic radiation suppressed. The basic instability-wave solution which is valid in the shear layer and the near-field region is constructed in terms of an asymptotic expansion using the method of multiple scales. This solution accounts for the effects of the slightly divergent mean flow. It is shown that the multiple-scales asymptotic expansion is not uniformly valid far from the shear layer. Continuation of this solution into the entire upper half-plane is described. The extended solution enables the near- and far-field pressure fluctuations associated with the instability wave to be determined. Numerical results show that the directivity pattern of acoustic radiation into the stationary medium peaks at 20 degrees to the axis of the shear layer in the downstream direction for supersonic flows. This agrees qualitatively with the observed noise-directivity patterns of supersonic jets.
    Keywords: ACOUSTICS
    Type: Journal of Fluid Mechanics; 98; May 29
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  • 6
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    Shock associated noise of inverted-profile coannular jets. I - Experiments (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The reduction of shock-associated noise in inverted-velocity-profile coannular jets is quantified and explained by conducting extensive optical and acoustic measurements for a suitable range of outer-stream and inner-stream presure-ratio combinations, and by interpreting the measured noise results with the aid of new theoretical models. It is shown that the shock noise from the outer stream is virtually eliminated when the inner stream is operated at a Mach number just above unity, regardless of all the other jet operating parameters. Furthermore, this reduction can be achieved at inverted-velocity-profile as well as normal-velocity-profile conditions.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration (ISSN 0022-460X); 98; 95-113
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  • 7
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    Shock associated noise of inverted-profile coannular jets. II - Condition for minimum noise. III - Shock structure and noise characteristic (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The generation of noise by the shock-turbulence interaction with shock cells in an inverted-profile coannular jet with nozzle exit velocity aligned with the jet axis is investigated analytically, interpreting the optical measurements of Tanna et al. (1985). The noise-intensity minimum at slightly supersonic primary-flow velocities is related to the weakness of the primary-stream shock-cell structure and the lack of such a pattern in the outer fan stream. The discrepancies between this finding and those of Dosanjh et al. (1977 and 1978) are attributed to nozzle design, the definition of minimum noise, and different interpretative approaches. A first-order shock-cell model is then developed to derive formulas for the peak frequencies and the scaling of noise intensity. The results of computations using these formulas are presented in graphs and found to be in good agreement with the experimental data.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration (ISSN 0022-460X); 98; 115-125
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  • 8
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    On the relationship between broadband shock associated noise and screech tones (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: It is recognized that the three principal components of the noise of shock containing supersonic jets, namely, the dominant part of turbulent mixing noise, the broadband shock associated noise and the screech tones are all generated directly by the large scale turbulence structures/instability waves of the jet flow. In this paper the relationship between broadband shock associated noise and screech tones is examined. It is shown that from the spectral characteristics point of view, the screech tones may be regarded as special cases of broadband shock associated noise. Theoretical calculations for the peak frequencies of broadband shock associated noise and the fundamental frequencies of stable screech tones are carried out. The calculated results are found to agree favorably with experimental measurements.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 84-2276
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  • 9
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    A study of sound transmission in curved duct bends by the Galerkin method (1976)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: A computational procedure based on the Galerkin method is developed to study the sound transmission and reflection characteristics of circularly curved bends of rectangular ducts. This procedure is adaptable to routine computer calculation. An important component of the computer program consists of a QR iterative algorithm which solves the matrix eigenvalue problem generated by the Galerkin method. The present procedure produced very satisfactory convergent results when applied even to cases where the incident wave has high frequency and high wave mode numbers. Some properties of the Galerkin solution are investigated. Its relationship to the classical solution by the method of separation of variables is discussed.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration; 45; Mar. 8
    Format: text
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  • 10
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    Shock-associated noise reduction from inverted-velocity-profile coannular jets (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The reduction of shock-associated noise in inverted velocity profile coannular jets is investigated and quantified by making optical and acoustic measurements as a function of outer and inner stream pressure ratio combinations and interpreting noise results with the aid of new theoretical models. Schlieren photographs are obtained in three series of test conditions with a varying primary stream pressure ratio, and acoustic experiments are conducted on a polar arc radius of 3.05 m over a frequency range of 200 Hz to 80 kHz. A constant-thrust comparison is made, and data are plotted and discussed with emphasis on the variation of jet noise levels with primary stream Mach number and equivalent single jets on an absolute basis. It is shown that shock noise from the outer stream is almost eliminated when the inner stream is operated at a Mach number above unity at inverted and normal velocity profile conditions, and optical measurements indicate that for a fixed supercritical fan stream pressure ratio, the repetitive shock structure in the outer stream is minimized when the primary stream ratio becomes supercritical.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 81-1972 , Aeroacoustics Conference; Oct 05, 1981 - Oct 07, 1981; Palo Alto, CA
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