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  • 1
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    Analysis of acoustic radiation in a jet flow environment (1974)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: The analysis of sound fields from arbitrary source distributions in terms of Legendre and spherical Hankel functions is well known. The purpose of this paper is to extend this classical method of analysis to environments such as jet flows where flow and flow gradients are inherently present. The wave-equation governing the radiation of sound in such an environment is derived. The steady state flow and flow gradients in the axial and transverse directions appear as coefficients in the terms of the wave-equation. A semi-numerical method is used to solve the wave-equation in terms of modified spherical harmonics yielding the phase velocities and the directivities of an infinite set of modes. The directivity of each mode is obtained in terms of modified Legendre functions by numerical integration. Some results of these directivity and phase-velocity calculations are presented for a limited number of frequency and flow parameters. Both convective and shear refraction are shown to be important.
    Keywords: AERODYNAMICS
    Type: Journal of Sound and Vibration; 36; Sept. 8
    Format: text
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  • 2
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    Sound propagation in and radiation from acoustically lined flow ducts: A comparison of experiment and theory (1973)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The results of an experimental and theoretical study of many of the fundamental details of sound propagation in hard wall and soft wall annular flow ducts are reported. The theory of sound propagation along such ducts and the theory for determining the complex radiation impedance of higher order modes of an annulus are outlined, and methods for generating acoustic duct modes are developed. The results of a detailed measurement program on propagation in rigid wall annular ducts with and without airflow through the duct are presented. Techniques are described for measuring cut-on frequencies, modal phase speed, and radial and annular mode shapes. The effects of flow velocity on cut-on frequencies and phase speed are measured. Comparisons are made with theoretical predictions for all of the effects studies. The two microphone method of impedance is used to measure the effects of flow on acoustic liners. A numerical study of sound propagation in annular ducts with one or both walls acoustically lined is presented.
    Keywords: PHYSICS, GENERAL
    Type: NASA-CR-2306
    Format: application/pdf
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  • 3
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    Sound measurements within and in the radiated field of an annular duct with flow. (1973)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Recent theory on sound propagation for flow ducts initiated a research program (sponsored by NASA-Langley, Contract NAS1-10472) which has the objective of comparing detailed acoustical measurements with the analytical predictions for sound propagation in a duct with sheared flow. The effects considered experimentally mean flow, shear, wall impedance, and source distribution on sound propagation within and radiation from finite lenght annular ducts. Results reported here are confined to acoustic distribution in hard annular ducts with and without mean flow and radiated field patterns for the same conditions. The acoustic distributions are shown for single and multi-mode excitation. The flow duct facility is also described.
    Keywords: FLUID MECHANICS
    Type: Journal of Sound and Vibration; 28; June 22
    Format: text
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  • 4
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    Effects of forward velocity on turbulent jet mixing noise (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Flight simulation experiments were conducted in an anechoic free jet facility over a broad range of model and free jet velocities. The resulting scaling laws were in close agreement with scaling laws derived from theoretical and semiempirical considerations. Additionally, measurements of the flow structure of jets were made in a wind tunnel by using a laser velocimeter. These tests were conducted to describe the effects of velocity ratio and jet exit Mach number on the development of a jet in a coflowing stream. These turbulence measurements and a simplified Lighthill radiation model were used in predicting the variation in radiated noise at 90 deg to the jet axis with velocity ratio. Finally, the influence of forward motion on flow-acoustic interactions was examined through a reinterpretation of the 'static' numerical solutions to the Lilley equation.
    Keywords: ACOUSTICS
    Type: NASA-CR-2702
    Format: application/pdf
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  • 5
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    Studies of the acoustic transmission characteristics of coaxial nozzles with inverted velocity profiles: Comprehensive data report (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The efficiency of internal noise radiation through a coannular exhaust nozzle with an inverted velocity profile was studied. A preliminary investigation was first undertaken (1) to define the test parameters which influence the internal noise radiation; (2) to develop a test methodology which could realistically be used to examine the effects of the test parameters; and (3) to validate this methodology. The result was the choice of an acoustic impulse as the internal noise source in the jet nozzles. Noise transmission characteristics of a coannular nozzle system were then investigated. In particular, the effects of fan convergence angle, core extension length to annulus height ratio and flow Mach numbers and temperatures were studied. Relevant spectral data only is presented in the form of normalized nozzle transfer function versus nondimensional frequency.
    Keywords: ACOUSTICS
    Type: NASA-CR-159628
    Format: application/pdf
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  • 6
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    Radiated sound field measurements from an annular flow duct facility (1973)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: A series of measured radiated field patterns correlated with defined in-duct conditions is presented, along with a second series of radiation measurements for multiple mode propagation, with individual mode amplitudes and phases at duct termination as well as with angular positions of higher-order stationary duct modes relative to the plane of the radiated field measurement. Though incomplete, these experimental data should prove useful for comparisons with theoretical predictions in testing analytical procedures and prediction techniques.
    Keywords: FACILITIES, RESEARCH, AND SUPPORT
    Type: Journal of Sound and Vibration; 30; Oct. 8
    Format: text
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  • 7
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    Measuring acoustic properties of materials and jet nozzles (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Method measures acoustic properties of sound-absorbent materials and jet-nozzle system. Advantages of impulse method over other methods are that test time and complication are reduced. Results obtained from impulse method have been compared with those from existing methods, both experimental and theoretical, and show excellent agreement.
    Keywords: MECHANICS
    Type: LEW-13265 , NASA Tech Briefs (ISSN 0145-319X); 4; 4; P. 523
    Format: text
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  • 8
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    Effect of jet temperature on jet and pure tone noise radiation (1969)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Temperature effects on turbofan and turbojet noise radiation
    Keywords: AIRCRAFT
    Type: NASA-CR-1472
    Format: application/pdf
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  • 9
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    A theoretical and experimental study of sound attenuation in an annular duct. (1973)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The flow duct used in the experiments for this study was described in an earlier paper by Plumblee and Dean (1972). Angular and radial standing modes were excited in isolation, and the modal attenuation and phase speed were measured in an annular duct with locally reacting liners. Contour maps of constant attenuation and phase speed were computed in the wall impedance plane for no-flow and several flow conditions, and experimental data were compared with some of these calculations. The effect of flow on modal attenuation and phase speed was measured and is theoretically verified. The major result of this study is the verification of theoretical predictions which ultimately can lead to a duct liner design optimization without resort to expensive and lengthy test programs.
    Keywords: FLUID MECHANICS
    Type: AIAA PAPER 73-1005 , Aero-Acoustics Conference; Oct 15, 1973 - Oct 17, 1973; Seattle, WA
    Format: text
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  • 10
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    Studies of the acoustic transmission characteristics of coaxial nozzles with inverted velocity profiles, volume 1 (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The efficiency of internal noise radiation through coannular exhaust nozzle with an inverted velocity profile was studied. A preliminary investigation was first undertaken to: (1) define the test parameters which influence the internal noise radiation; (2) develop a test methodology which could realistically be used to examine the effects of the test parameters; (3) and to validate this methodology. The result was the choice of an acoustic impulse as the internal noise source in the in the jet nozzles. Noise transmission characteristics of a nozzle system were then investigated. In particular, the effects of fan nozzle convergence angle, core extention length to annulus height ratio, and flow Mach number and temperatures were studied. The results are presented as normalized directivity plots.
    Keywords: ACOUSTICS
    Type: NASA-CR-159698 , LG79ER0178-VOL-1
    Format: application/pdf
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