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  • 1
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    Verification of a one-dimensional analysis of sound propagation in a variable area duct without flow (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: (Previously announced in STAR as N82-12891)
    Keywords: ACOUSTICS
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    NASA TECHNICAL REPORTS
  • 2
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    Pressure transfer function of a JT15D nozzle due to acoustic and convected entropy fluctuations (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: (Previously announced in STAR as N82-22951)
    Keywords: ACOUSTICS
    Type: Acoustical Society of America; vol. 72
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  • 3
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    Cross spectra between temperature and pressure in a constant area duct downstream of a combustor (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The feasibility of measuring pressure temperature cross spectra and coherence and temperature-temperature cross spectra and coherence at spatially separated points along with pressure and temperature auto-spectra in a combustion rig was investigated. The measurements were made near the inlet and exit of a 6.44 m long duct attached to a J-47 combustor. The fuel used was Jet A. The cross spectra and coherence measurements show the pressure and temperature fluctuations correlate best at low frequencies. At the inlet the phenomena controlling the phase relationship between pressure and temperature could not be identified. However, at the duct exit the phase angle of the pressure is related to the phase angle of the temperature by the convected flow time delay. Previously announced in STAR as N83-23116
    Keywords: ACOUSTICS
    Type: AIAA PAPER 83-0762
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  • 4
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    Analysis of pressure spectra measurements in a ducted combustion system (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Combustion noise propagation in an operating ducted liquid fuel combustion system is studied in relation to the development of combustion noise prediction and suppression techniques. The presence of combustor emissions in the duct is proposed as the primary mechanism producing the attenuation and dispersion of combustion noise propagating in an operating liquid fuel combustion system. First, a complex mathematical model for calculating attenuation and dispersion taking into account mass transfer, heat transfer, and viscosity effects due to the presence of liquid fuel droplets or solid soot particles is discussed. Next, a simpler single parameter model for calculating pressure auto-spectra and cross-spectra which takes into account dispersion and attenuation due to heat transfer between solid soot particles and air is developed. Then, auto-spectra and cross-spectra obtained from internal pressure measurements in a combustion system consisting of a J-47 combustor can, a spool piece, and a long duct are presented. Last, analytical results obtained with the single parameter model are compared with the experimental measurements. The single parameter model results are shown to be in excellent agreement with the measurements.
    Keywords: ACOUSTICS
    Type: NASA-TM-81583 , E-558
    Format: application/pdf
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  • 5
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    Pressure transfer function of a JT15D nozzle due to acoustic and convected entropy fluctuations (1982)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An acoustic transmission matrix analysis of sound propagation in a variable area duct with and without flow is extended to include convected entropy fluctuations. The boundary conditions used in the analysis are a transfer function relating entropy and pressure at the nozzle inlet and the nozzle exit impedance. The nozzle pressure transfer function calculated is compared with JT15D turbofan engine nozzle data. The one dimensional theory for sound propagation in a variable area nozzle with flow but without convected entropy is good at the low engine speeds where the nozzle exit Mach number is low (M=0.2) and the duct exit impedance model is good. The effect of convected entropy appears to be so negligible that it is obscured by the inaccuracy of the nozzle exit impedance model, the lack of information on the magnitude of the convected entropy and its phase relationship with the pressure, and the scatter in the data. An improved duct exit impedance model is required at the higher engine speeds where the nozzle exit Mach number is high (M=0.56) and at low frequencies (below 120 Hz).
    Keywords: ACOUSTICS
    Type: NASA-TM-82842 , E-1212 , NAS 1.15:82842 , Meeting of the Acoust. Soc. of Am.,; Apr 26, 1982 - Apr 30, 1982; Chicago, IL; United States
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  • 6
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    Dispersion of sound in a combustion duct by fuel droplets and soot particles (1979)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Dispersion and attenuation of acoustic plane wave disturbances propagating in a ducted combustion system are studied. The dispersion and attenuation are caused by fuel droplet and soot emissions from a jet engine combustor. The attenuation and dispersion are due to heat transfer and mass transfer and viscous drag forces between the emissions and the ambient gas. Theoretical calculations show sound propagation at speeds below the isentropic speed of sound at low frequencies. Experimental results are in good agreement with the theory.
    Keywords: ACOUSTICS
    Type: Meeting; Nov 26, 1979 - Nov 30, 1979; Salt Lake City, UT
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  • 7
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    Spectral structure of pressure measurements made in a combustion duct (1980)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A model for acoustic plane wave propagation in a combustion duct through a confined, flowing gas containing soot particles is presented. The model takes into account only heat transfer between the gas and soot particles. As a result, the model depends on only a single parameter which can be written as the ratio of the soot particle thermal relaxation time to the soot particle mass fraction. The model yields expressions for the attenuation and dispersion of the plane wave which depends only on this single parameter. The model was used to calculate pressure spectra in a combustion duct. The results were compared with measured spectra. For particular values of the single free parameter, the calculated spectra resemble the measured spectra. Consequently, the model, to this extent, explains the experimental measurements and provides some insight into the number and type of particles.
    Keywords: ACOUSTICS
    Type: NASA-TM-81471 , E-404 , Meeting of the Acoust. Soc. of Am.; Apr 21, 1980 - Apr 25, 1980; Atlanta
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  • 8
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    Pressure spectra and cross spectra at an area contraction in a ducted combustion system (1980)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Pressure spectra and cross-spectra at an area contraction in a liquid fuel, ducted, combustion noise test facility are analyzed. Measurements made over a range of air and fuel flows are discussed. Measured spectra are compared with spectra calculated using a simple analytical model.
    Keywords: ACOUSTICS
    Type: NASA-TM-81477 , E-411 , 1980 ASME Aerospace Conf.; Aug 13, 1980 - Aug 15, 1980; San Francisco
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  • 9
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    Analysis and correction of ground reflection effects in measured narrowband sound spectra using cepstral techniques (1975)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Ground reflections generate undesirable effects on acoustic measurements such as those conducted outdoors for jet noise research, aircraft certification, and motor vehicle regulation. Cepstral techniques developed in speech processing are adapted to identify echo delay time and to correct for ground reflection effects. A sample result is presented using an actual narrowband sound pressure level spectrum. The technique can readily be adapted to existing fast Fourier transform type spectrum measurement instrumentation to provide field measurements/of echo time delays.
    Keywords: ACOUSTICS
    Type: NASA-TM-X-71810 , E-8503 , Meeting of the Acoustical Soc. of Am.; Nov 04, 1975 - Nov 07, 1975; San Francisco, CA; United States
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  • 10
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    Analysis of ground reflection of jet noise obtained with various microphone arrays over an asphalt surface (1975)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Ground reflection effects on the propagation of jet noise over an asphalt surface are discussed for data obtained using a 33.02 cm (13-in.) diameter nozzle with microphones at several heights and distances from the nozzle axis. Analysis of ground reflection effects is accomplished using the concept of a reflected signal transfer function which represents the influence of both the reflecting surface and the atmosphere on the propagation of the reflected signal in a mathematical model. The mathematical model used as a basis for the computer program was successful in significantly reducing the ground reflection effects. The range of values of the single complex number used to define the reflected signal transfer function was larger than expected when determined only by the asphalt surface. This may indicate that the atmosphere is affecting the propagation of the reflected signal more than the asphalt surface. Also discussed is the selective placement of the reinforcements and cancellations in the design of an experiment to minimize ground reflection effects.
    Keywords: ACOUSTICS
    Type: Acoustical Society of America, Meeting; Apr 07, 1975 - Apr 11, 1975; Austin, TX
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