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  • 1
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    A Finite Element Propagation Model for Extracting Normal Incidence Impedance in Nonprogressive Acoustic Wave Fields (1996)
    Elsevier
    Details
    Publication Date: 1996-04-01
    Print ISSN: 0021-9991
    Electronic ISSN: 1090-2716
    Topics: Computer Science , Physics
    Published by Elsevier
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    PAPER CURRENT
  • 2
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    Sound transmission through a high-temperature acoustic probe tube (1992)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Keywords: ACOUSTICS
    Type: AIAA Journal (ISSN 0001-1452); 30; 318-323
    Format: text
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    NASA TECHNICAL REPORTS
  • 3
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    Evaluation of a multi-point method for determining acoustic impedance (1989)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: A multipoint method for determining acoustic impedance was evaluated in comparison with the traditional standing wave and two-microphone methods using 30 test samples covering the reflection factor magnitude range 0.004-0.999. The multipoint method is shown to combine the strengths of the standing wave and two-microphone methods while avoiding some of their inherent weaknesses. In particular, the results obtained suggest that the multipoint method will be less subject to flow induced random error than the two-microphone method in the presence of significant broadband noise levels associated with mean flow.
    Keywords: ACOUSTICS
    Type: Mechanical Systems and Signal Processing (ISSN 0888-3270); 3; 15-35
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    High Temperature Acoustic Liner Technology (1999)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: This paper describes work currently in progress at Langley on liner concepts that employ structures that may be suitable for broadband exhaust noise attenuation in high speed flow environments and at elevated temperatures characteristic of HSCT applications. Because such liners will need to provide about 10 dB suppression over a 2 to 3 octave frequency range, conventional single-degree-of-freedom resonant structures will not suffice. Bulk absorbers have the needed broadband absorption characteristic; however, at lower frequencies they tend to be inefficient.
    Keywords: Aircraft Propulsion and Power
    Type: First NASA/Industry High Speed Research Program Nozzle Symposium; 34-1 - 34-17; NASA/CP-1999-209423
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 5
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    Validation of a Numerical Method for Determining Liner Impedance (1996)
    In:  CASI
    Details
    Publication Date: 2018-06-05
    Description: This paper reports the initial results of a test series to evaluate a method for determining the normal incidence impedance of a locally reacting acoustically absorbing liner, located on the lower wall of a duct in a grazing incidence, multi-modal, non-progressive acoustic wave environment without flow. This initial evaluation is accomplished by testing the methods' ability to converge to the known normal incidence impedance of a solid steel plate, and to the normal incidence impedance of an absorbing test specimen whose impedance was measured in a conventional normal incidence tube. The method is shown to converge to the normal incident impedance values and thus to be an adequate tool for determining the impedance of specimens in a grazing incidence, multi-modal, nonprogressive acoustic wave environment for a broad range of source frequencies.
    Keywords: Aircraft Design, Testing and Performance
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 6
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    Experimental validation of a two-dimensional shear-flow model for determining acoustic impedance (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Tests were conducted to validate a two-dimensional shear-flow analytical model for determining the acoustic impedance of a liner test specimen in a grazing-incidence, grazing-flow environment. The tests were limited to a test specimen chosen to exhibit minimal effects of grazing flow so that the results obtained by using the shear-flow analytical model would be expected to match those obtained from normal-incidence impedance measurements. Impedances for both downstream and upstream sound propagation were generally consistent with those from normal-incidence measurements. However, sensitivity of the grazing-incidence impedance to small measurement or systematic errors in propagation constant varied dramatically over the range of test frequencies.
    Keywords: ACOUSTICS
    Type: NASA-TP-2679 , L-16203 , NAS 1.60:2679
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 7
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    Unsteady pressure loads in a generic high speed engine model (1992)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Unsteady pressure loads were measured along the top interior wall of a generic high-speed engine (GHSE) model undergoing performance tests in the combustion-Heated Scramjet Test Facility at the Langley Research Center. Flow to the model inlet was simulated at 72000 ft and a flight Mach number of 4. The inlet Mach number was 3.5 with a total temperature and pressure of 1640 R and 92 psia. The unsteady pressure loads were measured with 5 piezoresistive gages, recessed into the wall 4 to 12 gage diameters to reduce incident heat flux to the diaphragms, and distributed from the inlet to the combustor. Contributors to the unsteady pressure loads included boundary layer turbulence, combustion noise, and transients generated by unstart loads. Typical turbulent boundary layer rms pressures in the inlet ranged from 133 dB in the inlet to 181 dB in the combustor over the frequency range from 0 to 5 kHz. Downstream of the inlet exist, combustion noise was shown to dominate boundary layer turbulence noise at increased heat release rates. Noise levels in the isolator section increased by 15 dB when the fuel-air ratio was increased from 0.37 to 0.57 of the stoichiometric ratio. Transient pressure disturbances associated with engine unstarts were measured in the inlet and have an upstream propagation speed of about 7 ft/sec and pressure jumps of at least 3 psia.
    Keywords: ACOUSTICS
    Type: NASA-TP-3189 , L-16912 , NAS 1.60:3189
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 8
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    A finite element propagation model for extracting normal incidence impedance in nonprogressive acoustic wave fields (1995)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A propagation model method for extracting the normal incidence impedance of an acoustic material installed as a finite length segment in a wall of a duct carrying a nonprogressive wave field is presented. The method recasts the determination of the unknown impedance as the minimization of the normalized wall pressure error function. A finite element propagation model is combined with a coarse/fine grid impedance plane search technique to extract the impedance of the material. Results are presented for three different materials for which the impedance is known. For each material, the input data required for the prediction scheme was computed from modal theory and then contaminated by random error. The finite element method reproduces the known impedance of each material almost exactly for random errors typical of those found in many measurement environments. Thus, the method developed here provides a means for determining the impedance of materials in a nonprogressirve wave environment such as that usually encountered in a commercial aircraft engine and most laboratory settings.
    Keywords: ACOUSTICS
    Type: NASA-TM-110160 , NAS 1.15:110160
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 9
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    Evaluation of a multi-point method for determining acoustic impedance (1988)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: An investigation was conducted to explore potential improvements provided by a Multi-Point Method (MPM) over the Standing Wave Method (SWM) and Two-Microphone Method (TMM) for determining acoustic impedance. A wave propagation model was developed to model the standing wave pattern in an impedance tube. The acoustic impedance of a test specimen was calculated from a best fit of this standing wave pattern to pressure measurements obtained along the impedance tube centerline. Three measurement spacing distributions were examined: uniform, random, and selective. Calculated standing wave patterns match the point pressure measurement distributions with good agreement for a reflection factor magnitude range of 0.004 to 0.999. Comparisons of results using 2, 3, 6, and 18 measurement points showed that the most consistent results are obtained when using at least 6 evenly spaced pressure measurements per half-wavelength. Also, data were acquired with broadband noise added to the discrete frequency noise and impedances were calculated using the MPM and TMM algorithms. The results indicate that the MPM will be superior to the TMM in the presence of significant broadband noise levels associated with mean flow.
    Keywords: ACOUSTICS
    Type: NASA-TM-100637 , NAS 1.15:100637
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 10
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    Sound transmission through a high-temperature acoustic probe tube (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: An investigation was conducted of acoustic transmission through a tube subjected to an intense thermal gradient along its axis. The results are of interest in the interpretation of acoustic data from probe tube configurations designed to measure fluctuating pressures in high temperature environments. The measured transfer function across a localized heated region in the tube was compared to a computed transfer function based on a theoretical analysis of propagation through strong temperature gradients. Over the frequency range 0.4 kHz to 6.0 kHz, generally good agreement was obtained between the measured and calculated attenuation across the heated region with some discrepancy occurring at the attenuation minima. Agreement between measured and calculated phase difference was excellent to within the measurement resolution.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 90-3991
    Format: text
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    NASA TECHNICAL REPORTS
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