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  • 1
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    Validation of an Impedance Education Method in Flow (2004)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper reports results of a research effort to validate a method for educing the normal incidence impedance of a locally reacting liner, located in a grazing incidence, nonprogressive acoustic wave environment with flow. The results presented in this paper test the ability of the method to reproduce the measured normal incidence impedance of a solid steel plate and two soft test liners in a uniform flow. The test liners are known to be locally react- ing and exhibit no measurable amplitude-dependent impedance nonlinearities or flow effects. Baseline impedance spectra for these liners were therefore established from measurements in a conventional normal incidence impedance tube. A key feature of the method is the expansion of the unknown impedance function as a piecewise continuous polynomial with undetermined coefficients. Stewart's adaptation of the Davidon-Fletcher-Powell optimization algorithm is used to educe the normal incidence impedance at each Mach number by optimizing an objective function. The method is shown to reproduce the measured normal incidence impedance spectrum for each of the test liners, thus validating its usefulness for determining the normal incidence impedance of test liners for a broad range of source frequencies and flow Mach numbers. Nomenclature
    Keywords: Acoustics
    Type: AIAA-98-2279 , 4th AIAA/CEAS; Jun 02, 1998 - Jun 04, 1998; Toulouse; France
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  • 2
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    Improved Broadband Liner Optimization Applied to the Advanced Noise Control Fan (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The broadband component of fan noise has grown in relevance with the utilization of increased bypass ratio and advanced fan designs. Thus, while the attenuation of fan tones remains paramount, the ability to simultaneously reduce broadband fan noise levels has become more desirable. This paper describes improvements to a previously established broadband acoustic liner optimization process using the Advanced Noise Control Fan rig as a demonstrator. Specifically, in-duct attenuation predictions with a statistical source model are used to obtain optimum impedance spectra over the conditions of interest. The predicted optimum impedance information is then used with acoustic liner modeling tools to design liners aimed at producing impedance spectra that most closely match the predicted optimum values. Design selection is based on an acceptance criterion that provides the ability to apply increased weighting to specific frequencies and/or operating conditions. Constant-depth, double-degree of freedom and variable-depth, multi-degree of freedom designs are carried through design, fabrication, and testing to validate the efficacy of the design process. Results illustrate the value of the design process in concurrently evaluating the relative costs/benefits of these liner designs. This study also provides an application for demonstrating the integrated use of duct acoustic propagation/radiation and liner modeling tools in the design and evaluation of novel broadband liner concepts for complex engine configurations.
    Keywords: Acoustics
    Type: NF1676L-17755 , AIAA/CEAS Aeroacoustics Conference; Jun 16, 2014 - Jun 20, 2014; Atlanta, GA; United States
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  • 3
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    Experimental Validation of Numerical Simulations for an Acoustic Liner in Grazing Flow (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A coordinated experimental and numerical simulation effort is carried out to improve our understanding of the physics of acoustic liners in a grazing flow as well our computational aeroacoustics (CAA) method prediction capability. A numerical simulation code based on advanced CAA methods is developed. In a parallel effort, experiments are performed using the Grazing Flow Impedance Tube at the NASA Langley Research Center. In the experiment, a liner is installed in the upper wall of a rectangular flow duct with a 2 inch by 2.5 inch cross section. Spatial distribution of sound pressure levels and relative phases are measured on the wall opposite the liner in the presence of a Mach 0.3 grazing flow. The computer code is validated by comparing computed results with experimental measurements. Good agreements are found. The numerical simulation code is then used to investigate the physical properties of the acoustic liner. It is shown that an acoustic liner can produce self-noise in the presence of a grazing flow and that a feedback acoustic resonance mechanism is responsible for the generation of this liner self-noise. In addition, the same mechanism also creates additional liner drag. An estimate, based on numerical simulation data, indicates that for a resonant liner with a 10% open area ratio, the drag increase would be about 4% of the turbulent boundary layer drag over a flat wall.
    Keywords: Acoustics
    Type: AIAA Paper 2013-2222 , NF1676L-15497 , 19th AIAA/CEAS Aeroacoustics Conference; May 27, 2013 - May 29, 2013; Berlin; Germany
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  • 4
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    A Comparative Study of Four Impedance Eduction Methodologies Using Several Test Liners (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A comparative study of four commonly used impedance eduction methods is presented for a range of liner structures and test conditions. Two of the methods are restricted to uniform flow while the other two accommodate both uniform and boundary layer flows. Measurements on five liner structures (a rigid-wall insert, a ceramic tubular liner, a wire mesh liner, a low porosity conventional liner, and a high porosity conventional liner) are obtained using the NASA Langley Grazing Flow Impedance Tube. The educed impedance of each liner is presented for forty-two test conditions (three Mach numbers and fourteen frequencies). In addition, the effects of moving the acoustic source from upstream to downstream and the refractive effects of the mean boundary layer on the wire mesh liner are investigated. The primary conclusions of the study are that: (1) more accurate results are obtained for the upstream source, (2) the uniform flow methods produce nearly identical impedance spectra at and below Mach 0.3 but significant scatter in the educed impedance occurs at the higher Mach number, (3) there is better agreement in educed impedance among the methods for the conventional liners than for the rigid-wall insert, ceramic, or wire mesh liner, and (4) the refractive effects of the mean boundary layer on the educed impedance of the wire mesh liner are generally small except at Mach 0.5.
    Keywords: Acoustics
    Type: AIAA Paper 2013-2274 , NF1676L-15493 , 19th AIAA/CEAS Aeroacoustics Conference; May 27, 2013 - May 29, 2013; Berlin; Germany
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  • 5
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    Comparative Study of Impedance Eduction Methods, Part 2: NASA Tests and Methodology (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A number of methods have been developed at NASA Langley Research Center for eduction of the acoustic impedance of sound-absorbing liners mounted in the wall of a flow duct. This investigation uses methods based on the Pridmore-Brown and convected Helmholtz equations to study the acoustic behavior of a single-layer, conventional liner fabricated by the German Aerospace Center and tested in the NASA Langley Grazing Flow Impedance Tube. Two key assumptions are explored in this portion of the investigation. First, a comparison of results achieved with uniform-flow and shear-flow impedance eduction methods is considered. Also, an approach based on the Prony method is used to extend these methods from single-mode to multi-mode implementations. Finally, a detailed investigation into the effects of harmonic distortion on the educed impedance is performed, and the results are used to develop guidelines regarding acceptable levels of harmonic distortion
    Keywords: Acoustics
    Type: NF1676L-15560 , AIAA Paper 2013-2125 , 19th AIAA/CEAS Aeroacoustics Conference; May 27, 2013 - May 29, 2013; Berlin; Germany
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  • 6
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    Comparative Study of Impedance Eduction Methods (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The absorption efficiency of acoustic liners used in aircraft engines is characterized by the acoustic impedance. World wide, many grazing ow test rigs and eduction methods are available that provide values for that impedance. However, a direct comparison and assessment of the data of the di erent rigs and methods is often not possible because test objects and test conditions are quite di erent. Only a few papers provide a direct comparison. Therefore, this paper together with a companion paper, present data measured with a reference test object under similar conditions in the DLR and NASA grazing ow test rigs. Additionally, by applying the in-house methods Liner Impedance Non-Uniform ow Solving algorithm (LINUS, DLR) and Convected Helmhholtz Equation approach (CHE, NASA) on the data sets, similarities and differences due to underlying theory are identi ed and discussed.
    Keywords: Acoustics
    Type: NF1676L-15559 , 19th AIAA/CEAS Aeroacoustics Conference; May 27, 2013 - May 29, 2013; Berlin; Germany
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  • 7
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    ONERA-NASA Cooperative Effort on Liner Impedance Eduction (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: As part of a cooperation between ONERA and NASA, the liner impedance eduction methods developed by the two research centers are compared. The NASA technique relies on an objective function built on acoustic pressure measurements located on the wall opposite the test liner, and the propagation code solves the convected Helmholtz equation in uniform ow using a finite element method that implements a continuous Galerkin discretization. The ONERA method uses an objective function based either on wall acoustic pressure or on acoustic velocity acquired above the liner by Laser Doppler Anemometry, and the propagation code solves the linearized Euler equations by a discontinuous Galerkin discretization. Two acoustic liners are tested in both ONERA and NASA ow ducts and the measured data are treated with the corresponding impedance eduction method. The first liner is a wire mesh facesheet mounted onto a honeycomb core, designed to be linear with respect to incident sound pressure level and to grazing ow velocity. The second one is a conventional, nonlinear, perforate-over-honeycomb single layer liner. Configurations without and with ow are considered. For the nonlinear liner, the comparison of liner impedance educed by NASA and ONERA shows a sensitivity to the experimental conditions, namely to the nature of the source and to the sample width.
    Keywords: Acoustics
    Type: NF1676L-15558 , 19th AIAA/CEAS Aeroacoustics Conference; May 27, 2013 - May 29, 2013; Berlin; Germany
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  • 8
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    Segmented Liner to Control Mode Scattering (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The acoustic performance of duct liners can be improved by segmenting the treatment. In a segmented liner treatment, one stage of liner reduces the target sound and scatters energy into other acoustic modes, which are attenuated by a subsequent stage. The Curved Duct Test Rig is an experimental facility in which sound incident on the liner can be generated in a specific mode and the scatter of energy into other modes can be quantified. A series of experiments is performed in which the baseline configuration is asymmetric, that is, a liner is on one side wall of the test duct and the wall opposite is acoustically hard. Segmented liner treatment is achieved by progressively replacing sections of the hard wall opposite with liner in the axial direction, from 25% of the wall surface to 100%. It is found that the energy scatter from the (0,0) to the (0,1) mode reduces as the percentage of opposite wall treatment increases, and the frequency of peak attenuation shifts toward higher frequency. Similar results are found when the incident mode is of order (0,1) and scatter is into the (0,0) mode. The propagation code CDUCT-LaRC is used to predict the effect of liner segmenting on liner performance. The computational results show energy scatter and the effect of liner segmentation that agrees with the experimental results. The experiments and computations both show that segmenting the liner treatment is effective to control the scatter of incident mode energy into other modes. CDUCT-LaRC is shown to be a valuable tool to predict trends of liner performance with liner configuration.
    Keywords: Acoustics
    Type: NF1676L-15522 , AIAA Paper 2013-2078 , 19th AIAA/CEAS Aeroacoustics Conference; May 27, 2013 - May 29, 2013; Berlin; Germany
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  • 9
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    Communal Sensor Network for Adaptive Noise Reduction in Aircraft Engine Nacelles (2011)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Emergent behavior, a subject of much research in biology, sociology, and economics, is a foundational element of Complex Systems Science and is apropos in the design of sensor network systems. To demonstrate engineering for emergent behavior, a novel approach in the design of a sensor/actuator network is presented maintaining optimal noise attenuation as an adaptation to changing acoustic conditions. Rather than use the conventional approach where sensors are managed by a central controller, this new paradigm uses a biomimetic model where sensor/actuators cooperate as a community of autonomous organisms, sharing with neighbors to control impedance based on local information. From the combination of all individual actions, an optimal attenuation emerges for the global system.
    Keywords: Acoustics
    Type: NF1676L-12746 , IEEE Sensors 2011 Conference; Oct 28, 2011 - Oct 31, 2011; Limerick; Ireland
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  • 10
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    Broadband Liner Optimization for the Source Diagnostic Test Fan (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The broadband component of fan noise has grown in relevance with the utilization of increased bypass ratio and advanced fan designs. Thus, while the attenuation of fan tones remains paramount, the ability to simultaneously reduce broadband fan noise levels has become more appealing. This paper describes a broadband acoustic liner optimization study for the scale model Source Diagnostic Test fan. Specifically, in-duct attenuation predictions with a statistical fan source model are used to obtain optimum impedance spectra over a number of flow conditions for three liner locations in the bypass duct. The predicted optimum impedance information is then used with acoustic liner modeling tools to design liners aimed at producing impedance spectra that most closely match the predicted optimum values. Design selection is based on an acceptance criterion that provides the ability to apply increased weighting to specific frequencies and/or operating conditions. Typical tonal liner designs targeting single frequencies at one operating condition are first produced to provide baseline performance information. These are followed by multiple broadband design approaches culminating in a broadband liner targeting the full range of frequencies and operating conditions. The broadband liner is found to satisfy the optimum impedance objectives much better than the tonal liner designs. In addition, the broadband liner is found to provide better attenuation than the tonal designs over the full range of frequencies and operating conditions considered. Thus, the current study successfully establishes a process for the initial design and evaluation of novel broadband liner concepts for complex engine configurations.
    Keywords: Acoustics
    Type: NF1676L-13790 , 18th AIAA/CEAS Aeroacoustics Conference; Jun 04, 2012 - Jun 06, 2012; Colorado Springs, CO; United States
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