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  • 1
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    Investigation of acoustical shielding by a wedge-shaped airframe (2006)
    Elsevier
    Details
    Publication Date: 2006-06-01
    Print ISSN: 0022-460X
    Electronic ISSN: 1095-8568
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Elsevier
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    PAPER CURRENT
  • 2
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    Active control of flexural vibrations in beams (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The feasibility of using piezoelectric actuators to control the flexural oscillations of large structures in space is investigated. Flexural oscillations are excited by impulsive loads. The vibratory response can degrade the pointing accuracy of cameras and antennae, and can cause high stresses at structural node points. Piezoelectric actuators have the advantage of exerting localized bending moments. In this way, vibration is controlled without exciting rigid body modes. The actuators are used in collocated sensor/driver pairs to form a feedback control system. The sensor produces a voltage that is proportional to the dynamic stress at the sensor location, and the driver produces a force that is proportional to the voltage applied to it. The analog control system amplifies and phase shifts the sensor signal to produce the voltage signal that is applied to the driver. The feedback control is demonstrated to increase the first mode damping in a cantilever beam by up to 100 percent, depending on the amplifier gain. The damping efficiency of the control system when the piezoelectrics are not optimally positioned at points of high stress in the beam is evaluated.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA. Johnson Space Center, NASA(ASEE Summer Faculty Fellowship Program, 1987, Volume 1; 20 p
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    Active control of flexural vibrations in beams (1989)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: An analytical model of the feedback control system which estimates the voltage generated by the piezoelectric sensor as a function of the dynamic stress at the sensor location and the force exerted by the driver piezoelectric as a function of signal gain is developed. The analytical results are compared to measured results for a cantilever beam excited to vibrate in its first natural mode. The estimated increase in the first mode damping factor is in good agreement with the measured results.
    Keywords: CYBERNETICS
    Type: Journal of Aerospace Engineering (ISSN 0893-1321); 2; 141-154
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    Active vibration control in microgravity environment (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The low gravity environment of the space station is suitable for experiments or manufacturing processes which require near zero gravity. An experiment was fabricated to test the validity of the active control process and to verify the flow and control parameters identified in a theoretical model. Zero gravity is approximated in the horizontal plane using a low friction air bearing table. An analog control system was designed to activate calibrated air jets when displacement of the test mass is sensed. The experiment demonstrates that an air jet control system introduces an effective damping factor to control oscillatory response. The amount of damping as well as the flow parameters, such as pressure drop across the valve and flow rate of air, are verified by the analytical model.
    Keywords: MATERIALS PROCESSING
    Type: NASA. Lyndon B.; NASA. Lyndon B. John
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 5
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    Active control of fan-generated plane wave noise (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Subsonic propulsion systems for future aircraft may incorporate ultra-high bypass ratio ducted fan engines whose dominant noise source is the fan with blade passage frequency less than 1000 Hz. This low frequency combines with the requirement of a short nacelle to diminish the effectiveness of passive duct liners. Active noise control is seen as a viable method to augment the conventional passive treatments. An experiment to control ducted fan noise using a time domain active adaptive system is reported. The control sound source consists of loudspeakers arrayed around the fan duct. The error sensor location is in the fan duct. The purpose of this experiment is to demonstrate that the in-duct error sensor reduces the mode spillover in the far field, thereby increasing the efficiency of the control system. In this first series of tests, the fan is configured so that predominantly zero order circumferential waves are generated. The control system is found to reduce the blade passage frequency tone significantly in the acoustic far field when the mode orders of the noise source and of the control source are the same. The noise reduction is not as great when the mode orders are not the same even though the noise source modes are evanescent, but the control system converges stably and global noise reduction is demonstrated in the far field. Further experimentation is planned in which the performance of the system will be evaluated when higher order radial and spinning modes are generated.
    Keywords: ACOUSTICS
    Type: NASA-TM-109008 , NAS 1.15:109008
    Format: application/pdf
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  • 6
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    Application of beamform to sound propagation in the atmosphere (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The feasibility of applying beam-former technology (i.e., the technique that uses the combined signal from an array of sensors as a spatial filter to locate sound sources) in atmospheric acoustics is examined. Special attention is given to the microphone array, data collection, and signal processing technology as well as to an array processing algorithm. It is shown that this algorithm, which incorporates the maximum-likelihood method, is operational. Also discussed is a postprocessing algorithm for eliminating the ghosting sidelobes that arise from the moving sources.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 90-3990
    Format: text
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  • 7
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    Inlet Noise Reduction by Shielding for the Blended-Wing-Body Airplane (1999)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Noise shielding benefits associated with an advanced unconventional subsonic transport concept, the Blended-Wing-Body, were studied using a 4- percent scale, 3-engine nacelle model. The study was conducted in the Anechoic Noise Research Facility at NASA Langley Research Center. A high- frequency, wideband point source was placed inside the nacelles of the center engine and one of the side engines in order to simulate broadband engine noise. The sound field of the model was measured with a rotating microphone array that was moved to various stations along the model axis and with a fixed array of microphones that was erected behind the model. Ten rotating microphones were traversed a total of 22 degrees in 2-degree increments. Seven fixed microphones covered an arc that extended from a point in the exhaust exit plane of the center engine (and directly below its centerline) to a point 30 degrees above the jet centerline. While no attempt was made to simulate the noise emission characteristics of an aircraft engine, the model source was intended to radiate sound in a frequency range encompassing 1, 2, and 3 times the blade passage of a typical full-scale engine. In this study, the Blended-Wing-Body model was found to provide significant shielding of inlet noise. In particular, noise radiated downward into the forward sector was reduced by 20 to 25 dB overall in the full-scale frequencies from 2000 to 4000 Hz, decreasing to 10 dB or less at the lower frequencies. Also, it was observed that noise associated with the exhaust radiates into the sector directly below the model downstream to reduce shielding efficiency.
    Keywords: Aircraft Propulsion and Power
    Type: AIAA Paper 99-1937 , 5th AIAA/CEAS Aeroacoustics Conference; May 10, 1999 - May 12, 1999; Greater Seattle, WA; United States
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    NASA TECHNICAL REPORTS
  • 8
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    Single Mode Theory for Impedance Eduction in Large-Scale Ducts with Grazing Flow (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An impedance eduction theory for a rigid wall duct containing an acoustic liner with an unknown impedance and uniform grazing flow is presented. The unique features of the theory are: 1) non-planar waves propagate in the hard wall sections of the duct, 2) input data consist solely of complex acoustic pressures acquired on a wall adjacent to the liner, and 3) multiple higher-order modes may exist in the direction perpendicular to the liner and the opposite rigid wall. The approach is to first measure the axial propagation constant of a dominant higher-order mode in the liner sample section. This axial propagation constant is then used in conjunction with a closed-form solution to a reduced form of the convected Helmholtz equation and the wall impedance boundary condition to educe the liner impedance. The theory is validated on a conventional liner whose impedance spectrum is educed in two flow ducts with different cross sections. For the frequencies and Mach numbers of interest, no higher-order modes propagate in the hard wall sections of the smaller duct. A benchmark method is used to educe the impedance spectrum in this duct. A dominant higher-order vertical mode propagates in the larger duct for similar test conditions, and the current theory is applied to educe the impedance spectrum. Results show that when the theory is applied to data acquired in the larger duct with a dominant higher-order vertical mode, the same impedance spectra is educed as that obtained in the small duct where only the plane wave mode is present and the benchmark method is used. This result holds for each higher-order vertical mode that is considered.
    Keywords: Acoustics; Aircraft Propulsion and Power
    Type: NF1676L-17601 , (AVIATION 2014) AIAA Aviation and Aeronautics Forum and Exposition; Jun 16, 2014 - Jun 20, 2014; Atlanta, GA; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 9
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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
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 10
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    Configuration Effects on Liner Performance (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The acoustic performance of a duct liner depends not only on the intrinsic properties of the liner but also on the configuration of the duct in which it is used. A series of experiments is performed in the NASA Langley Research Center Curved Duct Test Rig (at Mach 0.275) to evaluate the effect of duct configuration on the acoustic performance of single degree of freedom perforate-over-honeycomb liners. The liners form the sidewalls of the duct's test section. Variations of duct configuration include: asymmetric (liner on one side and hard wall opposite) and symmetric (liner on both sides) wall treatment; inlet and exhaust orientation, in which the sound propagates either against or with the flow; and straight and curved flow path. The effect that duct configuration has on the overall acoustic performance, particularly the shift in frequency and magnitude of peak attenuation, is quantified. The redistribution of incident mode content is shown. The liners constitute the side walls of the liner test section and the scatter of incident horizontal order 1 mode by the asymmetric treatment and order 2 mode by the symmetric treatment into order 0 mode is shown. Scatter of order 0 incident modes into higher order modes is also shown. This redistribution of mode content is significant because it indicates that the liner design can be manipulated such that energy is scattered into more highly attenuated modes, thus enhancing liner performance.
    Keywords: Air Transportation and Safety
    Type: NF1676L-13625 , 18th AIAA/CEAS Aeroacoustics Conference; Jun 04, 2012 - Jun 06, 2012; Colorado Springs, CO; United States
    Format: application/pdf
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