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  • ACOUSTICS  (13)
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  • 1
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    Recent studies of rotorcraft blade-vortex interaction noise (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Recent results are presented from several research efforts aimed at the understanding of rotorcraft blade-vortex interaction (BVI) in terms of the noise generation, directivity, and control. The results are based on work performed by NASA Langley Research Center researchers, both alone and in collaboration with other research organizations. Based on analysis of a simplified physical model, the critical parameters controlling BVI noise generation have been identified. The detailed mapping of the acoustic radiation field of a model rotor in a wind tunnel has revealed the extreme sensitivity of directivity to rotor advance ratio and disk attitude. The control and reduction of BVI noise through the use of higher harmonic pitch control is discussed.
    Keywords: ACOUSTICS
    Type: Journal of Aircraft (ISSN 0021-8669); 31; 5; p. 1009-1015
    Format: text
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  • 2
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    Reduction of background noise induced by wind tunnel jet exit vanes (1985)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The NASA-Langley 4 x 7 m wind tunnel develops low frequency flow pulsations at certain velocity ranges during open throat mode operation, affecting the aerodynamics of the flow and degrading the resulting model test data. Triangular vanes attached to the trailing edge of flat steel rails, mounted 10 cm from the inside of the jet exit walls, have been used to reduce this effect; attention is presently given to methods used to reduce the inherent noise generation of the vanes while retaining their pulsation reduction features.
    Keywords: ACOUSTICS
    Type: AIAA Journal (ISSN 0001-1452); 23; 1631
    Format: text
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  • 3
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    Scaling of airfoil self-noise using measured flow parameters (1985)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Data from an airfoil broadband self-noise study are reported. Attention here is restricted to two-dimensional models at zero angle of attack to the flow. The models include seven NACA 0012 airfoil sections and five flat plate sections with chordlengths ranging from 2.54 to 60.96 cm. Testing parameters include flow velocity to 71.3 m/s and boundary-layer turbulence through natural transition and by tripping. Detailed aerodynamic measurements are conducted in the near-wake of the sharp trailing edges. The noise spectra of the self-noise sources are determined by the use of a cross-spectral technique. The acoustic data are normalized using the measured aerodynamic parameters in order to evaluate a commonly used scaling law. An examination of the Reynolds number dependence of the normalized overall levels has revealed a useful scaling result. This result appears to quantify the transition between turbulent boundary-layer trailing-edge noise and laminar boundary-layer vortex shedding noise.
    Keywords: ACOUSTICS
    Type: AIAA Journal (ISSN 0001-1452); 23; 207-213
    Format: text
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  • 4
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    Progress in rotor broadband noise research (1983)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The current status of research on various rotor broadband noise sources is reviewed. Theoretical modeling techniques which appear to have the most promise in the development of successful noise prediction capabilities are emphasized. The types of noise considered include: turbulence ingestion noise, blade self-noise due to turbulence passing the trailing edge, blade self-noise due to separated flow, and blade self-noise due to vortex shedding. Comparisons with experimental results are given, and present theoretical and experimental limitations are delineated to help identify areas of needed research.
    Keywords: ACOUSTICS
    Type: Vertica (ISSN 0360-5450); 7; 4, 19
    Format: text
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  • 5
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    Airfoil tip vortex formation noise (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Keywords: ACOUSTICS
    Type: AIAA Journal (ISSN 0001-1452); 24; 246-252
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  • 6
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    Trailing edge noise prediction from measured surface pressures (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Trailing edge (TE) noise is investigated for the case of a two-dimensional airfoil embedded in a uniform low Mach number flow, and the usefulness of several TE noise theories is examined by applying them to the measured data. The TE noise spectra and directivity are quantitatively determined for the case of a high Reynolds number and a fully turbulent boundary layer. Parameters include angle of attack, flow velocity and TE bluntness. Evanescent wave theories by Chase (1975) and Chandiramani (1974) are compared to the present results and show good agreement. Agreement of the near field pressure scatter phenomenon analysis with measurements implies that the basic assumptions used in the analysis are correct, i.e., the turbulent boundary layer (TBL) flow passes the trailing edge into the wake region. No hydrodynamic wake shedding activity is confirmed for the two-sided TBL flow, and a method incorporating the principles of the coherent output power method is used to determine the sound field. The near field edge scatter model is found to not only establish optimum sizing of edge treatment for noise control, but also to separate and identify the scattered field from the incident hydrodynamic field.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration; 78; Sept. 8
    Format: text
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  • 7
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    Airfoil tip vortex formation noise (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Spectral data are presented for the noise produced due to the turbulent three-dimensional vortex flow existing near the rounded tip of lifting airfoils. The results are obtained by the comparison of sets of two- and three-dimensional test data for different airfoil model sizes, angles of attack, and tunnel flow velocities. Microphone cross-correlation and cross-spectral methods were used to determine the radiated noise. Corrections were made for tunnel shear layer and source directivity effects. Interpretation of the results are aided by a three-dimensional flow analysis developed for this study which determines open tunnel and finite aspect ratio corrections heretofore neglected in tip vortex studies. Hot wire measurements were made in the tip vortex formation region for the specification of governing flow parameters. The spectral data is normalized in a format considered most useful for subsequent quantitative prediction of this noise mechanism for practical systems such as helicopter rotors. Comparison is made to the analysis of George and Chou. A recommended prediction method is given.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 84-2308
    Format: text
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  • 8
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    An experimental evaluation of the application of the Kirchhoff formulation for sound radiation from an oscillating airfoil (1977)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The Kirchhoff integral formulation is evaluated for its effectiveness in quantitatively predicting the sound radiated from an oscillating airfoil whose chord length is comparable with the acoustic wavelength. A rigid airfoil section was oscillated at samll amplitude in a medium at rest to produce the sound field. Simultaneous amplitude and phase measurements were made of surface pressure and surface velocity distributions and the acoustic free field. Measured surface pressure and motion are used in applying the theory, and airfoil thickness and contour are taken into account. The result was that the theory overpredicted the sound pressure level by 2 to 5, depending on direction. Differences are also noted in the sound field phase behavior.
    Keywords: ACOUSTICS
    Type: NASA-TP-1048 , L-11479
    Format: application/pdf
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  • 9
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    Feasibility of making sound power measurements in the NASA Langley V/STOL tunnel test section (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Based on exploratory acoustic measurements in Langley's V/STOL wind tunnel, recommendations are made on the methodology for making sound power measurements of aircraft components in the closed tunnel test section. During airflow, tunnel self-noise and microphone flow-induced noise place restrictions on the amplitude and spectrum of the sound source to be measured. Models of aircraft components with high sound level sources, such as thrust engines and powered lift systems, seem likely candidates for acoustic testing.
    Keywords: ACOUSTICS
    Type: NASA-TM-X-73934
    Format: application/pdf
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  • 10
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    Rotorcraft blade/vortex interaction noise - Its generation, radiation, and control (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-07-27
    Description: Recent results are presented from several research efforts aimed at the understanding of rotorcraft blade-vortex interaction noise generation, directivity, and control. The results are based on work performed by researches at the NASA Langley Research Center, both alone and in collaboration with other research organizations. Based on analysis of a simplified physical model, the critical parameters controlling the noise generation are identified. Detailed mapping of the acoustic radiation field reveals the extreme sensitivity of directivity to rotor advance ratio and disk attitude. A means of controlling blade-vortex interaction noise by higher harmonic pitch control is discussed.
    Keywords: ACOUSTICS
    Type: ICAS Congress; Sept. 9-14, 1990; Stockholm; Sweden
    Format: text
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