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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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    Airfoil trailing-edge flow measurements (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Keywords: AERODYNAMICS
    Type: AIAA Journal (ISSN 0001-1452); 24; 1245-125
    Format: text
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  • 3
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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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  • 4
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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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  • 5
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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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  • 6
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    Trailing edge noise prediction using Amiet's method (1981)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: Amiet's (1976, 1978) solution to the problem of airfoil trailing edge noise prediction is discussed in light of the results of evanescent wave theory's application to the measured surface pressure behavior near the trailing edge of an airfoil with a turbulent boundary layer. The method employed by Amiet has the advantage of incorporating the effect of finite chord in its solution. The assumed form of the pressure distribution is examined as well as the constant turbulent boundary layer convection assumption, which is found to be unnecessarily restrictive.
    Keywords: AERODYNAMICS
    Type: Journal of Sound and Vibration; 77; Aug. 8
    Format: text
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  • 7
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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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  • 8
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    A directional array approach for the measurement of rotor noise source distributions with controlled spatial resolution (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A special array system has been designed to examine noise source distributions over a helicopter rotor model. The particular measurement environment is for a rotor operating in the open jet of an anechoic wind tunnel. An out-of-flow directional microphone element array is used with a directivity pattern whose major directional lobe projects on the rotor disk. If significant contributions from extraneous tunnel noise sources in the direction of the side lobes are excluded, the dominant output from the array would be that noise emitted from the projected area on the rotor disk. The design incorporates an array element signal blending features which serves to control the spatial resolution of the size of the directional lobes. (Without blending, the resolution and side lobe size are very strong functions of frequency, which severely limits the array's usefulness).
    Keywords: AERODYNAMICS
    Type: Journal of Sound and Vibration (ISSN 0022-460X); 112; 192-197
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  • 9
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    An Aeroacoustic Study of a Leading Edge Slat Configuration (2002)
    In:  CASI
    Details
    Publication Date: 2018-06-11
    Description: Aeroacoustic evaluations of high-lift devices have been carried out in the Quiet Flow Facility of the NASA Langley Research Center. The present paper describes detailed flow and acoustic measurements that have been made in order to better understand the noise generated from airflow over a wing leading edge slat configuration, and to possibly predict and reduce this noise source. The acoustic database is obtained by a moveable Small Aperture Directional Array of microphones designed to electronically steer to different portions of models under study. The slat is shown to be a uniform distributed noise source. The data was processed such that spectra and directivity were determined with respect to a one-foot span of slat. The spectra are normalized in various fashions to demonstrate slat noise character. In order to equate portions of the spectra to different slat noise components, trailing edge noise predictions using measured slat boundary layer parameters as inputs are compared to the measured slat noise spectra.
    Keywords: Acoustics
    Type: Aeroacoustics; Volume 1; No. 3; 241-274
    Format: application/pdf
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  • 10
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    Reduction of Vane Noise in Wind-Tunnel Nozzles (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Vane-induced noise reduced by adhering thin layer of porous material such as foam to downwind surface of vanes, particularly near sharp trailing edges, to prevent occurrence of edge tones. Other modifications made to aerodynamically streamline vane structure were rounding leading edges and smoothly fairing trailing edges. Boundary layer trip applied to leading edge to prevent laminar tone shedding from trailing edge.
    Keywords: MECHANICS
    Type: LAR-13333 , NASA Tech Briefs (ISSN 0145-319X); 9; 4; P. 143
    Format: text
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