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  • 1985-1989  (5)
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  • 1
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    Prediction of helicopter rotor discrete frequency noise for three scale models (1988)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Keywords: ACOUSTICS
    Type: Journal of Aircraft (ISSN 0021-8669); 25; 420-427
    Format: text
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  • 2
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    The exact calculation of quadrupole sources for some incompressible flows (1988)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: This paper is concerned with the application of the acoustic analogy of Lighthill to the acoustic and aerodynamic problems associated with moving bodies. The Ffowcs Williams-Hawkings equation, which is an interpretation of the acoustic analogy for sound generation by moving bodies, manipulates the source terms into surface and volume sources. Quite often in practice the volume sources, or quadrupoles, are neglected for various reasons. Recently, Farassat, Long and others have attempted to use the FW-H equation with the quadrupole source and neglected to solve for the surface pressure on the body. The purpose of this paper is to examine the contribution of the quadrupole source to the acoustic pressure and body surface pressure for some problems for which the exact solution is known. The inviscid, incompressible, 2-D flow, calculated using the velocity potential, is used to calculate the individual contributions of the various surface and volume source terms in the FW-H equation. The relative importance of each of the sources is then assessed.
    Keywords: ACOUSTICS
    Type: NASA-TM-100623 , NAS 1.15:100623
    Format: application/pdf
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  • 3
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    A prediction of helicopter rotor discrete frequency noise for three scale models using a new acoustics program (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A new computer program which uses Farassat's most advanced subsonic time domain formulation has been written to predict helicopter rotor discrete frequency noise. A brief description of the program, WOPWOP, is followed by a comparison of predicted and experimentally measured acoustic pressure and spectra for a 1/4 scale UH-1 model rotor blade and a 1/7 scale OLS (AH-1G) model rotor blade. The C81 computer program was used to predict the spanwise loading on the rotor for aerodynamic input into the acoustic prediction. Comparisons are made for different flight conditions and microphone locations with good results. In general the acoustic pressure is underpredicted. The acoustic predictions for a tapered rotor blade and predictions for microphones well below the tip path plane show less underprediction. Finally, in-plane motion of the rotor blade is shown to significantly affect the peak-to-peak amplitude of the acoustic pressure for high advancing tip Mach numbers.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 87-0252
    Format: text
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  • 4
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    The uses and abuses of the acoustic analogy in helicopter rotor noise prediction (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The generation of noise by helicopter rotor blades is considered theoretically, reviewing recent analyses based on the acoustic analogy (where the effect of fluid motion is replaced by fictitious sources in an undisturbed fluid). The fundamental principles of the acoustic approach are explained and illustrated with diagrams; the governing Ffowcs-Williams/Hawkings equations are written with a reformulated quadrupole term; and the directivity of noise produced (1) by regions with steep gradients (such as shock surfaces) and (2) by boundary-layer quadrupoles (tip-vortex and blade wakes) is shown to be the same as that of thickness noise. The need to include both (1) and (2) in acoustic-analogy computations is indicated.
    Keywords: ACOUSTICS
    Type: National Specialists'' Meeting on Aerodynamics and Aeroacoustics; Feb 25, 1987 - Feb 27, 1987; Arlington, TX; United States
    Format: text
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  • 5
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    The Uses and Abuses of the Acoustic Analogy in Helicopter Rotor Noise Prediction (1987)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper is theoretical in nature and addresses applications of the acoustic analogy in helicopter rotor noise prediction. It is argued that in many instances the acoustic analogy has not been used with care in rotor noise studies. By this it is meant that approximate or inappropriate formulations have been used. By considering various mechanisms of noise generation, such abuses are identified and the remedy is suggested. The mechanisms discussed are thickness, loading, quadrupole, and blade-vortex interaction noise. The quadrupole term of the Ffowcs Williams-Hawkings equation is written in a new form which separates the contributions of regions of high gradients such as shock surfaces. It is shown by order of magnitude studies that such regions are capable of producing noise with the same directivity as the thickness noise. The inclusion of this part of quadrupole sources in current acoustic codes is quite practical. Some of the difficulties with the use of loading noise formulations of the first author in predictions of blade-vortex interaction noise are discussed. It appears that there is a need for development of new theoretical results based on the acoustic analogy in this area. Because of the impulsive character of the blade surface pressure, a time scale of integration different from that used in loading and thickness computations must he used in a computer code for prediction of blade-vortex interaction noise.
    Keywords: Acoustics
    Type: AHS Specialists'' Meeting on Aerodynamics and Aeroacoustics; Feb 25, 1987 - Feb 27, 1987; Arlington, TX; United States
    Format: application/pdf
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