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  • ACOUSTICS  (10)
  • Numerical Analysis  (1)
  • 1
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    Sensitivity of acoustic predictions to variation of input parameters (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The noise prediction code WOPWOP predicts the thickness and loading noise produced by a helicopter rotor, given the blade motion, rotor operating conditions, and fluctuating force distribution over the blade surface. However, the criticality of these various inputs, and their respective effects on the predicted acoustic field, have never been fully addressed. This paper examines the importance of these inputs, and the sensitivity of the acoustic predictions to a variation of each parameter. The effects of collective and cyclic pitch, as well as coning and flapping, are presented. Blade loading inputs are examined to determine the necessary spatial and temporal resolution, as well as the importance of the cordwise distribution. The acoustic predictions show regions in the acoustic field where significant errors occur when simplified blade motions or blade loadings are used. An assessment of the variation in the predicted acoustic field is balanced by a consideration of CPU time necessary for the various approximations.
    Keywords: ACOUSTICS
    Type: In: AHS and Royal Aeronautical Society, Technical Specialists' Meeting on Rotorcraft Acoustics(Fluid Dynamics, Philadelphia, PA, Oct. 15-17, 1991, Proceedings (A93-29401 10-71); 11 p.
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  • 2
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    Using finite volume methods for aeroacoustics (1990)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: This paper is concerned with the application of unsteady finite volume methods to the numerical calculation of aeroacoustic problems. Some discussion is made of how the acoustic analogy of Lighthill has led to a separation between the calculation of acoustic source, or flow, and the subsequent sound field. It is pointed out that for transonic flow this separation is not necessarily useful. The example problem of an impulsively started cylinder is given for speed of Mach .1 and .5. In the Mach .5 case a strong shock is followed as it develops. A further example of the cylinder stopping impulsively is given as well. MacCormack's explicit predictor-corrector method is used for all the examples.
    Keywords: ACOUSTICS
    Type: ; : Micro(
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  • 3
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    Energy considerations in computational aeroacoustics (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A finite-volume multistage time-stepping Euler code is used to investigate the use of CFD algorithms for the direct calculation of acoustics. The 2D compressible inviscid flow about an accelerating or decelerating circular cylinder is used as a model problem. The time evolution of the energy transfer from the cylinder to the fluid, as the cylinder is moved from rest to some nonnegligible velocity, is clearly seen. By examining the temporal and spatial characteristics of the numerical solution, a distinction can be made between the propagating acoustic energy, the convecting energy associated with the entropy change in the fluid, and the energy contained in the local aerodynamic field. Systematic variation of the cylinder acceleration shows that the radiated acoustic energy depends strongly upon the rate of acceleration or deceleration. The computational grid has a large effect on the ratio of acoustic energy to nonphysical entropy associated energy, while the role of the explicit artificial viscosity seems to be of second order. The entropy term was nearly negligible in all cases the cylinder was started slowly.
    Keywords: ACOUSTICS
    Type: ; : IR sensors; Procee
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  • 4
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    The influence of quadrupole sources in the boundary layer and wake of a blade on helicopter rotor noise (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: It is presently noted that, for an observer in or near the plane containing a helicopter rotor disk, and in the far field, part of the volume quadrupole sources, and the blade and wake surface quadrupole sources, completely cancel out. This suggests a novel quadrupole source description for the Ffowcs Williams-Hawkings equation which retain quadrupoles with axes parallel to the rotor disk; in this case, the volume and shock surface sourse terms are dominant.
    Keywords: ACOUSTICS
    Type: In: AHS and Royal Aeronautical Society, Technical Specialists' Meeting on Rotorcraft Acoustics(Fluid Dynamics, Philadelphia, PA, Oct. 15-17, 1991, Proceedings (A93-29401 10-71); 7 p.
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  • 5
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    Helicopter noise prediction - The current status and future direction (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The paper takes stock of the progress, assesses the current prediction capabilities, and forecasts the direction of future helicopter noise prediction research. The acoustic analogy approach, specifically, theories based on the Ffowcs Williams-Hawkings equations, are the most widely used for deterministic noise sources. Thickness and loading noise can be routinely predicted given good plane motion and blade loading inputs. Blade-vortex interaction noise can also be predicted well with measured input data, but prediction of airloads with the high spatial and temporal resolution required for BVI is still difficult. Current semiempirical broadband noise predictions are useful and reasonably accurate. New prediction methods based on a Kirchhoff formula and direct computation appear to be very promising, but are currently very demanding computationally.
    Keywords: ACOUSTICS
    Type: In: DGLR(AIAA Aeroacoustics Conference, 14th, Aachen, Germany, May 11-14, 1992, Proceedings. Vol. 2 (A93-19126 05-71); p. 724-735.
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  • 6
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    1991-92 AHS Technical Committee highlights - Acoustics (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Progress made in 1991-92 in rotorcraft noise technology is reviewed. Flight testing of the SV-15 Tiltrotor Research Aircraft fitted with Advanced Technology Blades, noise prediction research conducted at NASA Langley, and noise reduction studies which have demonstrated significant noise reduction using higher harmonic control are discussed.
    Keywords: ACOUSTICS
    Type: Vertiflite (ISSN 0042-4455); 38; 3, Ma
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  • 7
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    Prediction of helicopter rotor discrete frequency noise for three scale models (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Keywords: ACOUSTICS
    Type: Journal of Aircraft (ISSN 0021-8669); 25; 420-427
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  • 8
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    The exact calculation of quadrupole sources for some incompressible flows (1988)
    In:  CASI
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    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
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  • 9
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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
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    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
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  • 10
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    Analytic Formulation and Numerical Implementation of an Acoustic Pressure Gradient Prediction (2008)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Two new analytical formulations of the acoustic pressure gradient have been developed and implemented in the PSU-WOPWOP rotor noise prediction code. The pressure gradient can be used to solve the boundary condition for scattering problems and it is a key aspect to solve acoustic scattering problems. The first formulation is derived from the gradient of the Ffowcs Williams-Hawkings (FW-H) equation. This formulation has a form involving the observer time differentiation outside the integrals. In the second formulation, the time differentiation is taken inside the integrals analytically. This formulation avoids the numerical time differentiation with respect to the observer time, which is computationally more efficient. The acoustic pressure gradient predicted by these new formulations is validated through comparison with available exact solutions for a stationary and moving monopole sources. The agreement between the predictions and exact solutions is excellent. The formulations are applied to the rotor noise problems for two model rotors. A purely numerical approach is compared with the analytical formulations. The agreement between the analytical formulations and the numerical method is excellent for both stationary and moving observer cases.
    Keywords: Numerical Analysis
    Type: LF99-6143
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