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  • AERODYNAMICS  (2)
  • Aerodynamics  (1)
  • Fluid Mechanics and Thermodynamics  (1)
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  • 1
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    Sensitivity of acoustic predictions to variation of input parameters (1994)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: Rotor noise prediction codes predict 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 predicitions to a variation of each parameter. The effects of collective and cyclic pitch, as well as coning and cyclic flapping, are presented. Blade loading inputs are examined to determine the necessary spatial and temporal resolution, as well as the importance of the chordwise 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 Central Processing Unit (CPU) time necessary for the various approximations.
    Keywords: AERODYNAMICS
    Type: American Helicopter Society, Journal (ISSN 0002-8711); 39; 3; p. 43-52
    Format: text
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    NASA TECHNICAL REPORTS
  • 2
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    Computation of Sound Generated by Viscous Flow Over a Circular Cylinder (1997)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The Lighthill acoustic analogy approach combined with Reynolds-averaged Navier Stokes is used to predict the sound generated by unsteady viscous flow past a circular cylinder assuming a correlation length of 10 cylinder diameters. The two-dimensional unsteady flow field is computed using two Navier-Stokes codes at a low Mach number over a range of Reynolds numbers from 100 to 5 million. Both laminar flow as well as turbulent flow with a variety of eddy viscosity turbulence models are employed. Mean drag and Strouhal number are examined, and trends similar to experiments are observed. Computing the noise within the Reynolds number regime where transition to turbulence occurs near the separation point is problematic: laminar flow exhibits chaotic behavior and turbulent flow exhibits strong dependence on the turbulence model employed. Comparisons of far-field noise with experiment at a Reynolds number of 90,000, therefore, vary significantly, depending on the turbulence model. At a high Reynolds number outside this regime, three different turbulence models yield self-consistent results.
    Keywords: Aerodynamics
    Type: NASA-TM-110339 , NAS 1.15:110339
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    The personal aircraft: Status and issues (1994)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Paper summarizes the status of personal air transportation with emphasis upon VTOL and converticar capability. The former obviates the need for airport operations for personal aircraft whereas the latter provides both ground and air capability in the same vehicle. Fully automatic operation, ATC and navigation is stressed along with consideration of acoustic, environmental and cost issues.
    Keywords: AERODYNAMICS
    Type: NASA-TM-109174 , NAS 1.15:109174
    Format: application/pdf
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  • 4
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    An Analytical Comparison of the Acoustic Analogy and Kirchhoff Formulation for Moving Surfaces (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The Lighthill acoustic analogy, as embodied in the Ffowcs Williams-Hawkings (FW-H) equation, is compared with the Kirchhoff formulation for moving surfaces. A comparison of the two governing equations reveals that the main Kirchhoff advantage (namely nonlinear flow effects are included in the surface integration) is also available to the FW-H method if the integration surface used in the FW-H equation is not assumed impenetrable. The FW-H equation is analytically superior for aeroacoustics because it is based upon the conservation laws of fluid mechanics rather than the wave equation. This means that the FW-H equation is valid even if the integration surface is in the nonlinear region. This is demonstrated numerically in the paper. The Kirchhoff approach can lead to substantial errors if the integration surface is not positioned in the linear region. These errors may be hard to identify. Finally, new metrics based on the Sobolev norm are introduced which may be used to compare input data for both quadrupole noise calculations and Kirchhoff noise predictions.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: American Helicopter Society 53rd Annual Forum; Apr 29, 1997 - May 01, 1997; Virginia Beach, VA; United States
    Format: application/pdf
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