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  • 1
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    Analysis of the surface load and radiated sound of a vibrating airfoil with application to the experiment of Brooks (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A method is developed for calculating the surface load and radiated sound from a vibrating surface in a compressible viscous fluid. The method is applied to a thin two-dimensional elliptic cross-section. For large values of the viscous diffusion parameter, the surface load tends to an elliptic distribution in agreement with the results of inviscid theory when edge pressure continuity is enforced. For thin surfaces, the surface load is insensitive to variations in the thickness ratio. A three-dimensional spectral technique is developed to calculate the inviscid surface load and radiated sound from a thin vibrating airfoil. The inviscid theory predicts the correct form of the far field sound pressure and its phase. The actual levels are somewhat sensitive to the choice of theoretical spanwise surface pressure mode but are in better agreement with the experiment than the surface pressure. The comparison of theoretical and experimental surface pressure indicates that the viscous theory, used to validate the inviscid theory, is either inadequate or there is a source of experimental error.
    Keywords: ACOUSTICS
    Type: NASA-CR-3864 , NAS 1.26:3864 , ARAP-508
    Format: application/pdf
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  • 2
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    Viscous theory of surface noise interaction phenomena (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A viscous linear surface noise interaction problem is formulated that includes noise production by an oscillating surface, turbulent or vortical interaction with a surface, and scattering of sound by a surface. The importance of viscosity in establishing uniqueness of solution and partitioning of energy into acoustic and vortical modes is discussed. The results of inviscid two dimensional airfoil theory are used to examine the interactive noise problem in the limit of high reduced frequency and small Helmholtz number. It is shown that in the case of vortex interaction with a surface, the noise produced with the full Kutta condition is 3 dB less than the no Kutta condition result. The results of a study of an airfoil oscillating in a medium at rest are discussed. It is concluded that viscosity can be a controlling factor in analyses and experiments of surface noise interaction phenomena and that the effect of edge bluntness as well as viscosity must be included in the problem formulation to correctly calculate the interactive noise.
    Keywords: ACOUSTICS
    Type: NASA-CR-3331 , ARAP-419
    Format: application/pdf
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  • 3
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    Application of the Bernoulli enthalpy concept to the study of vortex noise and jet impingement noise (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A complete theory of aeroacoustics of homentropic fluid media is developed and compared with previous theories. The theory is applied to study the interaction of sound with vortex flows, for the DC-9 in a standard take-off configuration. The maximum engine-wake interference noise is estimated to be 3 or 4 db in the ground plane. It is shown that the noise produced by a corotating vortex pair departs significantly from the compact M scaling law for eddy Mach numbers (M) greater than 0.1. An estimate of jet impingement noise is given that is in qualitative agreement with experimental results. The increased noise results primarily from the nonuniform acceleration of turbulent eddies through the stagnation point flow. It is shown that the corotating vortex pair can be excited or de-excited by an externally applied sound field. The model is used to qualitatively explain experimental results on excited jets.
    Keywords: ACOUSTICS
    Type: NASA-CR-2987
    Format: application/pdf
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  • 4
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    A study of scattering, production, and stimulated emission of sound by vortex flows (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The basic theory of aeroacoustics of homentropic fluid media is applied to the problems of sound scattering, production, and stimulated emission. A general theory of scattering from low speed three-dimensional vortex flows is presented. Specific results are given for the horseshoe vortex and vortex ring. The noise of an elementary corotating vortex pair in various flows is calculated. It is shown that a potential flow and shear flow can substantially increase the basic pair noise. Small reverse shears can annihilate vortex pairs and eliminate the pair noise mechanism. The pair results are used to explain qualitatively the operation of noise suppression devices. The stimulated emission of a single vortex pair and four and six vortex arrays is demonstrated. The results for six vortices illustrate how external pure tones can amplify the broadband noise of a jet in agreement with recent experimental evidence.
    Keywords: ACOUSTICS
    Type: NASA-CR-3139 , ARAP-363
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  • 5
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    A study of production and stimulated emission of sound by vortex flows (1979)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The noise radiated by an elementary corotating vortex pair in a shear flow is calculated. It is shown that a small shear can substantially increase the noise while small reverse shears can annihilate vortex pairs and thus reduce the pair noise mechanism. The resonant excitation of an ensonified vortex pair and the broadband noise amplification of a six vortex cluster is calculated. The results are in qualitative agreement with recent experimental findings on jet broadband noise amplification.
    Keywords: ACOUSTICS
    Type: Mechanics of sound generation in flows; Aug 28, 1979 - Aug 31, 1979; Goettingen
    Format: text
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  • 6
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    A study of the viscous dissipation and surface loading on a vibrating surface (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The energy dissipated by viscosity at the edge of a vibrating flat plate is calculated and compared to the radiated acoustic energy. A correction to the Kirchhoff integral estimate of the noise is derived. For Helmholtz number of order unity and smaller the dissipation can be comparable to or greater than the acoustic energy. A viscous compressible theory of the load distribution on a vibrating two dimensional body is developed. First it is shown that load calculations based on potential theory and the Newmann uniqueness condition (continuity of potential or pressure on the surface) are not in agreement with experiment or the more correct viscous theory. For a flat plate airfoil the eigensolution of potential theory is indeterminant while viscous theory yields a unique solution that has square root singularities at the edges. It is also shown that for compact surfaces the far field acoustics depend only on the magnitude of the eigensolutions of potential theoy and so will be uniquely determined by the viscous theory. It is suggested that the general viscous theory of vibrating surfaces with cross sectional geometry will lead to results in agreement with expermentally measured load distributions.
    Keywords: ACOUSTICS
    Type: NASA-CR-3499 , ARAP-451
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  • 7
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    Interaction with and production of sound by vortex flows (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A description is presented of an improved version of a comprehensive theory of aeroacoustics which, in its original form, had first been reported by Yates and Sandri (1976). The new theory departs in several basic aspects from the concepts provided by Lighthill (1952), Ribner (1964), and Lilley et al. (1974). A detailed derivation of the theory is given for the case of constant entropy flows, taking into account the production of sound by a 'primary' fluid flow, the processing of sound by a flow, and the effects of sound on a primary flow. An investigation is conducted regarding the processing and the production of sound by vortex flow. Questions related to the excitation of a fluid flow by sound are also considered.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 77-1352 , Aeroacoustics Conference; Oct 03, 1977 - Oct 05, 1977; Atlanta, GA
    Format: text
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