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- W2337757103 abstract "i n t r o d u c t o r y r e m a r k s An analytical formulation of a suitable Green’s function has eluded jet noise researches to date. This paper explores the possibility of direct measurement of a Green’s function. To this end a point source is placed in the jet flow and the amplitude and phase of radiated signal are extracted. In principle, this information leads to a semi-empirical model Green’s function. j e t n o i s e r e f r a c t i o n (s h i e l d i n g ) Lighthill’s acoustic analogy1,2 yields less than satisfactory predictions when source and fluid motion are not accounted for. Whereas source motion can be dealt without great difficulty3,4,5, the interaction of the acoustic field and the hydrodynamic field poses far greater challenges. The influence of flow and temperature gradients on sound was demonstrated by a rather elegant set of experiments, wherein a small point source, emitting a single tone, was placed in a jet flow6,7. Schubert8, Mani9, Lilley10, Balsa11, and others have added their perspectives on refraction (or fluid shielding) effects. g r e e n ’s f u n c t i o n f o r j e t f l o w s Ribner12 described a ‘road-map’ for dealing with all the key features of jet noise including source convection and refraction by flow and temperature gradients, but no explicit solution was provided. It is assumed that the Green’s function G(x-y,t-t’,M)= GN Go(x-y,t-t’) is the product o f GN, the normalized Green’s function and Go the free field Green’s function (M=0). GN is measured for discrete frequencies, at it is possible to determine both amplitude and phase quite accurately using a lock-in amplifier technique. For discrete frequencies Gn =|GN|elS* applies. |Gn| is expected to be less than one whenever the field point x subtends a small angle with respect to the jet axis, At right angles to the jet axis the numerical value should tend to unity and the phase angle 8^ to zero. There is no reason to expect that the normalized Green’s function is invariant with the source location y. Both source location in the jet as well as the source-observer vector are parameters. (viz." @default.
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- W2337757103 date "2007-01-01" @default.
- W2337757103 modified "2023-09-23" @default.
- W2337757103 title "D ir e c t M e a s u r e m e n t f a G r e e n ' s F u n c t io n S u it a b l e f o r P r e d ic t io n o f S u b s o n ic J e t N o ise" @default.
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- W2337757103 hasPublicationYear "2007" @default.
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