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Old   November 10, 1998, 13:24
Default about noise control in nozzle flow
Alessandro Grossi
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I am going to study the noise generation from a nozzle flow. I have same questions. I use a K-espilon model for turbolence, which is the relation between noise and k or epsilon? Which is thew best turbolence model for this noise reduction problem? Where i can found some litterature on this argue?

I apologise for my english, and thanks for any help.
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Old   November 11, 1998, 11:48
Default Re: about noise control in nozzle flow
James Bridges
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This short answer is, there is no direct relationship between aerodynamic noise and k or epsilon. Mathematically speaking, there is an exact relationship between noise and high order spatial correlations of the velocity field; various turbulence models are introduced to relate these fourth-order correlations to the simpler single-point metrics such as k and/or epsilon. Then there is the propagation of this sound to the far-field through the turbulent flow field. That requires another model.

I'll give you three references. First, the classic Sir James Lighthill paper: M.J. Lighthill (1952). "On sound generated aerodynamically: General theory." Proc. Roy. Soc. A211, 1107. (For those who hadn't heard, Sir James drowned this year while making what was for him a routine swim around an island off the coast of England--a great loss.)

Second, a paper describing what is referred to as the "MGB method" after its developers, Ramani Mani, Phil Gliebe, and Tom Balsa: T.F. Balsa (1976), "The far field of high frequency convected singularities in sheared flows, with application to jet noise prediction," J. Fluid Mech., 330, 193.

And finally a more recent paper describing the implementation of this method using CFD results to prescribe the flow field: A.Khavaran, E.A.Krejsa, & C.M.Kim (1994), "Computation of supersonic jet mixing noise of an axisymmetric convergent-divergent nozzle," J. Aircraft, 31(3), 603.

For general knowledge, I'll cut to the chase and tell you that sound source strength scales as k^7/2. That should tell you how good your CFD turbulence model needs to be get the required accuracy in sound strength!

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