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 Chuck Leakeas April 13, 1999 16:30

Laminar Coaxial Jets?

Hello out there! Does anyone know anything about laminar coaxial jets or where to find journal articles on this topic. I have been searching the literature, but so far I've only encountered turbulent coaxial jets used to design jet engine nozzles. My application is strictly laminar, but I'm not sure exactly how to handle the developing shear layer between the coaxial flows. The shear layer may not even be a concern, but I need to read everything available before I design my problem. Any suggestions or recommendations would be greatly appreciated.

 John C. Chien April 14, 1999 13:16

Re: Laminar Coaxial Jets?

(1). Why not just run an axisymmetric coaxial jet calculation ? It is basically a 2-D problem and the computation should be relatively fast. (2). For laminar flow, the flow field is a function of the Reynolds number and ,I think, a function of the initial velocity profiles as well.

 Chuck Leakeas April 14, 1999 13:47

Re: Laminar Coaxial Jets?

John:

Thank you for answering my question. Are you implying that the shear layer between the coaxial jets is of no concern for laminar flows? What if the Re differs between the jets and/or the concentrations differ between the two jets? There may not be a great variation between them but could this lead to any problems such that there may be some instabilities in the flow field? Thanks for your time and consideration of my questions.

 John C. Chien April 14, 1999 14:33

Re: Laminar Coaxial Jets?

(1). In the coaxial jet case, you have :a). inlet velocity profile from the centerline to the wall of the inner jet. b). inlet velocity profile of the outer jet from the wall to the outer wall. ( assuming that the wall between the inner jet and the outer jet is relatively thin) But, if the dividing tube wall has finite thickness, then the inlet velocity profile between the outer edge of the inner jet and the inner edge of the outer jet is set equal to zero velocity. So the thickness of the dividing wall effect can be properly simulated. (2). For this problem to be computed properly, the inlet conditions are usually started at some location upstream of the jet exit plane and is inside the tube. In this way, the shear layer can be properly computed with the specified initial jet velocity profile inside the tube. The initial jet velocity profile can be either nearly uniform or fully developed depending on the length of the tube and design. (3). So, what I am saying is: the mixing between the two jets are important and the proper capturing of the shear layer is important, especially when the dividing wall thickness is finite ( that is, not a thin wall ). (4). Any CFD computation of jet mixing must handle the initial mixing region properly ( with enough resolution) in order to accurately prodict the down stream flow development because of the "parabolic nature" of the problem.

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