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Old   March 27, 2000, 06:22
Default Restrictions on time steps
  #1
Valdemir G. Ferreira
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Hi there,

I am working with Incompressible, viscous and time dependent flows. In recent days, I am implementing a Low-Reynolds turbulence model in a explicit code. If every one to know what is the restrictions on time steps in turbelent flows, or related papers, please let me know.

Thank you.
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Old   March 27, 2000, 07:49
Default Re: Restrictions on time steps
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Jonas Larsson
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There are a couple of papers by Kunz and Lakshminariayana on the stability of explicit solvers using the Low-Re Chien model. Their conclusion was that the Chien Low-Re k-epsilon model does not lead to any additional requirements. My real-life experience is not the same though... I'd say that if you want to use low-Re models you should consider moving to an implicit solver. The explicit Runge-Kutta solvers are not very suitable to solve this type of source-term dominated equations, IMHO.
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Old   March 27, 2000, 14:17
Default spelling correction
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clifford bradford
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look for Kunz and Lakshminarayana (Jonas got the second name wrong, but who can blame him)
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Old   March 28, 2000, 20:52
Default Re: Restrictions on time steps
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Bob Anderson
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Just a general observation, I don't have any particular references for you. The turbulence model introduces an eddy viscosity. If you already have a stability estimate based on viscosity considerations, that should work just fine with the combined eddy and molecular viscosity. The solver has no "knowledge" of any difference. This is how I have handled it before and it seemed to work just fine.

Also, a previous poster made reference to this being a "source term dominated" system. In what sense does \nu_t look like a source term? I had never conceptualized it as such.

Bob
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Old   March 29, 2000, 02:02
Default Re: Restrictions on time steps
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Jonas Larsson
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I meant that the k and epsilon equations are source term dominated. The effect on the other equations is only through \nu_t, as you say.
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Old   March 29, 2000, 02:24
Default Re: spelling correction
  #6
Jonas Larsson
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Heh, my misstake... I even took out his excellent turbomachinery book to check the spelling and I still got it wrong. Btw, the papers are in J. Comput. Physics, vol 103, p 141 and AIAA J. vol 30, no 1 p 13.
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