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LES unsteady : Turbulent Mixing Layer

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Old   December 7, 1999, 18:28
Default LES unsteady : Turbulent Mixing Layer
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Flav
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I'm studying the well-known Goebel and Dutton supersonic Mixing Layer under Fluent 5. My problem is 2D. The smallest element of the mesh is about 1e-5 m long. The domain is 0.5*0.048 m*m. The purpose of my study is to modelise the eddies produced by the turbulent mixing layer. I've already carried out a calculation with the k-eps model on a steady state, and it's allright. Then I set the solver to Viscous / LES. I use the "ideal gas" option, and the viscosity is set to "Suntherland".

Now, which solver do I have to choose ? Explicit or implicit ? Coupled or segregated ? Which time-step ? How many iterations per time step ?

I thought that the implicit solver was inconditionally stable, but it doesn't seem to be so. I can't find the correct settings to iterate towards a converged solution . (I also set the Courant Number to 0.5)

Concerning the inlets, I have input a profile for U along y. Do I have to add some kind of "noise" to U in order to trigger off eddies production ?

As you can see, I have many questions to ask. The fact is that the LES unsteady solver is not well explained in the Fluent Help/Tutorial files ...
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Old   December 7, 1999, 18:39
Default Re: LES unsteady : Turbulent Mixing Layer
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Jonas Larsson
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LES in 2D? LES must be run in 3D otherwise you will not get the right behaviour - turbulence is very much 3D.
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Old   December 8, 1999, 02:35
Default Re: LES unsteady : Turbulent Mixing Layer
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Flav
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I totally agree with you on this point, Jonas. But I therefore wonder why the LES option is enabled when running Fluent in 2D mode ? Is it just "some kind of LES" adapted to 2D ? Is it the same LES model used for both 2D or 3D ? If so, then it's true that running the LES model with a 2D case is just nonsense.
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Old   December 9, 1999, 05:22
Default Re: LES unsteady : Turbulent Mixing Layer
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Jonas Larsson
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Yep, my guess is that it is nonsense. I don't think that you can get any reasonable results running the LES model in 2D. What you are doing then is not LES - you are just using a very simple 0-equation turbulence model, which isn't adapted for this kind of simulation.

I don't know why Fluent has the LES model enabled in 2D. Any comments from Fluent?
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Old   December 10, 1999, 18:41
Default Re: LES unsteady : Turbulent Mixing Layer
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Paul Malan
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You are right that LES does not make much sense in 2D. We left it as an option mainly for the purpose of instruction. It's useful to be able to play with the mechanics of doing LES in 2D (running unsteady, gathering statistics, etc.). However, I agree that it can also be confusing to the uninitiated. So in a future release it will be disabled from the GUI, but with an option to turn it on through the text menu. Our documentation will also stress this issue more strongly.

-- Paul
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Old   December 13, 1999, 04:11
Default Re: LES unsteady : Turbulent Mixing Layer
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Jonas Larsson
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Paul, thanks for the clear answer! Hiding it in the text menu sounds like a better approach (For those of you who don't know who Paul Malan is - he is responsible for turbulence modeling in Fluent, so this is info straight from the source).
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