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Old   April 8, 2013, 05:18
Default Large Eddy Simulation - LES
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Hello all,

I want to used a LES to simulate the transport of smoke in an air flow. I know that LES is always unsteady. I have read in "Turbulence Modeling for CFD" by David C. Wilcox and could not find out why LES only can be used unsteady. Could anyone of you please tell me why it's not possible to use LES steady?

Thanks
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Old   April 8, 2013, 07:10
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The LES turbulence model is derived by modeling the small scales of the fluid. Formally, it looks like the RANS model, but it has nothing to do with that. Since no time integration / averaging is involved in LES, it just does not behave like that and thus results in transient solutions.
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Old   April 8, 2013, 07:28
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Quote:
Originally Posted by RodriguezFatz View Post
The LES turbulence model is derived by modeling the small scales of the fluid. Formally, it looks like the RANS model, but it has nothing to do with that. Since no time integration / averaging is involved in LES, it just does not behave like that and thus results in transient solutions.
I agree, it is simple to see the LES equation when you start from the NS equation and apply to each term a local filter. As a result, the time derivative of each conserved variable, e.g., mass, momentum, etc., does not vanish.
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Old   April 8, 2013, 08:05
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So because filtering is not equal to time averaging, the equation is time dependent and i have to regard the time derivative?
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Old   April 8, 2013, 08:19
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Yes. Better think of LES as a slight relaxation of computational efford (compared to DNS), that is bought by a loss of precision. But it's not too different from DNS.
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Old   April 8, 2013, 08:23
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Ok, thank you very much. I think i got it.
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