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LES near wall model & courant number

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Old   March 13, 2008, 11:11
Default LES near wall model & courant number
  #1
kasim
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Hi guys

I have two question:

1) Which wall model is used in LES for near wall modelling. Is it wall functions?

2) What is difference between average courant number and RMS courant number. In LES documentation they recommend courant number of 0.5-1 for LES, but i have Max courant and RMS courant during the run. No avearge courant number!

kasim
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Old   March 13, 2008, 15:58
Default Re: LES near wall model & courant number
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Andrew
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Hi kasim

1. For LES i think normal wall functions are used.

2. I am not sure

Andrew
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Old   March 13, 2008, 17:39
Default Re: LES near wall model & courant number
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Glenn Horrocks
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Hi,

I do not agree with Andrew's comments.

1. In general you have to integrate LES models to the wall, so wall functions are not used. With the SAS/DES approach you can use wall functions in some places. This is a very technical area, I recommend you do some reading on wall modelling for LES before deciding which way to go.

2. A dedicated LES CFD code usually uses explicit numerics which limits the courant number to less than 1.0. CFX is implicit and can therefore converge on courant numbers greater than one. However you need to do a timestep independence study to determine what time step size is OK for your model.

Glenn Horroks
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Old   March 14, 2008, 07:07
Default Re: LES near wall model & courant number
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kasim
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Thanks Andrew and Glenn. Glenn can you refer to me any literature that i can read on LES wall modelling. In documentation there is virtually nothing.

What is difference between RMS courant number and Max courant. How can one obtain average courant number?

Thank You Kasim
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Old   March 16, 2008, 17:50
Default Re: LES near wall model & courant number
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Glenn Horrocks
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Hi,

Here is some information for starters: http://www.cfd-online.com/Wiki/Near-...for_LES_models

Try the references from there.

The RMS is the root-mean-square (a type of average) but the max is the maximum value across the simulation.

Glenn Horrocks
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Old   March 16, 2008, 19:23
Default Re: LES near wall model & courant number
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kasim
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Thank you Mr. Horrocks
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