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-   -   LES near wall model & courant number (http://www.cfd-online.com/Forums/cfx/25445-les-near-wall-model-courant-number.html)

kasim March 13, 2008 11:11

LES near wall model & courant number
 
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

Andrew March 13, 2008 15:58

Re: LES near wall model & courant number
 
Hi kasim

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

2. I am not sure

Andrew

Glenn Horrocks March 13, 2008 17:39

Re: LES near wall model & courant number
 
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

kasim March 14, 2008 07:07

Re: LES near wall model & courant number
 
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

Glenn Horrocks March 16, 2008 17:50

Re: LES near wall model & courant number
 
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

kasim March 16, 2008 19:23

Re: LES near wall model & courant number
 
Thank you Mr. Horrocks


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