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-   -   Mesh in LES (http://www.cfd-online.com/Forums/openfoam/109072-mesh-les.html)

Djub November 8, 2012 12:07

Mesh in LES
 
Hi every Foamers!

I have read somewhere (but I can't remember where !!!) that mesh refinement was forbidden in LES, because the filtering operator has to permute with integral and derivative ones, which is possible only if delta is not constant. Does anybody confirm this?
I easily admit this. If there is a "large" cell close to a "small" one (let's say half), how to make comunicating the cells concerning the eddies modelled in the large cell and calculated in the small cell !?! Thus, I understand that a mesh has to be uniform. At least changing very smoothly.
On the other hand, I can read a lot of papers with refined meshs close to the walls and bluff bodies. How can this be ? :confused:

Anyone to explain me?

Djub November 8, 2012 12:48

I've got it:
Alberto Passalacqua wrote:
Quote:

You need good and prett uniform mesh anyways for LES, since you are assuming the filter operator and the differential operator are commutative, which is not true on non-uniform grids!
(see this thread)

Bernhard November 9, 2012 11:25

Quote:

Originally Posted by Djub (Post 391051)
On the other hand, I can read a lot of papers with refined meshs close to the walls and bluff bodies. How can this be ?

The point for a good LES simulation, is that you want to resolve 80% of the energy spectrum. Near the wall, you will have more energy in the smaller scales, so you need a fine grid there. Only in LES, you want to have cells with a low aspect ratio (which is not necessary in RANS), so you need to refine them in all three directions. If you apply that refinement globally, the amount of cells will get out of hand quite soon. So it is more a compromise to still perform LES.

Be sure to check out the PhD thesis by Eugene de Villiers, (see http://foamcfd.org/resources/theses.html ), he is addressing some of the issues you mention.


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