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-   -   How to mesh the 1st grid point (https://www.cfd-online.com/Forums/openfoam-meshing/61631-how-mesh-1st-grid-point.html)

lakeat October 7, 2007 09:50

How to mesh the 1st grid point
 
How to mesh the 1st grid point?

I am very young to CFD world. I'm trying to use oodles to learn LES techniques.
Now I have some problems about meshing.

I saw a paper on flow over a cylinder using Fluent, its meshing guideline is like this,
Reynolds Number | Flow Regime | Yplus/First Cell Height
Re < 1000 | Laminar | First Cell Height = Cylinder Radius/24
1000 < Re < 20000 | Turbulent, Enhanced Wall Treatment | Yplus < 10
Re > 20000 | Turbulent, Standard Wall Functions | Yplus > 30

Q-1.
Why mesh like this? Is there any references on how-to mesh under LES and DNS?
Sorry, I did not find help from <grid> and <fluent>...

Q-2.
Is that mean:
If I use wall function, then I can set the 1st grid point at the place of y+=30?
If I use not wall function, then I should set the 1st grid point at the place of y+=1?

Q-3.
I heard "Laminar LES"=DNS, what does it mean? Is it mean that their mesh is identical?

Q-4.
I see that the 1st grid point is decided by the flow state, but When I predict flow around a building on a certain Re for example, the laminar and turbulence coexist around the building, then how to decide the 1st grid point?

Thanks for your timehttp://www.cfd-online.com/OpenFOAM_D...part/happy.gif

Daniel

hadi October 8, 2007 07:13

Hey Daniel, I am new to LES
 
Hey Daniel,

I am new to LES too, and i am running channel flows, what i can tell you is that in LES you have two calculation strategies:

-A direct near wall resolution, the first point grid should lay in the viscous sublayer (y+<5, at least 3 points needed)
-Using wall functions to resolve wall region, with the 1st point grid at the limit of the log region(y+=30).

So your mesh will depend on the calculation strategy you choose.

Hope this helps.
good luck!
Hadi


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