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-   -   This is a dilema, isn't it? (https://www.cfd-online.com/Forums/openfoam-solving/64348-dilema-isnt.html)

lakeat May 8, 2009 00:02
This is a dilema, isn't it?
 
Hi Forum,
Suppose I have a flat plate case in wind tunnel, using RANS, it is required that the first cell height is around y+=30,
I have a dilemma here during refining the mesh near the boundary layer, on the one hand, modification of the simplegrading item will reduce the first cell height, on the other, if the average first cell height is below y+=30 (or y+=11, whatever), RANS will behave incorrectly.
So, how can I control the first cell height around a certain value, without breaking the wall-function guideline, when I try to refine the mesh?

marico May 8, 2009 01:43
Thats a problem of meshing. Which software do you use? For blockMesh you can change the grading... If you have enough resources take one of the lowRe-turbulence models, make a fine mesh and your y+ problem is solved...

lakeat May 8, 2009 01:55
  1. I am using blockMesh, if I increase the grading ratio, the first cell height is inevitably reduced and lower than y+=30 (or y+=11), don't you think this is a dilemma?
  2. How to use low Re-turbulence models, using turbFoam? But, my case is a very high Re flow in wind tunnel? Is komegaSST able to do that?
  3. Theoretically, I see no problem in combining S-A model with a fine mesh (y+=1), but other models in OpenFOAM are all integrated with wall function, so applying them to a very fine mesh is not correct, isn't it?

marico May 8, 2009 05:33
1. war and hunger is dilemma...
2. just specify in RASProperties file the model, you are in love with (incompressible). Low Re means locally near the wall low, NOT in free stream.
kOmegaSST is in standard release only with wall functions, but there is a version for incompressible LowRe in the wiki, if I remember rightly
3. Not all are with wall functions (USERS GUIDE)

Cheers
Marco

lakeat May 8, 2009 06:33
Thank you so much, I do remember someone talked about the komegaSST version without wall Function, and I havn't found it yet.

in fact, UserGuide is not detailed in this issue, even the RANS model is not listed completely. But most of the released OpenFOAM RANS model are implemented with wall function. Seems their idea is high Re simulation without wall function is not practical. :confused:

marico May 8, 2009 08:17
I'm very new to CFD, but as far as I experienced, it IS really not practical for "real" geometries...
Look in src/turbulenceModels, I think all LowRe models are named ***LowRe.
For more:

http://openfoamwiki.net/index.php/Si...bulence_models

paulo May 8, 2009 15:08
You can find some discussion on LowRe models in OF in this thread:

http://www.cfd-online.com/Forums/ope...tallmaras.html

Regards,

Paulo Rocha

lakeat May 8, 2009 21:54
That's exactly what I want, thank you Paulo. It is really a great discussion, I will follow that thread.


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