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Old   April 22, 2010, 10:31
Default RANS vs LES
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Remy Cavecchia
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Hi everybody,
I'm studying CFD and I want to learn a bit more about turbulences.
I want to compare a simulation done with a RANS model with Data I have for the same simulation but realised with a LES.
The idea is to see what can be do with RANS to achieve pretty good results.
I'd just like to know as a start point what kind of Mesh I need, to get acceptable results.
I mean what kind of value for Yplus?

My problem is about an turbulent pipe flow : D=140mm Re=80000.

Thanks for helping me.
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Old   April 22, 2010, 16:15
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There are many difference RANS models. Which one do you want to use?
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Old   April 23, 2010, 02:30
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Remy Cavecchia
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I'm gonna use the k-epsilon model.
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Old   April 23, 2010, 09:45
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The following is quoted directly from ANSYS online support:

For the k-e model, y+ should be less than 300. If y+ is below about 11 for the k-e model then it still works fine, but the model does not make use of the fine near wall mesh, so the mesh is wasted. Essentially the k-e model always uses the Scalable Wall Functions. The Scalable Wall Function approach is not valid below y+~=11, so the mesh is ignored below y+~= 11.There is no low-Re implementation of the k-e model in CFX. If there were, the low-Re k-e model would require y+<0.2 and has other problems too. k-e always uses Scalable Wall Functions, hence y+<300 is the requirement.

There's some additional information at
CFX 12.1: y+ value for SST

I hope this helps.
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Old   April 24, 2010, 07:41
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Ok, thank you for your help!!
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