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Old   August 6, 2005, 23:32
Default Y+
  #1
zi
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Hi all,

Does anybody know a way of gridding in Gambit such that your Y+ values be in a desired range? I am trying to use wall functions but no matter what I try, I can't get the Y+ in the desired range at all.

Note: Adaption does not really help!!!

Cheers,

Zi
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Old   August 7, 2005, 06:54
Default Re: Y+
  #2
Jim
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Hi Zi,

Probably the best control you will have over wall cell size is to use the boundary layer function, you can specify the depth of the first row precisely and even use aspect ratio based depth if you want to vary the thickness.

Otherwise break up the region next to your wall into various faces (in 2D) and when you mesh the edges of the face you can specify the 'first length'.

Hope this helps, J.
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Old   August 7, 2005, 09:10
Default Re: Y+
  #3
zi
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Hi,

Thanks for the reply. Unfortunately, I have tried using boundary layers, but due to the complexity of the geometry (a rotated circular mast attached to a sail at incidence) it behaves in a weird way. Also, I got the feeling that the second option you have told might not be suitable for a structured grid. What do you think?

Thanks,

Zi
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Old   August 7, 2005, 09:52
Default Re: Y+
  #4
Jim
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Hi Zi,

Not sure what you mean by weird way. Maybe give it a try breaking up the region into faces, if it doesn't work then use unstructured mesh close to the body. Unstructured meshes can be adapted in Fluent to give the best y+.

Unless you can provide more details not sure what else to suggest?

J.
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Old   August 7, 2005, 14:52
Default Re: Y+
  #5
zi
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Okay, now I'll try your method but this arises another question. In Fluent it is really hard to use the y+ adaption tool since y+ doesn't necessarily get in the desired range after adaption. What I mean is that y+ values in some cells drop to values that are below the range that you have specified. That results in a need for coarsening and I have never succeeded in that even in 2-D and conformal mode. Do you have any suggestions for this? Also, is it really possible to coarsen the grid in 3-D?

Thanks so much for your help...

Zi
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Old   August 7, 2005, 15:03
Default Re: Y+
  #6
Jim
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Depends on your range, you want y+ between 30 and 300, (although arguably as high as 500 can be okay).

If you adapt cells over 300 they should drop to about half - say 150 or so which is ideal. Shouldn't be a problem to coarsen though - go to grid adaption controls and ensure you are using hanging node (conformal is permanent I think) and that coarsen is selected or it will never work.

I haven't used 3D yet but don't see why it should be a prob, read the user guide in case it says so.

Good luck, J.
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