CFD Online Logo CFD Online URL
Home > Forums > CFX

y+ values

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   January 6, 2004, 19:00
Default y+ values
Mark Fahey
Posts: n/a
Hi, Can someone confirm my understanding of scaleable wall functions and the importance of y+ values. Is it true that there is no recommended minimum y+ value? So y+ values of around 1 to 3 are ok? If I want to resolve the flow profile in the boundary layer, then I should just make sure there are sufficient nodes, right?

Thanks in advance!

  Reply With Quote

Old   January 8, 2004, 12:27
Default Re: y+ values
Posts: n/a
Hi, Mark

To my understanding, yes, you are right. Scalable wall function can overcome the problems of inconsistencies with fine mesh (the first node is down to the sub-layer). If you are trying to resolve the boundary including the sub-layer, sub-layer models with y+ values lower than 2 are recommended.

Regards, Forrest
  Reply With Quote

Old   January 8, 2004, 15:38
Default Re: y+ values
Mark Fahey
Posts: n/a
  Reply With Quote

Old   January 9, 2004, 11:11
Default Re: y+ values
Posts: n/a
This is not the whole story.

k-e with scalable wall functions was a first step towards "y+ independent" meshes. In case of a y+ value below 11 anywhere in the model, y+ is set to 11 locally. So the node is shifted virtually out of the viscous sublayer. So wall functions are applied everywhere. This is a pragmatic approach but it leads to results better than ordinary k-e with meshes where y+ locally lies outside a range between 20 and 200, the well known y+ criterion.

The next step was the introduction of the SST model in which the wall treatment is managed automatically. Where y+ locally is below 11 the SST model switches from a wall function formulation to a low-Re formulation where the flow field is integrated down to the wall (no wall functions are used)

If you have a mesh where y+ is around 1 allover (which is better regarding accuracy), you also can use SST because the model then behaves as a full low-Re model
  Reply With Quote

Old   January 10, 2004, 00:05
Default Re: y+ values
Posts: n/a
Hi Helge,

I just look at the solver theory. It used Y* instead of Y+. ie. the alternative velocity scale is used instead of friction velocity in Y+.

I wonder if Y+ & Y* are the same. If I used friction velocity in the Y+ formula, can I still follow the criteria stated in the manual (i.e. 20<y*<100 if wall function approach is used)?

Thanks for advice.
  Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
TimeVaryingMappedFixedValue irishdave OpenFOAM Running, Solving & CFD 30 September 19, 2017 12:38
Interpolation values in nodes and cells mrestrepo30 FLUENT 0 April 27, 2010 09:20
max node values exceed max element values in contour plot jason_t FLUENT 0 August 19, 2009 11:32
exact face values RubenG Main CFD Forum 0 June 22, 2009 11:09
strange node values @ solid/fluid interface - help JB FLUENT 2 November 1, 2008 13:04

All times are GMT -4. The time now is 15:45.