# [Other] y+ in practice

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 June 9, 2012, 08:52 y+ in practice #1 Member   Robert Join Date: Mar 2011 Location: Warsaw Posts: 62 Rep Power: 7 Hello, Recently I have started meshing some simple geometries for heat-flow problems. I have encountered something like y+ coefficient. I understand basics of theory for this but I still have problems with usage of this. How should I use it? If I have simple turbulent flow in a tube of given diameter how can I calculate mesh size for it using y+? Can anyone explain it to me in the easy way?

 June 9, 2012, 09:09 #2 Super Moderator     Ghazlani M. Ali Join Date: May 2011 Location: Tokyo, Japan Posts: 1,366 Blog Entries: 23 Rep Power: 21 This can help you a lot : Which turbulent model is good for a diffuser? Is my mesh correct If yous search the forum in the y+ you will find a lot of threads about it. __________________ Regards, New to ICEM CFD, try this document --> https://goo.gl/NEbygB Ali

 June 9, 2012, 12:53 #3 New Member   Join Date: May 2012 Location: Moscow Posts: 29 Rep Power: 6 I would also recommend reading this: http://tinyurl.com/bs6rla7 Sixkillers likes this.

 June 9, 2012, 14:56 #4 Member   Robert Join Date: Mar 2011 Location: Warsaw Posts: 62 Rep Power: 7 I am familiar with this article and OK, I understand for which turbulence what value of y+ should be used. But my question is how can I calculate size of the first cell next to the wall using this value and obtain size in mm? I have found CFD Online y+ estimator and there is sth like boundary layer length - is this length of the entire boundary layer or just let's say length of the turbulent part? In the simple pipe will it be its length?

 June 9, 2012, 18:33 #5 Senior Member   Ananthakrishnan.A.S Join Date: Feb 2012 Location: Mumbai (Bombay), India Posts: 157 Rep Power: 8 Y+ should not ideally lie in the range of 5 to 30.. this is where laminar to turbulence transition takes place. It should not lie in this range bcos capturing physics in the transition layer is quite a task in itself. So i generally run the fluent for around 50 iterations and then check the Y+ value. If Y+ has to changed then i come back to my meshing software to redo the mesh as required (to either coarsen or refine)..may be you can try this procedure.. Concept of Y+ calculation can be read separately...

June 10, 2012, 08:51
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 Originally Posted by Kwiaci I am familiar with this article and OK, I understand for which turbulence what value of y+ should be used. But my question is how can I calculate size of the first cell next to the wall using this value and obtain size in mm? I have found CFD Online y+ estimator and there is sth like boundary layer length - is this length of the entire boundary layer or just let's say length of the turbulent part? In the simple pipe will it be its length?
layer length is basically characteristic length. For a pipe it is usually its hydraulic diameter. For a flat plane it is its length and so on.