y+ in practice
 

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?

This can help you a lot :
http://www.cfd-online.com/Forums/flu...-diffuser.html

http://www.cfd-online.com/Forums/flu...h-correct.html

If yous search the forum in the y+ you will find a lot of threads about it.

I would also recommend reading this: http://tinyurl.com/bs6rla7

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?

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...

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.