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May 11, 2012, 04:36 
Y+ Range

#1 
Member
prince
Join Date: Jun 2011
Posts: 56
Rep Power: 7 
Dear friends
I am modeling turbulent flow heat transfer model. Re no is in the range of 30,000. Geometry is little complicated. Inlet is simple pipe flow and then it meet an hemispherical geometry in mid way and flow split. From top of hemispherical surface a solid vacuum tube is attached (there is no flow in the tube). Therefore, an annuls geometry formed from that point on. Hemispherical geometry is little peculiar. It has the thickness of 1.5mm at mid point and 3mm at the top surface. It is again solid and modeled as a wall. I have used kepsilon model along with standard wall function in fluent. My y+ values for the flow rate of 3kg/s is in the range 0f 30<y+<75 which is acceptable. but for flow rate of 35kg/s its range increase from 30<y+<745. I have tried adapting the grid. And after that i get maximum y+ is 381 but i did not notice any change in my temperature profile. I have tried with komega model (which does not require wall function) and there also not much change. My question is whether my modeling is valid inspite of y+>300 as my reynolds no is very large ?? If not what should be right range of y+. As 30<y+<300 is valid for flat plate... and my geometry is not different. Also I did not understand why there is an upper limit for y+ 

May 11, 2012, 04:45 

#2  
Super Moderator

http://www.linkedin.com/groups/yin...032.S.85359857
Few clarifications. If you are predicting heat transfer then you need to resolve the thermal boundary layer as well. So try to keep the low Y+ as possible (~1) and sufficient no of nodes in boundary layer. Quote:


May 11, 2012, 08:03 

#3 
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prince
Join Date: Jun 2011
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Hey far,
Thanks for taking interest. I get your point Far. As heat transfer near walls are important my grid should be very fine to capture thermal boundary layer. And as my Y+ values are very much larger than 1, that means my results are not at all good. That imply i should not use wall function at all and for this i need to opt for low Re number models. . That mean i need to remesh my geometry and start from the scratch ?? Way to go.. I need to clear one point if u can help me out. In fluent i do not find any wall function for Komega model. Through some literature i also get to know that komega is better in predicting near wall profile. but if its the case why my temperature profile is not changing with Komega model ?? I also get literature which suggest wall function for komega model. Little contradiction here.. and i am confused. if u can tell me about it or point out some reference. 

May 11, 2012, 08:16 

#4  
Super Moderator

Quote:
However with y+ greater than 30, fluent switches to wall function approach automatically. 

May 12, 2012, 01:29 

#5  
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,291
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Quote:
Quote:
That said, the temperature boundary conditions at the wall are handled very similar to the the velocity boundary conditions so the use of large y+ is not totally wrong. The wall modelling approach for both high and low Re schemes for the energy equation is nearly identical to the velocity. Last edited by LuckyTran; May 12, 2012 at 01:57. 

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