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July 15, 2018, 02:58 |
T Plus
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#1 |
Senior Member
Sasan Ghomi
Join Date: Sep 2012
Location: Denmark
Posts: 292
Rep Power: 15 |
Dear Friends,
I have a question about T+. I cannot find any option in CFX in order to monitor T+ values. I am wondering if CFX applies wall functions for heat transfer. We always talk about the acceptable values for Y+ but T+ values are not discussed. Could anyone do me a favor and cast light on this ambiguity? Best Regards |
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July 15, 2018, 20:21 |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,871
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This is discussed in the CFX theory documentation, under wall treatments. Yes, the thermal equivalent of y+ is T+. But wall functions on thermal modelling are known to be problematic and are not as widely used as for the momentum equations. Many researchers don't use wall functions if they require accurate heat transfer results and resort to integration to the wall. Hence parameters like T+ are not widely used.
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July 16, 2018, 17:09 |
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#3 |
Senior Member
Join Date: Jun 2009
Posts: 1,880
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If you look into the theory documentation for "Near Wall Modelling" chapter (as Glenn suggested) you will find the formula used for Tplus,
Then, you can write an expression for Tplus as a function of Yplus, and the related properties. From the formula, and your knowledge of the Prandtl number you can infer Tplus behavior for all cases. You will need a material with extreme variations of properties along a wall to have very different correlation behavior between Yplus for every node. |
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July 21, 2018, 07:41 |
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#4 |
Senior Member
Sasan Ghomi
Join Date: Sep 2012
Location: Denmark
Posts: 292
Rep Power: 15 |
Thank you so much for your responses.
I did not get any conclusion that whether or not CFX uses heat transfer wall functions by default? My second question is that when we use wall functions for velocity domain, does it necessarily mean that we are using heat transfer wall functions as well? Also, I can remember that in reference books about turbulent flow it was a must to take care of the first node's position of mesh in boundary layer. In fact, the first node should not be positioned in the buffer layer (5<Yplus<30). However, I have not seen any recommendation like that in CFX software program. What is the secret behind that? Could anyone help me with these questions please? Best Regards |
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July 21, 2018, 08:04 |
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#5 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,871
Rep Power: 144 |
If your setup is using wall functions for the momentum equation it is using wall functions for heat transfer as well. So if your turbulence model uses wall functions by default it will use them for heat transfer as well.
The model used in wall functions only applies to the logarithmic layer, from about y+=11 and out. It does not apply in the sublayer. So if you apply wall functions in y+<11 you are applying them in a regime where they are not applicable. The scalable wall functions in CFX have removed this lower limit problem: https://sharcnet.ca/Software/Ansys/1.../i1313358.html
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Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
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