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-   -   Is wall ajacent temperature equal to conservative temperature of the wall? (http://www.cfd-online.com/Forums/cfx/111122-wall-ajacent-temperature-equal-conservative-temperature-wall.html)

shenying0710 December 27, 2012 23:43

Is wall ajacent temperature equal to conservative temperature of the wall?
 
hello, my friends!
CFX manual says “Wall Adjacent Temperature, is the average temperature in the element adjacent to the wall. ” From this sentences, it seems that Wall Adjacent Temperature is just conservative temperature of the wall.
But in all my cases, I found that on adiabat wall, Wall Adjacent Temperature is equal to hybrid temperature of the wall, not conservative temperature! And on non-adiabatic wall, Wall Adjacent Temperature is neither equal to hybrid temperature nor conservative temperature of the wall!
Would you answer my confusion?
Thank you in advance!
Shenying

ghorrocks December 28, 2012 05:25

No, the wall temperature is the temperature in the wall boundary face of the element, wall adjacent temperature is the conservative temperature of the element. The centroid of the element is off the wall, so it is representative of the flow near the wall.... hence the name.

Are you using wall functions? Or a laminar flow?

shenying0710 December 29, 2012 01:07

Thanks, Glenn, you give good interpretation of the difference between hybrid temperature and conservative temperature of the wall. But my question is "Is Wall Ajacent Temperature the same as conservative temperature of the wall?", and you said yes, "Wall Adjacent Temperature is the conservative temperature of the element". But I found that on adiabat wall, Wall Adjacent Temperature is equal to hybrid temperature of the wall, not conservative temperature. And on non-adiabatic wall, Wall Adjacent Temperature is neither equal to hybrid temperature nor conservative temperature of the wall.
I'm really very confused about the three variables.

shenying0710 December 29, 2012 01:14

I used wall functions for turbulent flow. I guess what you will say may be like this page: http://www.eureka.im/3679.html . That's really my another question.
It says "For turbulent flow, Tnw is the conservative temperature for the wall node. This is what is used to close the wall heat flux when using turbulent wall functions. For laminar flow, Tnw is taken from averaging the vertex temperatures in the element adjacent to the wall."
I'm sorry that I can't understand. What's the difference between "conservative temperature for the wall node" and "averaging the vertex temperatures in the element adjacent to the wall" ? I think they are the same

ghorrocks December 29, 2012 05:50

I do not know off the top of my head what wall functions do to the parameters you are looking at. Laminar flows use simple wall boundaries which should behave as expected in the issues you are looking at, but wall functions are deliberate distortions of this to recreate the turbulent boundary layer.

Can you run a sample flow with laminar flow? Hopefully that should behave as you expect.

shenying0710 December 30, 2012 13:05

I tried sample flow with laminar flow, and you guess what happened. All the three variables, hybrid temperature of the wall, conservative temperature of the wall and Wall Adjacent Temperature, are equal to each other!
Wow, now I'm lost in the confusion deeply, ha-ha.

ghorrocks December 30, 2012 16:12

Wall thermal wall boundary condition did you apply to the laminar flow model?

shenying0710 January 4, 2013 01:14

Quote:

Originally Posted by ghorrocks (Post 399482)
Wall thermal wall boundary condition did you apply to the laminar flow model?

I applied adiabatic condition to all the walls.
Anyhow, Ill try to summarize what I get during these days:
(1) for laminar flow, on adiabatic wall:
Wall Adjacent Temperature ≠ hybrid temperature ,
hybrid temperature= conservative temperature ,
The denser the mesh near the wall is, the closer Wall Adjacent Temperature is to hybrid temperature and conservative temperature.
(2) for turbulent flow, on adiabatic wall:
Wall Adjacent Temperature = hybrid temperature ,
hybrid temperature ≠ conservative temperature ,
The denser the mesh near the wall is, the closer hybrid temperature is to conservative temperature.
(3) for turbulent flow, on non-adiabatic wall:
Wall Adjacent Temperature ≠ hybrid temperature ,
Wall Adjacent Temperature ≠ conservative temperature ,
hybrid temperature ≠ conservative temperature .

ghorrocks January 4, 2013 05:03

For laminar flow and adiabatic wall has zero temperature gradient, so of course all the temperatures will be the same. This is as expected. Also everything getting closer together with smaller mesh is to be expected.

I cannot explain the differences between the temperatures you are getting for the different combinations.


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