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-   -   Thermal Boundary layer (http://www.cfd-online.com/Forums/fluent/98915-thermal-boundary-layer.html)

leomec88 March 21, 2012 22:01

Thermal Boundary layer
 
Hello guys!
I have been working in a simulation that consists in a air flow over a heated rotating cylinder. I' m trying to visualize the thermal boundary layer. I have used the Coutours- Static temperature, but the results doesn't look good, the thermal boudary layer is quite big. On the other hand, I have used the wall temperature ( outer surface) and the results look closer to the expected, thinner. But my doubt is if I can use the wall temperature ( outer surface) to visualize the thermal bounday layer instead of the static temperature. Does anybody know the difference between the 2 methods?? Thanks!!

leomec88 March 21, 2012 22:06

and what about the velocity bounday layer? Does anybody know how to visualize ???
thanks once more

leomec88 March 22, 2012 23:04

Does anybody know?

robboflea March 23, 2012 10:28

The velocity boundary layer you can visualize it with normal velocity contours or vector plots. I don't think that the wall temperature can be used. I have no idea on how Fluent defines it.
What is the value of your y+ at the wall? How you obtain the rotation of the cylinder?

Cheers,

Rob

LuckyTran March 24, 2012 01:01

Quote:

Originally Posted by leomec88 (Post 350787)
Hello guys!
I have been working in a simulation that consists in a air flow over a heated rotating cylinder. I' m trying to visualize the thermal boundary layer. I have used the Coutours- Static temperature, but the results doesn't look good, the thermal boudary layer is quite big. On the other hand, I have used the wall temperature ( outer surface) and the results look closer to the expected, thinner. But my doubt is if I can use the wall temperature ( outer surface) to visualize the thermal bounday layer instead of the static temperature. Does anybody know the difference between the 2 methods?? Thanks!!

For thermal boundary layer, plot static temperature was you were doing
For velocity boundary layer, you need to plot the appropriate velocity.

You shouldn't be using wall temperature, since that is a surface/wall condition and has nothing to do really with your fluid. Boundary layers strictly refer to the fluid profiles. Plot static temperature, the answer is there. It's possible the thermal boundary layer from the simulation is wrong if the simulation was poorly done. It's also possible the simulation is correct and that there is nothing wrong with the solution.

As an aside on the wall temperature and inner/outer surface business...
When energy is involved, the way the heated/cooled walls is handled is to apply two very thin solid regions very close to the wall. Two regions are necessary to define both the surface temperature and the surface heat flux. I forget how each surface is treated specifically but in general the two temperatures of the inner and outer surface are different. The difference in temperature is what provides the gradient in the solid necessary to drive the heat flux. Read the fluent manual if you're really interested in this aspect.


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