Learning Forum

[nkg5399]

What I understand about the periodic boundary conditions in the fluent manual is that wall temperatures or wall fluxes should be the same for all walls present in the geometry.

My questions are following

1. If we consider a rectangular duct, is there any way we can keep the top, and the bottom wall temperatures different in the periodic BC?

2. Is the upstream bulk temperature always 300K? or can we change that according to the geometry? If so, how to calculate that?

3. I ran a couple of cases for the same wall temperatures(400K) and upstream bulk T=300K. Surprisingly, I got minimum T in the flow field is about 270K. How is the temperature dropping below 300K? The net heat flux isn't 0. I've observed similar kind of T-plot in the fluent manual (Periodic BC) Post-processing. I think it's wrong?

Please give some suggestions. 

Thanks. 

[Rob]

You can have a different heat flux for the two surfaces, but not different temperatures. Periodic flows are intended as a "simple" way to reduce domain size, and have several limitations (as you've read). 

There's a temperature option in the Periodic Boundary settings, so you can adjust the upstream bulk temperature. This explains the temperature being lower elsewhere, from the 2019R1 User's Guide:

 

13.4.6. Postprocessing for Periodic Heat Transfer

The actual temperature field predicted by ANSYS Fluent in periodic models will not be periodic, and viewing the temperature results during postprocessing will display this actual temperature field ( of Equation 13–22). The displayed temperature may exhibit values outside the range defined by the inlet bulk temperature and the wall temperature. This is permissible since the actual temperature profile at the inlet periodic face will have temperatures that are higher or lower than the inlet bulk temperature.