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Tensian September 16, 2013 11:18

BCs compatibility modified simpleFoam
 
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Hi OF community,

I am triying to make the simulations of a complicated channel using simpleFoam, adding temperature equation as stated in somewhere in this forum (I don remember where, but it was something like tutorial "how to add temperature to icoFoam").

The geometry consists in two inlets, an outlet and a wall.

When I started with simulations I found that if I set "fixedValue" for inlets (293 K), outlet (304 K) and wall (306 K), then when I postprocess results I saw something strange near the outlet (some cold zones with no physcal meaning, because in this place the fluid should have been conditioned to hotter temperatures, at least 304 K I guess due to the BC imposed at wall and outlet).

Due to this issue, I changed the BC at outlet and I set it as "zeroGradient". With this change the results are physically correct.

My question is why the first set up is wrong?FLUENT does not give any problem, in addition, it specifies temperature by default if user does not do it (I guess).
Anybody knows if these BCs are adequate?Or maybe the problem is the mesh? (the same mesh gives good results with the second setup and I have tried several meshes, always with the same results when I use fixedVale at outlet). I guess that implementation of solver simpleFoam with temperature is OK, but if it's needed I can post it.

I attach two pictures that should ilustrate my problems.

Best regards and thanks in advance for all answers,

James

kornickel September 16, 2013 13:59

I think your first setup doesn't work out because it's physically not correct. You can't tell the solver which temperature to have at the outlet if you already did it at the inlet since the outlet temperature is depending on the inlet temperature. Same as pressure, velocity etc...

Correct me if I'm wrong :confused:

Tensian September 16, 2013 14:52

Thanks for the answer, but I am not sure about what you say...FLUENT allows to specify temperature on every zone with BC asigned. It doesnŽt look phisically meaningless for me...

Tensian September 26, 2013 06:18

Hi Frank,

I have been talking with other engineers and they agree with you. No temperature should be imposed at outlet, it has to be calculed by the solver based on the inlet and wall temperature of fluid.

Thanks!


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