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Specifying operating temperature in buoyancy flow

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Old   June 28, 2006, 04:21
Default Specifying operating temperature in buoyancy flow
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PAD
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Hi,

Could someone please explain to me how to specify the operating temperature in buoyant flow using the Boussinesq approximation.

If modelling a long tube with pressure inlet / outlet and a cold tube part in the middle section how should the To value be specified? Same as temperature on pressure inlet or same as temperature on cold wall?

The results will differ depending on which value I choose.

Regards, PAD
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Old   June 28, 2006, 11:41
Default Re: Specifying operating temperature in buoyancy f
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Evan Rosenbaum
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For an open system, the value doesn't matter much. Don't use Boussinesq for a closed system.
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Old   June 29, 2006, 01:20
Default Re: Specifying operating temperature in buoyancy f
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PAD
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Hi Evan,

I tried simulating a long vertical tube 0.05 m ID, 1.0 m long. Middle section (0.2 m) was held at 273 K while pressure inlet (top of tube) was held at 293 K / 0 gauge and pressure out (bottom of tube) was held at 293 K / 0 gauge as well.

The results varied dependent on the operating temperature chosen which according to you it shouldn't. Am I doing something wrong?

Thanks in advance!

Regards, PAD
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Old   June 29, 2006, 05:45
Default Re: Specifying operating temperature in buoyancy f
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PAD
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Just a thought - would it be possible / useful to make a UDF which applies the Boussinesq approx. locally, i.e. the buoyancy force for a cell is calculated from the mean temperature of the surrounding cells?

Regards, PAD
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Old   June 29, 2006, 14:44
Default Re: Specifying operating temperature in buoyancy f
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Evan Rosenbaum
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Well, when I said is shouldn't matter too much, I was thinking in terms of a specific open system with unidirectional flow from inlet to outlet. My error. The situation you describe would be dependent. In an external flow situation (like the outside wall of a building), you would use the ambient temperature for the Boussinesq. In your situation, I'm not sure there is a steady-state solution. The fluid would try to rise in the bottom half of the tube and sink in the top half. You may have an unsteady solution.
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