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Old   February 6, 2010, 13:17
Default Boundary Conditions
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Jérémy Bulle
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Hello,

I'm new to fluent and I'm working on a case with a "U" pipe. I want to predict the pressure drop for a certain mass flow. I have problems with openfoam so I try to run the case on fluent to see where I did wrong with openfoam.

So I'd like to set the static pressure at the inlet & outlet
(I know, fluent has BC to match a mass flow but I'd like to compare results with OpenFoam wich doesn't have this type of boundary conditions so I do it this way)

In openfoam, there is no droplist for the boundary conditions and you can do whatever you want, it won't tell you if you do wrong things.
Here in fluent, I see that I can't set "static pressure" at the inlet but only total pressure and initial gauge pressure.
Why? Is it an error to only give static pressure @inlet & oulet? You absolutly have to set total pressure? Because in openfoam I set static pressure at the inlet & outlet and maybe my mistake is here so I'd like to know ...

I don't know if my question is very clear but thanks for your help!
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Old   February 7, 2010, 04:50
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Jouke de Baar
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the initial gauge pressure is used for supersonic flows only
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Old   February 7, 2010, 08:30
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Jérémy Bulle
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Ok thanks

but why isn't there an option "inlet static pressure" for boundary condition? Why am I forced to set "inlet total pressure" ?
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Old   February 7, 2010, 09:28
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Jouke de Baar
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i think it has to do with the fysical aspect of your boundary condition. i can imagine a boundary at a fixed total pressure (like the atmospheric pressure or something). but i can't imagine a fysical boundary that can discriminate static pressure from dynamic pressure. a fysical boundary will just 'see' the total pressure?

anyway i'm not sure, just a gues ...
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Old   February 7, 2010, 09:33
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Jérémy Bulle
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Thanks for the hint.

I see things like this : Imposing a static pressure difference between inlet/outlet and you will have a flow ... This is why I don't understand why you abslutly have to set total pressure... There must be a reason but I don't understand ...

In fact I try to compare fluent with openfoam because in openfoam you can set whatever you want as boundary conditions even things wich are completely wrong so I try to "copy/paste" fluent's BC to openfoam to be sure that my case is OK at boundary condition level.
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Old   February 7, 2010, 20:37
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Hmm....perhaps some manual calculations to get a rough idea of the pressure drops involved would be helpful. But yeah, thats how FLUENT works
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