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Why should we define both P and U for boundaries?

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Old   October 17, 2010, 22:50
Lightbulb Why should we define both P and U for boundaries?
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Maysam
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Dear Foamers,

What should be velocity boundary condition (U) type for outlet of a problem which we want set pressure (P) as fixedValue.


We can't suppose it zeroGradient because it is not fully developed at outlet. and i don't know why should we define both of P and U for boundaries in OpenFOAM.
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Old   October 18, 2010, 03:20
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Hi,

you have to define BC for each field you are solving, so you need a BC for U and p. Other CFD software like fluent or cfx does the same (without telling you).

zeroGradients is the correct BC (or better inletOutlet) for U at the outlet. Of course your BC has an effect on the solution (as in all CFD software), so you must place the outlet BC far (at least a bit) away from the area of interest.

Regards,
Christian
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Old   October 18, 2010, 03:55
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Quote:
Originally Posted by Chris Lucas View Post
Hi,

you have to define BC for each field you are solving, so you need a BC for U and p. Other CFD software like fluent or cfx does the same (without telling you).

zeroGradients is the correct BC (or better inletOutlet) for U at the outlet. Of course your BC has an effect on the solution (as in all CFD software), so you must place the outlet BC far (at least a bit) away from the area of interest.

Regards,
Christian
Thanks for your reply.
the problem is in processing algorithm depend on its form, in boundary cell we set pressure and the algorithm try to calculate a velocity which satisfy continuity and ...
so setting both of U and P leads to have more equations than unknowns and this is the cause that other softwares like Fluent don't need to define both of them.
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