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-   -   Why should we define both P and U for boundaries? (http://www.cfd-online.com/Forums/openfoam/81131-why-should-we-define-both-p-u-boundaries.html)

maysmech October 17, 2010 22:50

Why should we define both P and U for boundaries?
 
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.:confused:

Chris Lucas October 18, 2010 03:20

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

maysmech October 18, 2010 03:55

Quote:

Originally Posted by Chris Lucas (Post 279542)
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:confused: and this is the cause that other softwares like Fluent don't need to define both of them.


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