Why should we define both P and U for boundaries?

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 October 17, 2010, 22:50 Why should we define both P and U for boundaries? #1 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 10 Sponsored Links 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.

 October 18, 2010, 03:20 #2 Senior Member   Christian Lucas Join Date: Aug 2009 Location: Braunschweig, Germany Posts: 202 Rep Power: 10 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

October 18, 2010, 03:55
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 Originally Posted by Chris Lucas 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
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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