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Old   January 13, 2011, 16:09
Default pRef and adjustPhi
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Santiago Marquez Damian
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Hi all, I was studying adjustPhi and it is clear what is done, but I have a more basic question,

1. What is the reason of this adjustment and how it is related to pRef (the code is not run if pressure is fixed in some boundary)?

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Old   January 16, 2011, 15:03
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Hello Santiago,

Yes you are right. In case pressure has to be calculated, adjustPhi is necessary to obey continuity and allow a correct pressure solution. If pressure is already known (fixed) there's no reason for using adjustPhi.

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Old   January 17, 2011, 22:48
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Santiago Marquez Damian
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José, thx for the answer. I know that part, the background question is:

Why can appear mass imbalances if pressure is not fixed at any boundary?

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Old   January 18, 2011, 09:27
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David Boger
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Keep in mind that the system of equations for the pressure Poisson equation is singular if there are Neumann conditions on all boundaries. On the other hand, if pressure is specified at one point or more, the matrix is non-singular. In the latter case, the matrix can be inverted without any problem, regardless of the right-hand side. In the former case, the matrix is singular and solutions to the system are more limited. In particular, they require that mass is already globally conserved, which amounts to one less condition that the solution needs to overcome.

That's a pretty non-technical explanation, but I hope it helps. For more info, see section 1.3 (compatibility conditions) in this link.
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Old   January 18, 2011, 12:22
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Santiago Marquez Damian
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David thanks for your answer. It's true with all Neumann BC's, compatibility conditions must be satisfied. Nevertheless in FOAM due the pressure referencing you never solve a singular matrix. If you have a pressure boundary then there isn't a problem, on the other hand if pressure have to be referenced then you have to give a pRefValue and pRefCell or pRefPoint. With this data pressure value is enforced in that point by pEqn transformation, via fvMatrix::setReference() method. Looking at code, in fvMatrix.C


Code:
00475 template<class Type>
00476 void Foam::fvMatrix<Type>::setReference
00477 (
00478     const label celli,
00479     const Type& value,
00480     const bool forceReference
00481 )
00482 {
00483     if ((forceReference || psi_.needReference()) && celli >= 0)
00484     {
00485         source()[celli] += diag()[celli]*value;
00486         diag()[celli] += diag()[celli];
00487     }
00488 }
00489
it's clear that p value is enforced weakly. When you solve pEqn you don't obtain in pRefPoint the exact value you want. It would be completely different if you replace the equation for p en the cell corrresponding to pRefPoint doing diag()[celli]=1; and source()[celli]=pRef; then you recover the exact value of p.

May be this difference is the reason of the correction when you enforce the value of pRef. I don't know, but it's a detail we've found here with my advisor.

Regards.
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Old   January 18, 2011, 12:56
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As far as the weak enforcement, I think Hrv has discussed the reasoning previously on the forum. These touch on the topic (adjustPhi and the reference pressure), but if you dig around on the forum some more, I think there is a more explicit discussion somewhere already.
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Old   January 18, 2011, 14:11
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David, thx for the linx, I did some searching before but I couldn't arrive to these threads. Things aren't enterely clear yet. Hrv discusses the necessity of adjustPhi but not why it is necessary even if you set a reference. I'll post my conclusions later.

Regards.
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Old   January 29, 2013, 13:14
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Quote:
Originally Posted by santiagomarquezd View Post
David, thx for the linx, I did some searching before but I couldn't arrive to these threads. Things aren't enterely clear yet. Hrv discusses the necessity of adjustPhi but not why it is necessary even if you set a reference. I'll post my conclusions later.

Regards.
Dear Santiago

did you find the answer of your question ?
if yes, please share it me.

Regards
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