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 January 13, 2011, 16:09 pRef and adjustPhi #1 Senior Member     Santiago Marquez Damian Join Date: Aug 2009 Location: Santa Fe, Santa Fe, Argentina Posts: 418 Rep Power: 14 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)? Regards __________________ Santiago MÁRQUEZ DAMIÁN, Ph.D. Post-doctoral Fellow Research Center for Computational Mechanics (CIMEC) - CONICET/FICH-UNL T.E.: 54-342-4511594 Ext. 1005 Güemes 3450 - (3000) Santa Fe Santa Fe - Argentina http://www.cimec.org.ar

 January 16, 2011, 15:03 #2 Member   Usit McCarra Join Date: Oct 2010 Posts: 51 Blog Entries: 2 Rep Power: 6 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. Regards!

 January 17, 2011, 22:48 #3 Senior Member     Santiago Marquez Damian Join Date: Aug 2009 Location: Santa Fe, Santa Fe, Argentina Posts: 418 Rep Power: 14 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? Regards. __________________ Santiago MÁRQUEZ DAMIÁN, Ph.D. Post-doctoral Fellow Research Center for Computational Mechanics (CIMEC) - CONICET/FICH-UNL T.E.: 54-342-4511594 Ext. 1005 Güemes 3450 - (3000) Santa Fe Santa Fe - Argentina http://www.cimec.org.ar

 January 18, 2011, 09:27 #4 Senior Member   David Boger Join Date: Mar 2009 Location: Penn State Applied Research Laboratory Posts: 146 Rep Power: 8 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. sharonyue and abtinansari like this. __________________ David A. Boger

 January 18, 2011, 12:22 #5 Senior Member     Santiago Marquez Damian Join Date: Aug 2009 Location: Santa Fe, Santa Fe, Argentina Posts: 418 Rep Power: 14 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 00476 void Foam::fvMatrix::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. __________________ Santiago MÁRQUEZ DAMIÁN, Ph.D. Post-doctoral Fellow Research Center for Computational Mechanics (CIMEC) - CONICET/FICH-UNL T.E.: 54-342-4511594 Ext. 1005 Güemes 3450 - (3000) Santa Fe Santa Fe - Argentina http://www.cimec.org.ar

 January 18, 2011, 12:56 #6 Senior Member   David Boger Join Date: Mar 2009 Location: Penn State Applied Research Laboratory Posts: 146 Rep Power: 8 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. __________________ David A. Boger

 January 18, 2011, 14:11 #7 Senior Member     Santiago Marquez Damian Join Date: Aug 2009 Location: Santa Fe, Santa Fe, Argentina Posts: 418 Rep Power: 14 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. __________________ Santiago MÁRQUEZ DAMIÁN, Ph.D. Post-doctoral Fellow Research Center for Computational Mechanics (CIMEC) - CONICET/FICH-UNL T.E.: 54-342-4511594 Ext. 1005 Güemes 3450 - (3000) Santa Fe Santa Fe - Argentina http://www.cimec.org.ar

January 29, 2013, 13:14
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Quote:
 Originally Posted by santiagomarquezd 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

if yes, please share it me.

Regards

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