# Differences in solution method for pisoFoam and buoyantBoussinesqPisoFoam

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 December 30, 2009, 17:56 Differences in solution method for pisoFoam and buoyantBoussinesqPisoFoam #1 Member   Matthew J. Churchfield Join Date: Nov 2009 Location: Boulder, Colorado, USA Posts: 49 Rep Power: 18 To whom can help, I am trying to reconcile some differences between pisoFoam and buoyantBoussinesqPisoFoam. In the UEqn.H file of buoyantBoussinesqPisoFoam the following equation is set up to predict velocity: UEqn == fvc::reconstruct((fvc::interpolate(rhok)*(g & mesh.Sf()) - fvc::snGrad(p)*mesh.magSf())) However, in pisoFoam, this is the equation: UEqn == -fvc::grad(p) Aside from the inclusion of the gravity term in buoyantBoussinesqPisoFoam, why are the face values used to reconstruct the cell centered values, whereas in pisoFoam the cell center values are used directly? The same situation occurs in setting up the pressure equation. In buoyantBoussinesqPisoFoam, the equation is: volScalarField rUA("rUA", 1.0/UEqn.A()); surfaceScalarField rUAf("(1|A(U))", fvc::interpolate(rUA)); fvm::laplacian(rUAf, p) == fvc::div(phi) whereas in pisoFoam, it is: volScalarField rUA = 1.0/UEqn.A(); fvm::laplacian(rUA, p) == fvc::div(phi) buoyantBoussinesqPisoFoam is solving for pressure on the faces, and pisoFoam is solving for cell centered pressure. Does this make a difference? Why are the two codes different in this manner? Thank you rajibroy likes this.

 January 10, 2010, 17:27 #2 Senior Member   Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,912 Rep Power: 36 Hi, in pisoFoam you have the standard implementation, in buoyantBoussinesqPisoFoam the solution algorithm is modified as follows: - you reconstruct the gravity and the pressure gradient contributions from the corresponding contribution to the flux - you solve a "pseudo-staggered" version of the pressure equation - you correct the flux - you obtain the velocity correction reconstructing from the flux again (remember the flux is always continuous) This technique tries to mimic a staggered grid arrangement. It is applied to the gravity term too, since it is included in the pressure equation. Best, Alberto kaifu, MPJ, mechy and 7 others like this. __________________ Alberto Passalacqua GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541) OpenQBMM - An open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. Last edited by alberto; January 10, 2010 at 17:28. Reason: removed quote

March 12, 2012, 06:12
#3
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Samuele Z
Join Date: Oct 2009
Location: Mozzate - Co - Italy
Posts: 520
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Quote:
 Originally Posted by alberto Hi, in pisoFoam you have the standard implementation
Dear Alberto,

I am trying to understand the different solvers. So, for simpleFoam I've found this wiki..
As far as the pisoFoam is concerned, you wrote that it present the standard implementation.
What does this mean? Is there a reference page?

Thanks a lot,
Samuele

March 12, 2012, 06:53
#4
Senior Member

Alberto Passalacqua
Join Date: Mar 2009
Location: Ames, Iowa, United States
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Rep Power: 36
Quote:
 Originally Posted by samiam1000 Dear Alberto, I am trying to understand the different solvers. So, for simpleFoam I've found this wiki.. As far as the pisoFoam is concerned, you wrote that it present the standard implementation. What does this mean? Is there a reference page? Thanks a lot, Samuele
Hi Samuele,

unfortunately I don't think there is a reference page. In pisoFoam you have the standard PISO algorithm you find in books, without body force term and without any particular treatment, except the Rhie-Chow interpolation. You can take a look at the icoFoam page on the wiki and you'll see many similarities.

My statement has to be read in the context of the comparison between the two solvers in the topic, where buoyantBoussinesqPisoFoam uses flux reconstruction to improve the solution procedure when body force terms are included.

Best,
__________________
Alberto Passalacqua

GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541)
OpenQBMM - An open-source implementation of quadrature-based moment methods.

To obtain more accurate answers, please specify the version of OpenFOAM you are using.

 March 12, 2012, 06:57 #5 Senior Member   Samuele Z Join Date: Oct 2009 Location: Mozzate - Co - Italy Posts: 520 Rep Power: 18 That's great, thanks. And what about the pimpleFoam solver? Do you know which solver is embedded in such a solver? Thanks a lot, Samuele

 March 12, 2012, 07:00 #6 Senior Member   Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,912 Rep Power: 36 The "pimple" solvers use a "combination of PISO and SIMPLE", which is not that far from the flavors you find in other codes with different names (unsteady SIMPLE, iterative PISO, depending on the creativity of the authors :-)). In short, it is an iterative solution method with sub-iterations over the set of equations to improve the robustness of the algorithm using under-relaxation, and to speed-up transient simulations or perform pseudo-transient simulations. Best, samiam1000 likes this. __________________ Alberto Passalacqua GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541) OpenQBMM - An open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

September 10, 2013, 23:24
#7
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Dongyue Li
Join Date: Jun 2012
Location: Beijing, China
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Quote:
 Originally Posted by alberto Hi, in pisoFoam you have the standard implementation, in buoyantBoussinesqPisoFoam the solution algorithm is modified as follows: - you reconstruct the gravity and the pressure gradient contributions from the corresponding contribution to the flux - you solve a "pseudo-staggered" version of the pressure equation - you correct the flux - you obtain the velocity correction reconstructing from the flux again (remember the flux is always continuous) This technique tries to mimic a staggered grid arrangement. It is applied to the gravity term too, since it is included in the pressure equation. Best, Alberto
Dear Alberto,

Can I say that I have to reconstruct it when there is a body force? Is it a must?

for example, in interFoam's UEqn:
Code:
```solve
(
UEqn
==
fvc::reconstruct
(
(
) * mesh.magSf()
)
);```
Can I code it like:
Code:
```solve
(
UEqn
==

);```

August 6, 2023, 09:12
#8
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Reviewer #2
Join Date: Jul 2015
Location: Knoxville, TN
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Quote:
 Originally Posted by alberto Hi, in pisoFoam you have the standard implementation, in buoyantBoussinesqPisoFoam the solution algorithm is modified as follows: - you reconstruct the gravity and the pressure gradient contributions from the corresponding contribution to the flux - you solve a "pseudo-staggered" version of the pressure equation - you correct the flux - you obtain the velocity correction reconstructing from the flux again (remember the flux is always continuous) This technique tries to mimic a staggered grid arrangement. It is applied to the gravity term too, since it is included in the pressure equation. Best, Alberto
Alberto,

What is the reason that this "pseudo-staggered" version of the pressure equation is not needed in the standard psioFoam, but needed by the buoyantBoussinesqPisoFoam?

Thanks,
Rdf