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November 30, 2010, 05:15 
I need explanations about fixedFluxPressure

#1 
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Cyprien
Join Date: Feb 2010
Location: Stanford University
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Hello!
I had a look at the fixedFluxPressure boundary condition and I am not quite sure to well understand what this BC does. I guess I have to use this BC if I want to evaluate the pressure according to a velocity input. Am I wrong ? Furthermore, in the code source, one can read: Code:
gradient() = (phip  (patch().Sf() & Up))/patch().magSf()/rAp; All enlightments are welcome! Regards, Cyp 

August 19, 2013, 20:36 
buoyantPressure vs. fixedFluxPressure

#2 
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Jose Rey
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I realize this is an old thread, but it is still very relevant.
I was having terrible problems trying to get buoyantBoussinesqSimpleFoam to converge. I changed the p_rgh BC's from buoyantPressure to fixedFluxPressure and the model started to converge. Anybody? 

August 1, 2014, 04:42 

#3 
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Srivathsan N
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Location: India
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Same question here. What is the difference between buoyantPressure and fixedFLuxPressure boundary conditions?
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Regards, Srivaths 

October 16, 2014, 02:23 
OpenFAOM 2.3.0 doesn't have buoyantPressure BC

#4 
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Fatema Zandi Goharrizi
Join Date: Mar 2009
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Hi
OpenFAOM version 2.3.0 doesn't have buoyantPressure libraries. Am I right? what is the difference between buoyantPressure and fixedFluxPressure? why fixedFluxPressure converge better? is it the only difference between them? Please help 

October 16, 2014, 03:46 

#5 
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Fabian Roesler
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OpenFOAM 2.2.2 has both boundary conditions. In newer Version, buoyantPressure boundary was dismissed. For clarification have a look into the code or into Doxygen.
From buoyantPressureFvPatchScalarField we learn that: This boundary condition sets the pressure gradient appropriately for buoyant flow. If the variable name is one of pd, p_rgh or ph_rgh, we assume that the pressure variable is and the gradient set using: Otherwise we assume that it is the static pressure, and the gradient calculated using: From fixedFluxPressureFvPatchScalarField we see that: This boundary condition adjusts the pressure gradient such that the flux on the boundary is that specified by the velocity boundary condition. The predicted flux to be compensated by the pressure gradient is evaluated as , both of which are lookedup from the database, as is the pressure diffusivity used to calculate the gradient using: 

October 16, 2014, 04:21 

#6 
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Fabian Roesler
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Some additional remarks:
The fixedFluxPressure boundary is known do have a better convergence. This is in my opinion due to the more pressure related calculation of the gradient: phi and phiHbyA (predicted flux field) are directly linked to pressure equation. On a wall, the flux difference tends to zero and so the boundary condition turns to zeroGradient. The buoyanPressure boundary is more the physical method to describe the pressure gradient with Archimedes' principle. I hope my explanation is understandable and correct. Cheers Fabian 

November 5, 2014, 17:06 

#7 
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Santiago Marquez Damian
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Hi folks, I'm dealing with fixedFluxPressure BCs also and this thread was helpful. Checking the code things have changed at least in FOAM 2.3.0. The class definition is now more simple
fixedFluxPressureFvPatchScalarField.C Code:
125 void Foam::fixedFluxPressureFvPatchScalarField::updateCoeffs 126 ( 127 const scalarField& snGradp 128 ) 129 { 130 if (updated()) 131 { 132 return; 133 } 134 135 curTimeIndex_ = this>db().time().timeIndex(); 136 137 gradient() = snGradp; 138 fixedGradientFvPatchScalarField::updateCoeffs(); 139 } Code:
27 // Update the fixedFluxPressure BCs to ensure flux consistency 28 setSnGrad<fixedFluxPressureFvPatchScalarField> 29 ( 30 p_rgh.boundaryField(), 31 ( 32 phiHbyA.boundaryField() 33  (mesh.Sf().boundaryField() & U.boundaryField()) 34 )/(mesh.magSf().boundaryField()*rAUf.boundaryField()) 35 ); Hope this helps for 2.3.0 users. Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar Last edited by santiagomarquezd; November 7, 2014 at 06:25. Reason: Add equation 

April 14, 2015, 16:46 

#8 
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anonymous
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I would like to comment out a bit, why does OpenFOAM use the expression given by santiagomarquezd (gracias por el análisis del código fuente) for the pressure surface normal gradient in the boundaries.
First of all we need to write down the momentum equation discretized using the RhieChow interpolation method. This reads: Where the f subscript means interpolate to the face, is the variable p_rgh whose value is , and the vector u is the velocity. Now multiply the equation by the face surface vector where the vector n is the surface normal: Thus the following equation for the pressure surface gradient can be obtained: Hope this is useful for you foamers 

October 14, 2015, 13:55 

#9 
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Angel Monsalve
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Thanks for the explanation!!


August 31, 2016, 14:35 
value uniform 0;

#10 
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Jon Elvar Wallevik
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Dear all, thank you for the above discussion
There is one thing I don't understand fully. In the damBreak example for interFoam (say of OF 4.0) the b.c. for p_rgh is given as... leftWall { type fixedFluxPressure; value uniform 0; } so the question is, what does "value uniform 0;" stand for? There is one constructor in fixedFluxPressureFvPatchScalarField.C which has 3 arguments: Foam::fixedFluxPressureFvPatchScalarField::fixedFl uxPressureFvPatchScalarField ( const fvPatch& p, const DimensionedField<scalar, volMesh>& iF, const dictionary& dict ) ... and in that function body, there is if (dict.found("value") && dict.found("gradient")) But in interFoam, this constructor is not used, since it is the constructor with two arguments that is used (actually the first constructor), c.f. in pEqn.H setSnGrad<fixedFluxPressureFvPatchScalarField> ( p_rgh.boundaryField(), ( phiHbyA.boundaryField()  fvOptions.relative(mesh.Sf().boundaryField() & U.boundaryField()) )/(mesh.magSf().boundaryField()*rAUf.boundaryField() ) ); So am I understanding this correctly: When using interFoam, then the "value uniform 0;" is actually not used? Any comment would be helpful J. 

November 23, 2016, 18:45 

#11 
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Declan
Join Date: Oct 2016
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Hi Jon,
I had a similar question about flowRateInletVelocity which requires an input value that doesn't appear to do anything. As explained by Roman in the below thread some derived boundary conditions necessarily inherit a placeholder value like this one because of their basic structure. http://www.cfdonline.com/Forums/ope...tvelocity.html I think the value uniform 0; you asked about is like this and could be assigned any value you like without affecting your simulation. 

January 18, 2017, 05:46 

#12 
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Mido
Join Date: Mar 2011
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So the bottom line, The difference between this bc and zeroGradient is in term of the convergence (HOW MANY ITER TO SOLVE THE PRESSURE EQN ) not the accuracy or whether the solver will converge or not.


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