Continuity equation in coupledFvMatrix

bastil · June 8, 2010, 04:51

HI all,

I try to write a coupled solver that uses continuity equation. I tried to do it that way:

Code:

        coupledFvScalarMatrix coupledEqns(1);

        coupledEqns.set
            (    
            0,
            new volScalarField
            (
                fvc::div(phi)
            )
        );

Does not seem to work that way. Do I need to convert to fvScalarMatrix? How? Thanks.

Regards Bastian

l_r_mcglashan · June 9, 2010, 05:07

Just make it of type fvScalarMatrix:

Code:

coupledFvScalarMatrix coupledEqns(1);

coupledEqns.set
(    
   0,
   new fvScalarMatrix
   (
       fvm::ddt(rho)
       fvc::div(phi)
   )
);

where phi = rho*U interpolated to the cell faces. You can't solve what you had previously because fvc::grad calculates the gradient explicitly; there is no variable to solve for!

bastil · June 9, 2010, 10:40

Thanks Laurence,

since my code is steady-state and incompressible I do not really need fvm::ddt(rho). So I tried:

Code:

coupledFvScalarMatrix coupledEqns(1);

coupledEqns.set
(    
   0,
   new fvScalarMatrix
   (
       fvc::div(phi)
   )
);

This throws an error:

Code:

Making dependency list for source file coupledFoam.C
SOURCE=coupledFoam.C ;  g++ -m64 -Dlinux64 -DDP -DFOAM_DEV_REVISION_NUMBER=1716 -Wall -Wno-strict-aliasing -Wextra -Wno-unused-parameter -Wold-style-cast -O3  -DNoRepository -ftemplate-depth-40 -I/opt/OpenFOAM/OpenFOAM-1.5-dev/src/finiteVolume/lnInclude -I/opt/OpenFOAM/OpenFOAM-1.5-dev/src/turbulenceModels/RAS -I/opt/OpenFOAM/OpenFOAM-1.5-dev/src/coupledMatrix/lnInclude -I/opt/OpenFOAM/OpenFOAM-1.5-dev/src/transportModels -IlnInclude -I. -I/opt/OpenFOAM/OpenFOAM-1.5-dev/src/OpenFOAM/lnInclude -I/opt/OpenFOAM/OpenFOAM-1.5-dev/src/OSspecific/Unix/lnInclude   -fPIC -c $SOURCE -o Make/linux64GccDPOpt/coupledFoam.o
In file included from coupledFoam.C:65:
coupledEqn.H: In function ‘int main(int, char**)’:
coupledEqn.H:14: error: no matching function for call to ‘Foam::fvMatrix<double>::fvMatrix(Foam::tmp<Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> >)’
/opt/OpenFOAM/OpenFOAM-1.5-dev/src/finiteVolume/lnInclude/fvMatrix.C:327: note: candidates are: Foam::fvMatrix<Type>::fvMatrix(Foam::GeometricField<Type, Foam::fvPatchField, Foam::volMesh>&, Foam::Istream&) [with Type = double]
/opt/OpenFOAM/OpenFOAM-1.5-dev/src/finiteVolume/lnInclude/fvMatrix.C:266: note:                 Foam::fvMatrix<Type>::fvMatrix(const Foam::tmp<Foam::fvMatrix<Type> >&) [with Type = double]
/opt/OpenFOAM/OpenFOAM-1.5-dev/src/finiteVolume/lnInclude/fvMatrix.C:235: note:                 Foam::fvMatrix<Type>::fvMatrix(const Foam::fvMatrix<Type>&) [with Type = double]
/opt/OpenFOAM/OpenFOAM-1.5-dev/src/finiteVolume/lnInclude/fvMatrix.C:188: note:                 Foam::fvMatrix<Type>::fvMatrix(Foam::GeometricField<Type, Foam::fvPatchField, Foam::volMesh>&, const Foam::dimensionSet&) [with Type = double]
/opt/OpenFOAM/OpenFOAM-1.5-dev/src/finiteVolume/lnInclude/initContinuityErrs.H:38: warning: unused variable ‘cumulativeContErr’
make: *** [Make/linux64GccDPOpt/coupledFoam.o] Error 1

I don't really understand why.

hjasak · June 9, 2010, 13:15

Easy - you are trying to make a matrix and you are only giving it a field: fvc::div(phi).

That has no matrix coefficients - how do you expect to solve the system of equations that says

[0] * [x] = [b]

Hrv

bastil · June 9, 2010, 18:33

Quote:

Originally Posted by hjasak

That has no matrix coefficients - how do you expect to solve the system of equations that says

[0] * [x] = [b]

Thanks Hrv,

that makes sence. I am not yet deep enough in it. So how is the correct formulation of the continuity equation?

mikeP · February 10, 2012, 08:40

Hello Mr. Jasak,

This is interesting. Could you also explain this code (fvm::div(phi, U) == 0) in the matrix form, similar to what you have written before as [0] * [x] = [b] ?

All the best

mikeP · February 10, 2012, 10:24

another question - how is this possible? divergence of a scalar?

fvc::div(phi)

T.D. · October 13, 2014, 10:05

Hi,

fvm: (div,phi) == 0

Briefly explaining,

fvm: stands for implicit. A discretization in space is done first (via the FVM) and in time if necessary and all the coeffs. are placed in a matrix form [O], then the resultant of the discretization of the source terms are placed to the right hand side of the equation in a vector [S].where the unknowns are the velocity field vector components on the mesh.
All above, are such that [O]*[U] = [S] which is solved to find [U] vector field components.

For the scalar issue: you should see 4.4.5 in:
http://www.openfoam.org/docs/user/fvSchemes.php

Regards,

T.D.

June 8, 2010, 04:51	Continuity equation in coupledFvMatrix	#1
bastil Senior Member BastiL Join Date: Mar 2009 Posts: 530 Rep Power: 20	HI all, I try to write a coupled solver that uses continuity equation. I tried to do it that way: Code: coupledFvScalarMatrix coupledEqns(1); coupledEqns.set ( 0, new volScalarField ( fvc::div(phi) ) ); Does not seem to work that way. Do I need to convert to fvScalarMatrix? How? Thanks. Regards Bastian

June 9, 2010, 05:07		#2
l_r_mcglashan Senior Member Laurence R. McGlashan Join Date: Mar 2009 Posts: 370 Rep Power: 23	Just make it of type fvScalarMatrix: Code: coupledFvScalarMatrix coupledEqns(1); coupledEqns.set ( 0, new fvScalarMatrix ( fvm::ddt(rho) fvc::div(phi) ) ); where phi = rho*U interpolated to the cell faces. You can't solve what you had previously because fvc::grad calculates the gradient explicitly; there is no variable to solve for! __________________ Laurence R. McGlashan :: Website

June 9, 2010, 13:15		#4
hjasak Senior Member Hrvoje Jasak Join Date: Mar 2009 Location: London, England Posts: 1,907 Rep Power: 33	Easy - you are trying to make a matrix and you are only giving it a field: fvc::div(phi). That has no matrix coefficients - how do you expect to solve the system of equations that says [0] * [x] = [b] Hrv __________________ Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk

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February 10, 2012, 08:40		#6
mikeP Member Join Date: Jun 2011 Posts: 42 Rep Power: 15	Hello Mr. Jasak, This is interesting. Could you also explain this code (fvm::div(phi, U) == 0) in the matrix form, similar to what you have written before as [0] [x] = [b]* ? All the best

February 10, 2012, 10:24		#7
mikeP Member Join Date: Jun 2011 Posts: 42 Rep Power: 15	another question - how is this possible? divergence of a scalar? fvc::div(phi)

October 13, 2014, 10:05	Hi	#8
T.D. Senior Member Join Date: Sep 2010 Posts: 226 Rep Power: 17	Hi, fvm: (div,phi) == 0 Briefly explaining, fvm: stands for implicit. A discretization in space is done first (via the FVM) and in time if necessary and all the coeffs. are placed in a matrix form [O], then the resultant of the discretization of the source terms are placed to the right hand side of the equation in a vector [S].where the unknowns are the velocity field vector components on the mesh. All above, are such that [O]*[U] = [S] which is solved to find [U] vector field components. For the scalar issue: you should see 4.4.5 in: http://www.openfoam.org/docs/user/fvSchemes.php Regards, T.D.