CFD Online Discussion Forums - BuoyantBoussinesq(Pimple/Piso)Foam, changed equation from OF 1.6 to 1.7 ???

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- - BuoyantBoussinesq(Pimple/Piso)Foam, changed equation from OF 1.6 to 1.7 ??? (https://www.cfd-online.com/Forums/openfoam/80466-buoyantboussinesq-pimple-piso-foam-changed-equation-1-6-1-7-a.html)

BuoyantBoussinesq(Pimple/Piso)Foam, changed equation from OF 1.6 to 1.7 ???

Hi Folks,

one question regarding the changes from OF 1.6 to 1.7 in the buoyantBoussinesq(Piso/Pimple)Foam solver. I think the implementation of splitting the pressure according to p = p_rgh + rho*g.x is not fully correct.

The pressure equation in OF 1.7 seems OK, since in the Rhie-Chow formulation the contributions from the static pressure cancel and we are left with:

Code:

phi = (fvc::interpolate(U) & mesh.Sf())

        + fvc::ddtPhiCorr(rUA, U, phi);



surfaceScalarField buoyancyPhi = rUAf*ghf*fvc::snGrad(rhok)*mesh.magSf();

phi -= buoyancyPhi;

instead of

Code:

 surfaceScalarField phiU

    (

        (fvc::interpolate(U) & mesh.Sf())

      + fvc::ddtPhiCorr(rUA, U, phi)

    );



    phi = phiU + rUAf*fvc::interpolate(rhok)*(g & mesh.Sf());

in OF 1.6.

In contrast, in the momentum equation the term for the buoyancy ( and gravity) force has changed from (OF 1.6)

Code:

fvc::reconstruct( ...

                    fvc::interpolate(rhok)*(g & mesh.Sf())  ...)

to (OF 1.7)

Code:

fvc::reconstruct( ...

  - ghf*fvc::snGrad(rhok)*mesh.magSf() ...)

These are two different expressions: F ~ rhok * g vs. F ~ gradT * g.h

Is this really a bug or do I miss some point? Thanks for any comment...

Regards,
Jan

Edit:
In the meantime I am pretty sure this is indeed a bug and I submitted a bugreport.

Edit:
Problem is solved. This bug was fixed a few days ago in the git repository.

Dear Foamers,
can someone explain me why OF1.7 still uses

HTML Code:

- ghf*fvc::snGrad(rhok)*mesh.magSf()

instead of

HTML Code:

fvc::interpolate(rhok)*(g & mesh.Sf())

as it was used in the previous version OF1.6.
I do not understand why the gradient of the densitiy is used while in the equations I find in literature it has to be just the densitiy multiplied with the constant g.
Can someone help me? Is this still a bug or do I overlook something?

Quote:

Originally Posted by myself (Post 276790)

Code:

phi = (fvc::interpolate(U) & mesh.Sf())

        + fvc::ddtPhiCorr(rUA, U, phi);



surfaceScalarField buoyancyPhi = rUAf*ghf*fvc::snGrad(rhok)*mesh.magSf();

phi -= buoyancyPhi;

instead of

Code:

 surfaceScalarField phiU

    (

        (fvc::interpolate(U) & mesh.Sf())

      + fvc::ddtPhiCorr(rUA, U, phi)

    );



    phi = phiU + rUAf*fvc::interpolate(rhok)*(g & mesh.Sf());

in OF 1.6.

In contrast, in the momentum equation the term for the buoyancy ( and gravity) force has changed from (OF 1.6)

Code:

fvc::reconstruct( ...

                    fvc::interpolate(rhok)*(g & mesh.Sf())  ...)

to (OF 1.7)

Code:

fvc::reconstruct( ...

  - ghf*fvc::snGrad(rhok)*mesh.magSf() ...)

Hallo Jan!

You wrote that the term in OF17 was formulated with an error, am I correct?

But still if i look OF171 (Ubuntu package installed via apt-get):

Code:

            UEqn

         ==

            fvc::reconstruct

            (

                (

                  - ghf*fvc::snGrad(rhok)

                  - fvc::snGrad(p_rgh)

                )*mesh.magSf()

            )

Is this term still there? And how it should be?

Regards,
Alexander