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-   -   problem when decomposing the velocity flux according to the x, y and z directions.. (http://www.cfd-online.com/Forums/openfoam-programming-development/84234-problem-when-decomposing-velocity-flux-according-x-y-z-directions.html)

Cyp January 24, 2011 06:48

[solved]problem when decomposing velocity flux according to the x, y and z directions
 
Hi!

I would like to decompose the velocity flux phi into 3 flux, one for each direction:

Code:

phi = phi_x + phi_y + phi_z
To do that, I defined phi_x, phi_y and phi_z as:
Code:

surfaceScalarField phi_x = linearInterpolate(U.component(vector::X)*vector(1,0,0)) & mesh.Sf();
surfaceScalarField phi_y = linearInterpolate(U.component(vector::Y)*vector(0,1,0)) & mesh.Sf();
surfaceScalarField phi_z = linearInterpolate(U.component(vector::Z)*vector(0,0,1)) & mesh.Sf();

Then I update these fields at each time step with the above definition. However, the field
Code:

phiTest = phi - (phi_x+phi_y+phi_z)
is not equal to "zero". Which (I suppose) yields in divergence for my applications...

Is someone know how to do such a thing ?

I was wondering is there is a way to get these fields from the fvScalarMatrix pEqn exactly in the same manner we update the phi flux field :
In OpenFOAM, we get the phi flux from the pEqn matrix:
Code:

fvScalarMatrix pEqn
(
    fvm::laplacian(-M,p)
);
pEqn.setReference (....);
pEqn.solve();
phi = pEqn.flux();

I was wondering is there is a way to get these fields (phi_x, phi_y, phi_z) from the fvScalarMatrix pEqn in similar manner ??

Best regards,
Cyp

PS: in my example, M is a volTensorField

Cyp January 24, 2011 15:46

I found a solution : from phi = pEqn.flux() I can create phix, phiy and phiz.


First, I defined a flux phi_ex with 1 on the faces concerning x and 0 elsewhere. I can create this flux with this snippet:

Code:

    volVectorField E
    (
        IOobject
        (
            "E",
            runTime.timeName(),
            mesh,
            IOobject::NO_READ,
            IOobject::AUTO_WRITE
        ),
        mesh,
      dimensionedVector ("E",dimensionSet(0,0,0,0,0),vector::zero)
    );
    E = vector(1,0,0);
    surfaceScalarField phi_ex = linearInterpolate(E) & mesh.Sf() / mesh.magSf();

then I can get phix from phi using :
Code:

phix = phi * phi_ex;
And I repeat the operation for y and z.

However, I still have problem on my outlet BC....


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