CFD Online Discussion Forums - Mass flux update in simpleFoam and rhoSimpleFoam

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Mass flux update in simpleFoam and rhoSimpleFoam

Hello to all,

I would like to know if anyone can clarify why the mass flux in simpleFoam is calculated as:

Code:

phi = phiHbyA - pEqn.flux();

And in rhoSimpleFoam:

Code:

phi = phiHbyA + pEqn.flux();

Why does the sign change?

in simpleFoam it comes from the discretization of the continuity equation:

$\vec{v}_P = a_P^{-1} H(\vec{v}) - a_P^{-1}\nabla P$

$\nabla \cdot \vec{v} = 0$

$F = \vec{v}_f \cdot \vec{S_f} = \left (a_P^{-1} H(\vec{v}) \right )_f- \left (a_P^{-1}\nabla P \right )_f$

Where the .flux() function is used to rebuild the fluxes accounting for the implicit and explicit part resulting from the discretization of the laplacian operator (https://www.cfd-online.com/Forums/op...ux-method.html)

For the rhoSimpleFoam part, I would appreciate the help!