[bieshuxuhe]

hi,
I often see two kinds of UEqn in "Foam of multiphase flow"(e.g. interFoam ,MRFInterFoam ......) ! these are :
the first is

   surfaceScalarField muEff
    (
        "muEff",
        mixture.muf()
      + fvc::interpolate(rho*turbulence->nut())
    );

    fvVectorMatrix UEqn
    (
        fvm::ddt(rho, U)
      + fvm::div(mixture.rhoPhi(), U)
      - fvm::laplacian(muEff, U)
      - (fvc::grad(U) & fvc::grad(muEff))
    //- fvc::div(muEff*(fvc::interpolate(dev(fvc::grad(U))) & mesh.Sf()))
    );
this kind of UEqn often used for porous or MRF ...

the second is

    fvVectorMatrix UEqn
    (
        fvm::ddt(rho, U)
      + fvm::div(rhoPhi, U)
      + turbulence->divDevRhoReff(rho, U)
    );

to make divDevRhoReff() more clear ,look down (for kEpsilon ):

00195 tmp<fvVectorMatrix> kEpsilon::divDevRhoReff
00196 (
00197     const volScalarField& rho,
00198     volVectorField& U
00199 ) const
00200 {
00201     volScalarField muEff("muEff", rho*nuEff());
00202 
00203     return
00204     (
00205       - fvm::laplacian(muEff, U)
00206       - fvc::div(muEff*dev(T(fvc::grad(U))))
00207     );
00208 }
this kind of UEqn often used for interFoam, InterDyMFoam ...

I think the red part in the code is the main difference ! That is the diffution term ! could you talk about how to make a choice between the two kinds of UEqn ?
for example : if we take the first UEqn for InterDyMFoam , will it be unsuitable for some reason ?

[bieshuxuhe]

you could discuss whatever you want about the discretization of diffution term .