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Why is new convection theme used is the rhoEqn ?

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Old   December 3, 2014, 00:28
Question Why is new convection theme used is the rhoEqn ?
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Hi Everyone,

While I am reading the code I got from my friend, I find following code:

Code:
tmpPhi=phi;
tmp<fv::convectionScheme<scalar> > mvConvection
(
    fv::convectionScheme<scalar>::New
    (
        mesh,
        fields,
        tmpPhi,
        mesh.divScheme("div(phi,Yh)")
    )
);

//Density equation
fvScalarMatrix rhoEqn
(
    fvm::ddt(rho) +  mvConvection->fvmDiv(tmpPhi, rho)
);

rhoEqn.solve();
It seems that this code has created a new convection themes, and apply it like
Code:
turbulence->divDevRhoReff(rho, U)
My first question is that what is the difference between this new rho equation and its original form?

And latter, the new convection theme is used again as following:

Code:
fvScalarMatrix YEqn
(
    fvm::ddt(rho,Y) 
    + urho*mvConvection->fvmDiv(tmpPhi, Y)
    + fvm::laplacian(Dcoeff,Y)
);
where urho is defined as

Code:
const dimensionedScalar urho  //for tmpPhi = phi || rhoPhi
    (
        "urho",
        dimensionSet(1,-3,0,0,0,0,0),
        scalar(1.0)
    );
My second question is why the new convection theme is used again here and why do we need the "urho".

Have anyone met this before?
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