[K.C.]

Hello everybody,

actually I have a problem with the discretisation of UEqn in InterFoam.
As you can see from the source code, the term "fvc::grad(U)&fvc::grad(nu)" is subtracted; i guess to take into account the variable viscosity.
But if I look deep into the source code of fvm::laplacian(nu,U), then the variable viscosity should already be treated.

The volume integral of "Gradient(nu*Gradient(U))" is euqal to the surface integral over "nu*Gradient(U)" (Gauus theorem). And nu is interpolated to face centers, before matrix coefficients are calculated. Therefore the varibale viscosity is already taken into account, isn't it?
So where does the explicit correction term comes from?

Generall question:
How can the Gradient of a vectorField be calculated?
I followed the source code up to Foam::fv::gradScheme<Type>::grad.
Then my skills in C++ metaprogramming fail.
I can see the call of calcGrad in some cases (I don't really understand the if-condition), which would involve interpolation of U to surface centers and sum up the face area vectors*U:

forAll(owner, facei)
{
GradType Sfssf = Sf[facei]*issf[facei];

igGrad[owner[facei]] += Sfssf;
igGrad[neighbour[facei]] -= Sfssf;
}

Then I dont see the diverence to fvc::div(U), which should be zero all the time (pressure fields makes velocity field divergence free).

Previous thanks for any answer.