[Sun]

Hi,
I am going through the compressibleInterFoam solver (VOf for 2 compressible, isothermal immiscible fluids). In the sub-cycling of the alpha equation (alphaEqnsSubCycle.H) there is this line of code:

volScalarField divU(fvc::div(fvc::absolute(phi, U)));

I cannot figure out what this line does. Any explanations or helps would be really appreciated.
Thanks.
Cheers!

[Sun]

As far as I understand, it returns the given relative flux in absolute form. And i think this is how it does it. (fvcMeshPhi.C from line 151 to 183)

Foam::tmp<Foam::surfaceScalarField> Foam::fvc::absolute
 (
    const tmp<surfaceScalarField>& tphi,
    const volVectorField& U
)
{
    if (tphi().mesh().moving())
     {
         return tphi + fvc::meshPhi(U);
     }
     else
     {
         return tmp<surfaceScalarField>(tphi, true);
      }
   }
   
   
   Foam::tmp<Foam::surfaceScalarField> Foam::fvc::absolute
   (
       const tmp<surfaceScalarField>& tphi,
       const volScalarField& rho,
       const volVectorField& U
   )
   {
       if (tphi().mesh().moving())
       {
           return tphi + fvc::interpolate(rho)*fvc::meshPhi(rho, U);
       }
       else
       {
           return tmp<surfaceScalarField>(tphi, true);
       }
   }

I still don't know, though, why we need it for the alpha sub-cycling?