CFD Online Discussion Forums - Signs used in advecting volume fractions in interfoam

Hi,

I am aware of the standard equation used to advect volume fractions and about the derivation regarding "interface compression" that interfoam solver uses.

Coming to the code in "alphaEqn.H", I have the following two questions:

Code:

surfaceScalarField phic = mag(phi/mesh.magSf());                      

phic = min(interface.cAlpha()*phic, max(phic));                      

 surfaceScalarField phir = phic*interface.nHatf();                        

   for (int aCorr=0; aCorr<nAlphaCorr; aCorr++)                      

     {                       

      surfaceScalarField phiAlpha =   

                                  fvc::flux ( phi,  

                                  alpha1,                       

                                  alphaScheme  )

                               + fvc::flux ( -fvc::flux(-phir, scalar(1) - alpha1, alpharScheme),

                                               alpha1,

                                              alpharScheme );

Q1) I don't really understand the sign convention used for the compression term.
In the relative flux term: -fvc::flux(-phir, ---). So, arent you finally adding an additional convective term to the first term of "phi*alpha1"?

Q2) If C-alpha =1 and considering the relative velocity acts as a shock front against the original convective term, wouldnt both the flux term and the relative flux term balance each other and cancel out leading to no advection?

I do realise my imagination is wrong as I do see correct flow behaviour. Can anyone enlighten me.

Thanks;
Katt