Phase field: calculation of the |grad f| div(grad f/|grad f|) term
 

The equation i've to solve (Y.Sun,C. Beckermann, Sharp interface tracking using phase-field equation, Journal of computational physics) is:

df/dt+V*grad f=a*[ laplacian f+f*(1-f)/b-|grad f| div [ grad f/|grad f|]]
with a,b parameters (or constants).

how can I calculate the "|grad f| div(grad f/|grad f|) " term in an esplicit way in OF?

This is how I discretised the equation (except the term i'm talking about) neglecting, for the moment, the transport term:

fvScalarMatrix phaseEqn
(
fvm::Sp(1./runTime.deltaT(),phase)
-ophase/runTime.deltaT()
-a*fvm::laplacian(phase)
-1./b*ophase*(1.-ophase*ophase)
+?????
);
phaseEqn.relax();
iterations=solve(phaseEqn).nIterations();

f=phase and ophase is the old time step value of the field (i know i can use ddt directly but i need to perform some coupling later so i prefer to be "free" to discretise by myself the temporal derivative)

Thank you!