[Alucard]

Goodmoring
I'm working in the development of my mesoscopic solidification model.
In few words I solve three coupled equations:
1 for the phase (phi) :
d(phi)/dt+v_front*mag(grad(phi))=[lapl(phi)+phi*(1-phi)*(1-2*phi)-mag(grad(phi))*div[grad(phi)/mag(grad(phi))]]
where phi=0 in the liquid
phi=1 in the solid
and varies as an hyperbolic arctg in between (the right hand side term imposes a fixed depth surface thickness)
Results for a growing circle are shown below (just to have an idea of what I'm trying to do):




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2 i solve a diffusion equation for the solute :
dCl/dt=lapl(Cl)+S

3. i couple with an entalpy equation:

dH/dt=lapl(T)
.
My problem is to impose the v_front velocity cause it is function of a parameter
omega=Cl(phi=0.9)-Cl(phi=0.5)
in the interface phi ]0,1[ domain.
WHAT I NEED:
is to know the position of the isocontours 0.9 and 0.5 in the domain and to evaluate so omega.
I can try to do it by bilinear interpolation by hands but I suppose (?) that perhaps there are some "tools" already developed in OpenFOAM (and that I don't find..).
Can someone please help me or give me some ideas in order to do it in a proper way?
Thank you