\-This model implements two-\/phase flow of two immiscible fluids $\alpha\in\{ w, n \}$ using a standard multiphase \-Darcy approach as the equation for the conservation of momentum, i.\-e. \[ v_\alpha=-\frac{k_{r\alpha}}{\mu_\alpha}\textbf{K}\left(\textbf{grad}\, p_\alpha-\varrho_{\alpha}{\textbf g}\right)\]
\-By inserting this into the equation for the conservation of the phase mass, one gets \[\phi\frac{\partial\varrho_\alpha S_\alpha}{\partial t}-\text{div}\left\{\varrho_\alpha\frac{k_{r\alpha}}{\mu_\alpha}\mbox{\bf K}\left(\textbf{grad}\, p_\alpha-\varrho_{\alpha}\mbox{\bf g}\right)\right\}- q_\alpha=0\;, \]
\-This equations are by a fully-\/coupled vertex centered finite volume (box) scheme as spatial and the implicit \-Euler method as time discretization.
\-By using constitutive relations for the capillary pressure $p_c = p_n - p_w$ and relative permeability $k_{r\alpha}$ and taking advantage of the fact that $S_w + S_n =1$, the number of unknowns can be reduced to two. \-Currently the model supports choosing either $p_w$ and $S_n$ or $p_n$ and $S_w$ as primary variables. \-The formulation which ought to be used can be specified by setting the {\ttfamily\-Formulation} property to either {\ttfamily\-Two\-P\-Common\-Indices\-::p\-Ws\-N} or {\ttfamily\-Two\-P\-Common\-Indices\-::p\-Ns\-W}. \-By default, the model uses $p_w$ and $S_n$.
