* where, \f$p_n\f$ denotes the wetting phase pressure, \f$\boldsymbol{K}\f$ the absolute permeability, \f$\lambda_n\f$ the non-wetting phase mobility, \f$\rho_n\f$ the non-wetting phase density and \f$g\f$ the gravity constant.
* As in the two-phase pressure equation a total flux depending on a total velocity is considered one has to be careful at neumann flux boundaries. Here, a phase velocity is only uniquely defined, if
* the saturation is at the maximum (\f$1-S_{rw}\f$, \f$\boldsymbol{v}_{total} = \boldsymbol{v}_n\f$) or at the minimum (\f$ S_{rn} \f$, \f$\boldsymbol{v}_n = 0\f$)
* \f[\boldsymbol{v} = -\frac{1}{\mu} \boldsymbol{K} \left(\text{grad}\, p + \rho g \text{grad}\, z\right),\f]
* where, \f$p\f$ is the pressure, \f$\boldsymbol{K}\f$ the absolute permeability, \f$\mu\f$ the viscosity, \f$\rho\f$ the density and \f$g\f$ the gravity constant.
* where, \f$p_n\f$ denotes the wetting phase pressure, \f$\boldsymbol{K}\f$ the absolute permeability, \f$\lambda_n\f$ the non-wetting phase mobility, \f$\rho_n\f$ the non-wetting phase density and \f$g\f$ the gravity constant.
* As in the two-phase pressure equation a total flux depending on a total velocity is considered one has to be careful at neumann flux boundaries. Here, a phase velocity is only uniquely defined, if
* the saturation is at the maximum (\f$1-S_{rw}\f$, \f$\boldsymbol{v}_{total} = \boldsymbol{v}_n\f$) or at the minimum (\f$ S_{rn} \f$, \f$\boldsymbol{v}_n = 0\f$)
* The wetting or the non-wetting phase pressure, or the global pressure has to be given as piecewise constant cell values.
* The phase velocities are calculated following Darcy's law as
* where \f$p_\alpha\f$ denotes the pressure of phase \f$_\alpha\f$ (wetting or non-wetting), \f$\boldsymbol{K}\f$ the absolute permeability, \f$\lambda_\alpha\f$ the phase mobility, \f$\rho_\alpha\f$ the phase density and \f$g\f$ the gravity constant.
* The total velocity is either calculated as sum of the phase velocities
//! Class including the variables and data of discretized data of the constitutive relations.
/*! The variables of two-phase flow, which are one pressure and one saturation are stored in this class.
* Additionally, a velocity needed in the transport part of the decoupled two-phase flow is stored, as well as discretized data of constitutive relationships like
* mobilities, fractional flow functions and capillary pressure. Thus, they have to be callculated just once in every time step or every iteration step.