One of the driving forces in porous media flow is the capillary pressure. In standard models, it is given depending on the saturation. However, recent experiments have shown disagreement between measurements and numerical solutions using such simple models. Hence, we consider in this paper two extensions to standard capillary pressure relationships. Firstly, to correct the nonphysical behavior, we use a recently established saturation-dependent retardation term. Secondly, in the case of heterogeneous porous media, we apply a model with a capillary threshold pressure that controls the penetration process. Mathematically, we rewrite this model as inequality constraint at the interfaces, which allows discontinuities in the saturation and pressure. For the standard model, often finite-volume schemes resulting in a nonlinear system for the saturation are applied. To handle the enhanced model at the interfaces correctly, we apply a mortar discretization method on nonmatching meshes. Introducing the flux as a new variable allows us to solve the inequality constraint efficiently. This method can be applied to both the standard and the enhanced capillary model. As nonlinear solver, we use an active set strategy combined with a Newton method. Several numerical examples demonstrate This work was supported in part by IRTG NUPUS. the efficiency and flexibility of the new algorithm in 2D and 3D and show the influence of the retardation term.Keywords Porous media · Entry pressure · Variational inequality · Mortar · Active set strategy
MotivationFlow processes in porous media involving two immiscible fluids need to be understood and predicted when dealing with subsurface hydrosystems or industrial applications. For example, in the unsaturated zone, the spatial distribution of the water and air phase, as well as their fluxes, serves as a basis for modeling transport of contaminants, such as pesticides or heavy metals (e.g., [8]). As examples for industrial applications in two-phase flow, the movement of fluids through a filter or the infiltration of ink into paper (see [24]) can be considered. All these applications have in common that the porous media structure is, in general, highly heterogeneous. The challenges are to combine the complex multiphase flow processes with the heterogeneity distribution of the porous media properties.The physical-mathematical model underlying simulations of two-phase flow on the Darcy scale usually requires a constitutive relationship between (wetting phase) saturation S w and the capillary pressure p c . Traditionally, one assumes that this relationship is determined under quasistatic or steady-state conditions but can also be applied to any transient flow processes fulfilling the Reynolds number criterion. However, recently, some works have questioned this assumption (see, e.g., [25]). The authors were able to improve numerical simulation results by applying a model