Capillary Pressure Models
The capillary pressure can be either a user defined expression, or given by the van Genuchten or Brooks and Corey models.
Van Genuchten Model
The van Genuchten (Ref. 49) model gives the capillary pressure pc (SI unit: Pa), as well as the liquid water relative permeability κrl (dimensionless), and the vapor relative permeability κrv (dimensionless). These quantities are expressed as functions of the liquid water saturation sl (dimensionless), the entry capillary pressure pec (SI unit: Pa), and two dimensionless constitutive constants mvG and lvG.
The liquid water relative permeability is used in the computation of the liquid water capillary flux, while the vapor relative permeability is used to account for the presence of liquid water in the pores for the calculation of the moist air Darcy velocity. See the Moisture Flow multiphysics coupling.
Brooks and Corey model
The Brooks and Corey (Ref. 50) model gives the capillary pressure pc (SI unit: Pa), as well as the liquid water relative permeability κrl (dimensionless), and the vapor relative permeability κrv (dimensionless). These quantities are expressed as functions of the liquid water saturation sl (dimensionless), the entry capillary pressure pec (SI unit: Pa), and the pore distribution index λp (dimensionless).
The liquid water relative permeability is used in the computation of the liquid water capillary flux, while the vapor relative permeability is used to account for the presence of liquid water in the pores for the calculation of the moist air Darcy velocity. See the Moisture Flow multiphysics coupling.