The liquid saturation, sl, which describes the amount of liquid water within the pores, is defined from the specification of a moisture storage function
, which specifies the total moisture content (left hand side of the equation below) as a function of the relative humidity
:
where εp (dimensionless) is the porosity.
In the equilibrium formulation, the evaporation and condensation source terms from the moist air and liquid water mass conservation equations cancel out. However, the Hygroscopic Porous Medium node stores the evaporation source term into the
mt.G_evap variable, to be accounted for as a mass source in the fluid flow equations for the gas phase in a moisture flow model. This mass source is automatically handled by the
Moisture Flow coupling node.
The default Temperature Τ and
Absolute pressure pA are
User defined. When additional physics interfaces are added to the model, the temperature and absolute pressure variables defined by these physics interfaces can also be selected from the list. For example, if a
Heat Transfer in Moist Porous Media interface is added, you can select
Temperature (ht) from the list. If a
Laminar Flow interface is added, you can select
Absolute pressure (spf) from the list.
If the node was added automatically after selecting the Heat and Moisture Transport predefined multiphysics interface, the temperature of the
Heat and Moisture multiphysics node is used by default and the input field is not editable. To edit the
Temperature field, click
Make All Model Inputs Editable (
).
Select a coordinate system from the Coordinate system list for the interpretation of directions in anisotropic material properties. The default is the
Global coordinate system, and the list contains any additional coordinate system (except boundary coordinate systems) added under the
Definitions node.
The Hygroscopic Porous Medium node and its subnodes inherit these coordinate system settings. In particular, the
Moist air velocity field (in
Moist Air (Hygroscopic Porous Medium) subnode), the
Permeability (in
Porous Matrix (Hygroscopic Porous Medium) subnode), and the
Capillary liquid flux density (in
Hygroscopic Porous Medium node) should be set according to the coordinate system selected in this section.
The Moisture storage function 
should be set to characterize the relationship between the amount of accumulated water and the relative humidity in the material. The default
Moisture storage function is taken
From material. For
User defined, enter another value or expression.
When a Porous Material node is defined on the domains where the
Hygroscopic Porous Medium feature is active, the
Moisture storage function and the
Moisture diffusivity can be set in the
Homogenized Properties section of the
Porous Material node.
