Settings for the Moisture Transport Interface
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is mt.
Physical Model
In 2D and 1D axisymmetric components, set the Thickness dz, which is the thickness of the domain in the out-of-plane direction. The default value is 1 m.
In 1D components, set the Cross sectional area Ac and the Cross sectional perimeter Pc of the domain. Default values are 1 m2 and , respectively.
Set the Reference vapor concentration which is used when the moisture transport interface is coupled to a fluid flow interface with the Incompressible flow option selected in the Compressibility list. In this case, the fluid density is evaluated at the Reference pressure level (defined in the Physical Model section of the fluid flow interface), at the reference temperature defined in the moisture transport interface (293.15 K), and at the Reference vapor concentration specified in this section.
Choose the Mixture type for moist air:
By default, a Diluted species formulation is used in the Moisture Transport in Air interface and in the Moisture Transport in Building Materials interface. This formulation is well adapted for small vapor concentration conditions. In this case, the density of moist air is supposed not to change with variations of moisture content, and the moisture content variation is expressed through the transport of vapor concentration cv.
For larger vapor concentration conditions, the variation of moist air density are modeled, and the Concentrated species formulation should be used. In this case, the moisture content variation is expressed through the transport of vapor mass fraction ωv.
Note that in hygroscopic porous media domains, the Concentrated species formulation is always used, and this setting is not used. In building materials domains, this setting has no effect either.
Finally, select the Account for gravity in liquid velocity check box to enable the Gravity feature.
Consistent Stabilization
The Streamline diffusion check box is selected by default and should remain selected for optimal performance for applications that include a convective or translational term. Crosswind diffusion provides extra diffusion in regions with sharp gradients. The added diffusion is orthogonal to the streamlines, so streamline diffusion and crosswind diffusion can be used simultaneously. The Crosswind diffusion check box is also selected by default.
Inconsistent Stabilization
The Isotropic diffusion check box is not selected by default. To add isotropic diffusion, select the Isotropic diffusion check box. The field for the tuning parameter δid then becomes available. The default value is 0.25; increase or decrease the value of δid to increase or decrease the amount of isotropic diffusion.
In the COMSOL Multiphysics Reference Manual:
Advanced Settings
To display this section, click the Show More Options button () and select Advanced Physics Options. These settings only apply to the domains in which either a Moist Air feature or an Hygroscopic Porous Medium feature is active. Select a Convective termNonconservative form (the default) or Conservative form. The latter should be used for compressible flow.
Discretization
To display all settings available in this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. The shape functions used for the relative humidity are by default Quadratic Lagrange in building materials and porous media, and Linear in moist air.