COMSOL 6.4 - Settings for the Moisture Transport Interface

The 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 field. The first character must be a letter.

The default (for the first physics interface in the model) is mt.

In 2D and 1D axisymmetric components, set the 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 Ac and the Pc of the domain. Default values are 1 m2 and

, respectively.

Set the , which is used when the moisture transport interface is coupled to a fluid flow interface with the option selected in the list. In this case, the fluid density is evaluated at the (defined in the section of the fluid flow interface), at the temperature defined in the domain features of the moisture transport interface (293.15 K), and at the specified in this section.

Choose the :

•

By default, a formulation is used in the Moisture Transport in Air, the Moisture Transport in Building Materials, and the Moisture Transport in Free and Porous Media interfaces. 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, the compressibility effects are neglected.

•

For larger vapor concentration conditions, the variation of moist air density are modeled, and the formulation should be used. This is the default option in the Equilibrium Moisture Transport in Porous Media and the Nonequilibrium Moisture Transport in Porous Media interfaces. In this case, the compressibility effects are taken into account.

Note that this setting impacts only moist air domains, and has no effect on building materials and hygroscopic porous media domains.

Finally, select the checkbox to enable the Gravity feature.

This section is available by clicking the button (

) and selecting in the dialog.

The checkbox is selected by default and should remain selected for optimal performance for applications that include a convective or translational term. 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 checkbox is also selected by default and is only available when the checkbox is selected.

This section is available by clicking the button (

) and selecting in the dialog.

The option (not selected by default) adds an additional term to the diffusion coefficient to dampen spurious oscillations caused by convective terms. Increasing the δid amplifies the effect of the isotropic diffusion. As a rule of thumb, the tuning parameter should be large enough to ensure convergence, while remaining as small as possible as it reduces the accuracy of the original problem.

To display this section, click the button (

) and select . These settings only apply to the domains in which a feature is active. Select a — (the default) or . The latter should be used for compressible flow.

Note that and formulations are equivalent when using , they can both be used for large vapor concentrations.

To display all settings available in this section, click the button (

) and select in the dialog. The shape functions used for the relative humidity are by default in building materials, and in moist air and porous media. Note that when a porous medium is used with set to , an additional dependent variable is solved for, the liquid saturation, which uses the same type of shape functions as the relative humidity.