Set the Diffusion coefficient D (SI unit: m2/s) for the binary diffusion of vapor in air. The default value 2.6·10−5 m²/s is valid at T = 298 K and pA = 1 atm. You may set a temperature or pressure-dependent diffusion coefficient through the use of the variables mt.T and mt.pA.

Specify the Effective diffusivity model used to account for the porosity and the tortuosity of the porous medium in the diffusion coefficient, Deff (SI unit: m2/s). The available options are Millington and Quirk model (the default), Bruggeman model, Tortuosity model, and No correction.

For the Tortuosity model option, enter a value for the Tortuosity factor, τ (dimensionless). The default is 1.

Then, set the Moist air velocity field, ug, that should be interpreted as the Darcy velocity, that is, the volume flow rate per unit cross sectional area. For User defined enter values or expressions for the components based on space dimensions. Or select an existing velocity field in the component (for example, Velocity field (br) from a Brinkman Equations interface).

When the parent Hygroscopic Porous Medium node is coupled to a Porous Medium node in a fluid flow interface through the Moisture Flow coupling, an additional property, the relative moist air permeability κrg (dimensionless) can be set to account for the presence of liquid water in the computation of the Darcy velocity of moist air. The default value is 1-mt.sl. Depending on the chosen capillary model, this setting can be automatically calculated, and thus is not shown. See Liquid Water (Hygroscopic Porous Medium) for more details.