Under the Constituent Materials tab, the diffusivity of the diffusing species in the constituent materials must be defined. The Diffusivity Input Mode determines how the diffusivity is found. It can either be entered directly (by Effective Diffusivity) or it is computed by Porosity / Tortuosity.

If porous constituent materials are present:

• by Porosity / Tortuosity might be chosen if the diffusion inside of the porous constituent material is described by a small Knudsen number.
• by Effective Diffusivity must be chosen if the diffusion inside of the porous constituent material is described by a large or intermediate Knudsen number, i.e. the diffusivity inside of this material must be determined with Bosanquets approximation.
• by Effective Diffusivity must be chosen, if the diffusion does not happen solely in the fluid phase, but also in the solid materials.

In the example presented here, the Diffusivity Input Mode is set to by Effective Diffusivity and the diffusivity of the porous material is set to 2.5e-6 m²/s, which is the diffusivity computed for the MPL model (Example 3) with the help of Bosanquet’s approximation. The corresponding diffusion coefficient in air, 1.051e-5 m²/s is computed through equation (133) using a mean free path nm and a mean thermal velocity m/s.

Inside a porous material, the diffusivity can be independent of the space direction (Isotropic) or the internal pore structure might prefer a certain direction of the diffusion. In this case, selecting Transverse Isotropic or Orthotropic allows to enter different diffusivities for the different directions.

In cases where the diffusion inside of the porous constituent (Material ID 03 in this example) is described by a small Knudsen number, by Porosity / Tortuosity can be used as input mode.

In this case, for all present fluids, the Diffusion Coefficient in (Fluid) must be entered. This corresponds to the bulk diffusivity inside this fluid constituent material, as can be seen here for Material ID 00, where the porosity is automatically set to 100% and the tortuosity is set to 1. For all porous constituents that contain the fluid, the effective diffusivity is now computed as

Here, denotes the entered diffusion coefficient, the porosity and the tortuosity factor. For all solid materials, the diffusivity is automatically set to 0. The default value pre-entered in the dialog, 20.86 mm²/s is the diffusivity of oxygen in nitrogen at 25°C and 1013 mbar (see Becker et al).