
For isotropic diffusion, in Equation 8.61 is equal to a scalar times the identity matrix and the equation reduces to
You can specify isotropic diffusivity as a single userdefined function that applies to all UDS transport equations. For this case, choose userdefined from the dropdown list for UDS Diffusivity in the Materials panel (Figure 8.6.1).
Define Materials...
If you have previously loaded a compiled UDF library or have interpreted the UDF, then the UserDefined Functions panel will open, allowing you to hook the DEFINE_DIFFUSIVITY UDF to FLUENT. If no functions have been loaded, you will get an error message. Refer to the separate UDF Manual for details on userdefined functions. Isotropic diffusion coefficients can also be defined on a perscalar basis by selecting definedperuds from the dropdown list for UDS Diffusivity in the Materials panel (Figure 8.6.2). This will open the UDS Diffusion Coefficients panel (Figure 8.6.3).
In the UDS Diffusion Coefficients panel, select a scalar equation (e.g., uds0) and then choose a constant, polynomial, or userdefined function from the Coefficient dropdown list. (For the default fluid (air), the constant diffusion coefficient is kg/ms.) If you choose polynomial, the Polynomial Profile panel will open and you can specify your coefficients as a function of temperature. See Section 8.2.1 for details.
When you select the userdefined option, the UserDefined Functions panel will open allowing you to hook a DEFINE_DIFFUSIVITY UDF only if you have previously loaded a compiled UDF library or interpreted a UDF. Otherwise, you will get an error message. Refer to the separate UDF Manual for details on userdefined functions.