For isotropic diffusion, in Equation 8.6-1 is equal to a scalar times the identity matrix and the equation reduces to
You can specify isotropic diffusivity as a single user-defined function that applies to all UDS transport equations. For this case, choose user-defined from the drop-down list for UDS Diffusivity in the Materials panel (Figure 8.6.1).
If you have previously loaded a compiled UDF library or have interpreted the UDF, then the User-Defined 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 user-defined functions. Isotropic diffusion coefficients can also be defined on a per-scalar basis by selecting defined-per-uds from the drop-down 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., uds-0) and then choose a constant, polynomial, or user-defined function from the Coefficient drop-down list. (For the default fluid (air), the constant diffusion coefficient is kg/m-s.) 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 user-defined option, the User-Defined 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 user-defined functions.