8.6.1 Isotropic Diffusion

For isotropic diffusion, $\Gamma$ in Equation  8.6-1 is equal to a scalar $\Gamma$ 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 Create/Edit Materials dialog box

Materials

If you have previously loaded a compiled UDF library or have interpreted the UDF, then the User-Defined Functions dialog box will open, allowing you to hook the DEFINE_DIFFUSIVITY UDF to ANSYS FLUENT. If no functions have been loaded, you will get an error message. More information about user-defined functions can be found in the separate UDF Manual.

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 Create/Edit Materials dialog box. This will open the UDS Diffusion Coefficients dialog box (Figure  8.6.1).

Figure 8.6.1: The UDS Diffusion Coefficients Dialog Box

In the UDS Diffusion Coefficients dialog box, 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 $1$ kg/m-s.) If you choose polynomial, the Polynomial Profile dialog box 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 dialog box 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. More information about user-defined functions can be found in the separate UDF Manual.