8.6 User-Defined Scalar (UDS) Diffusivity

There are two types of UDS diffusivity that you can specify in ANSYS FLUENT: isotropic and anisotropic. Diffusion is isotropic when it is the same in all directions. Isotropic diffusion coefficients can be specified in two ways: either as a single user-defined that applies to all UDS transport equations defined for your model; or on a per-scalar basis as constants, polynomial functions of temperature, or user-defined functions.

Diffusion is anisotropic when the diffusion coefficients are different in different directions. Anisotropic diffusion can be specified by a tensor diffusion coefficient matrix $\Gamma$ (Equation  8.6-1) for each UDS (in both fluid and solid zones) in four different ways: general anisotropic, orthotropic, cyl-orthotropic, and user-defined-anisotropic. All UDS diffusivity parameters are set from the Create/Edit Materials dialog box and are discussed below. Note that details about how to define and use UDFs in UDS transport equations is discussed in the separate UDF Manual.

The second-order diffusion term in the most general form is

where $\Gamma$ is a 3 $\times$ 3 tensor in 3D.