2.7.1 Introduction

For each of the $N$ scalar equations you specified in your ANSYS FLUENT model you can supply a unique UDF for the diffusion coefficients, flux, and unsteady terms in the scalar transport equation. For multiphase you have the added benefit of specifying UDFs on a per-phase basis in both fluid and solid zones. Additionally, you can specify a UDF for each source term you define for a given scalar equation as well as boundary conditions on wall, inflow, and outflow boundaries.

Diffusion Coefficient UDFs

For each of the $N$ scalar equations you have specified in your ANSYS FLUENT model using the User-Defined Scalars dialog box you can supply a unique user-defined function (UDF) for isotropic and anisotropic diffusivity for both fluid and solid materials. Recall that ANSYS FLUENT computes the diffusion coefficient in the UDS equation.

Isotropic diffusivity UDFs are defined using the DEFINE_DIFFUSIVITY macro (Section  2.3.3) and anisotropic coefficients UDFs are defined using DEFINE_ANISOTROPIC_DIFFFUSIVITY (Section  2.7.2). Additional pre-defined macros that you can use when coding UDS functions are provided in Section  3.2.8.

Flux UDFs

For each of the $N$ scalar equations you have specified in your ANSYS FLUENT model using the User-Defined Scalars dialog box you can supply a unique user-defined function (or UDF) for the advective flux term. Recall that ANSYS FLUENT computes the flux in the UDS equation.

UDS Flux UDFs are defined using the DEFINE_UDS_FLUX macro
(Section  2.7.3). Additional pre-defined macros that you can use when coding scalar flux UDFs are provided in Section  3.2.8.

Unsteady UDFs

For each of the $N$ scalar equations you have specified in your ANSYS FLUENT model using the User-Defined Scalars dialog box you can supply a unique UDF for the unsteady function. Recall that ANSYS FLUENT computes the unsteady term in the UDS equation.

Scalar Unsteady UDFs are defined using the DEFINE_UDS_UNSTEADY macro (Section  2.7.4). Additional pre-defined macros that you can use when coding scalar unsteady UDFs are provided in Section  3.2.8.

Source Term UDFs

For each of the $N$ scalar equations you have specified in your ANSYS FLUENT model using the User-Defined Scalars dialog box you can supply a unique UDF for each source. Recall that ANSYS FLUENT computes the source term in the UDS equation.

Scalar source UDFs are defined using the DEFINE_SOURCE macro and must compute the source term, $S_{\phi_k}$, and its derivative $\frac{\partial S_{\phi_k}}{\partial \phi_k}$ (Section  2.3.19). Additional pre-defined macros that you can use when coding scalar source term UDFs are provided in Section  3.2.8.

Fixed Value Boundary Condition UDFs

For each of the $N$ scalar equations you have specified in your ANSYS FLUENT model using the User-Defined Scalars dialog box you can supply a fixed value profile UDF for fluid boundaries.

Fixed value UDFs are defined using the DEFINE_PROFILE macro. See Section  2.3.15 for details. Additional pre-defined macros that you can use for coding scalar transport equation UDFs are provided in Section  3.2.8.

Wall, Inflow, and Outflow Boundary Condition UDFs

For each of the $N$ scalar equations you have specified in your ANSYS FLUENT model using the User-Defined Scalars dialog box you can supply a specified value or flux UDF for all wall, inflow, and outflow boundaries.

Wall, inflow, and outflow boundary UDFs are defined using the DEFINE_PROFILE macro (Section  2.3.15). Additional pre-defined macros that you can use for coding scalar transport equation UDFs are provided in Section  3.2.8.