Prandtl-K Macro: Prandtl Number

Prandtl-K Macro, UDF, Prandtl Number — ANSYS Fluent CFD Simulation

Description

This project implements the Prandtl-K macro through a User-Defined Function (UDF) to customize turbulence modeling in ANSYS Fluent, demonstrated on turbulent flow through a channel with obstacles. UDFs let you extend Fluent's built-in turbulence models, and here the DEFINE_PRANDTL macro is used to supply custom Prandtl numbers to the turbulence kinetic energy and dissipation-rate equations — giving you direct control over parameters the standard interface fixes. As the opening project of the User-Defined Functions (UDF): Beginner CFD Training Package, it introduces the UDF workflow — writing, compiling, and hooking a macro into the solver — through a fundamental turbulence-modeling customization.

Methodology

The 2D geometry is modeled in Design Modeler and meshed in ANSYS Meshing with an unstructured grid of 97,972 cells, with obstacles placed in the channel to induce turbulence. The core of the project is a custom implementation of the Prandtl-number calculations for the turbulence kinetic energy (TKE) and turbulent dissipation rate (TDR), based on the RNG turbulence model, applied to the standard k-epsilon model through the DEFINE_PRANDTL macro. The UDF process is covered step by step: writing the custom Prandtl-number functions for TKE and TDR, implementing them in the DEFINE_PRANDTL macro, compiling and loading the UDF into ANSYS Fluent, and setting up the turbulence model to use the custom functions.

Analysis

After running the simulations, a comparative study evaluates the effect of the custom UDF on the turbulence modeling, using comparative bar graphs of key parameters with and without the UDF, turbulence-variable contours, and velocity and pressure distribution comparisons. From these results you can see how the customized Prandtl numbers change the predicted turbulent flow around the obstacles, and assess the improvement in fidelity the UDF provides. This kind of customization enhances accuracy for complex turbulent flows and lets turbulence models be adapted to specific flow conditions, with applications from aerospace to industrial fluid dynamics. By the end of this project, you'll be able to write custom Prandtl-number functions, implement the DEFINE_PRANDTL macro, compile and load a UDF into ANSYS Fluent, and interpret a comparative study that quantifies how a UDF improves turbulence modeling.