UDF CFD Simulation

What is User-Defined Function (UDF)?

In some simulations, certain parameters depend on other parameters. We can use User-Defined Functions (UDFs) to define these relationships in these cases.

UDFs applications

• Post-processing: Create custom outputs for simulation results that are not available by default. For instance, UDFs can be utilized to calculate and output dimensionless numbers such as the Reynolds number (Re) and the Nusselt number (Nu).
• Material property: In many simulations, considering a constant value for the material properties leads to inaccurate results. Therefore, the material properties need to be defined as functions of pressure, temperature, etc. In such cases, ANSYS Fluent provides the possibility of defining these properties using simple functions inside the software. If the desired function is not inside the software, it is necessary to define a UDF.

Edit window of material properties, ANSYS Fluent

• Enhance existing ANSYS FLUENT models: Customize the behavior of built-in models to better suit specific simulation needs. For instance, modifying the discrete phase model to include a new type of particle interaction.
• Initialize with a previous solution: Start a new simulation using data from a previous run to save computational time.
• Customize boundary conditions: Define unique boundary conditions unavailable in the standard setup. For instance, creating a custom inlet velocity profile that varies with time.

In each iteration, it is possible to use the UDF with the available results and apply the output of the function in the next iteration.

The features of a typical UDF are as follows

• Must have an include statement for the udf.h file: At the beginning of your UDF code, include #include “udf.h” to allow the use of FLUENT-specific functions and macros.
• Must be defined using DEFINE macros supplied by FLUENT: Use the predefined DEFINE macros to define the function and its type. For instance, define an adjust function using DEFINE_ADJUST(adjust_function, domain) to modify solution variables at each iteration
• UDFs are executed as interpreted or compiled functions: Interpreted functions are executed line by line at runtime, offering flexibility but slower performance. Compiled functions are precompiled into optimized machine code, providing better performance and stability, though requiring setup overhead during development.
• UDFs are hooked to ANSYS FLUENT solver using graphical user interface dialog boxes: ANSYS FLUENT allows users to integrate User-Defined Functions (UDFs) through GUI dialog boxes, streamlining the process of linking custom code to solver actions. These dialog boxes enable users to specify when and how the UDF should execute during the simulation, such as at the end of iterations or during initialization. This graphical approach simplifies the incorporation of custom functionality into FLUENT simulations, enhancing flexibility and ease of use.
• Programming language: User-defined functions in ANSYS FLUENT are primarily written in the C programming language. However, ANSYS Fluent also supports UDFs written in C++ and Fortran.

CFD LAND projects of UDF CFD Simulation

Looking to write UDFs in ANSYS Fluent simulations? Dive into our CFD SHOP, take a look at the projects at the top of this page, where you will discover many projects that include UDFs. Moreover, you can easily order your project at Order Project; our experts can write and utilize the most complex UDFs.