Writing a User Defined Function (UDF) for CFD Modelinglearncax.com /blog/2013/06/03/writ ing-a-user-def ined-f unction-udf - in-ansys-f luent/

Writing a UDF i.e. a user defined function, has always been a challenging stuff for CFD engineers those have nevertried out one, nevertheless has been a matter of interest for all. Hereby we present a short article to demonstrate thesignificance of writing a UDF in CFD modeling with a simple test case that would help us all to develop anunderstanding at the introductory level.

Prerequisite: The reader is already familiar with ANSYS FLUENT software and C programming language.

The blog shall give us an overview of user defined function (UDF) with a short demo session explaining how UDFare written using C programming and how they are interpreted or compiled in a commercial CFD software likeANSYS FLUENT, and will finally explain how they are called in the software.

What is a UDF ?

A UDF is basically a C program or a C function that can be dynamically loaded with ANSYS FLUENT to enhance itsstandard features. The features of a typical UDF are as follows:

Must be defined using DEFINE macros supplied by FLUENT

Must have an include statement for the udf.h file

Use predefined macros and functions to access FLUENT solver data and to perform other tasks

Are executed as interpreted or compiled functions

Are hooked to ANSYS FLUENT solver using a graphical user interface dialog boxes

UDF can be used for a number of applications like:

1. Customize boundary conditions

2. Material property definitions

3. Adjust computed values on a once-per- iteration basis

4. Initialize with a previous solution

5. Execute at the end of an iteration

6. Upon exit from ANSYS FLUENT

7. Enhance post-processing

8. Enhance existing ANSYS FLUENT models (such as discrete phase model, multiphase mixture model,discrete ordinates radiation model)

Def ining a UDF:

UDFs are implemented in FLUENT code as macros. The general format for a defined macros is the macroname thatis define followed by a underscore with the udf name in parenthesis followed by a coma and then mentioning thevariables that are to be passed, as shown in examples below.

The definitions for DEFINE macros are contained in the udf.h header file, hence it is not possible to have somethingelse defined in the UDF that is outside the udf.h header file.

Letus say for example, if we want to define a customized velocity profile to a inlet we have to define something likeas shown below.

Example:

The udf.h header file contains the following:

Definitions for DEFINE macros

#include compiler directives for C library function header files

Header files (for example, stdio.h) for other ANSYS FLUENT-supplied macros and functions

A UDF should begin with #include followed with “udf.h” followed by whichever macro to be used can be defined,shown below.

Compiling & Interpret ing UDFs:

Now let us understand how a UDF iscompiled or interpreted i.e. how a UDFis exactly included in the FLUENTsoftware. The file containing the UDFcan be either interpreted or compiledin FLUENT. The way in which the source code is compiled and the code that results from the compilation process isdifferent for both cases. In compiled UDFs, the UDFs are built in same way as that the ANSYS FLUENT softwareitself is built, whereas in the interpreted UDFs the source code is compiled into a machine code that executes on aninternal emulator whenever the UDF is invoked.

Let us now understand how UDFs are compiled. Basically they are compiled either by graphical user interphase(GUI) or the text user interphase (TUI) . In the case of GUI it is a two step process i.e. a window called compiledUDF flashes on the screen whenever we try to compile. The input it requires are:

1. Built a shelf library object file from a source file

2. Load the shared library into FLUENT.

So to summarize we create a library file from our UDF C program, later we load the library file into FLUENT as if it isa part of FLUENT.

Now let us see how how interpreted UDF differ from compiled ones. This is a single step process, occuring atruntime only and involves use of interpreted UDFs dialog box.

Basically what happens when we interpret a UDF, is that the source code is compiled into an intermediatearchitecture machine code using a preprocessor. Now this machine code is executed on an internal emulator, orinterpreter when the UDF is invoked . As an additional function is to be called during execution, it therefore incurs areduction in performance.

Types of UDF:

As per the requirement the basic/ general practice goes as :

1. For small, straightforward functions we use interpreted UDFs

2. For complex functions that have high significant CPU requirement and require access to a shared library we usecomplied UDFs. Because compiled UDF is like a part of ANSYS FLUENT software itself.

Hooking UDFs into a problem setup :

We can either interpret or compile the UDF source file. Now the function contained in the interpreted code or sharedlibrary will appear in the dropdown list in dialogue boxes. We may choose this function to activate or “hook” our UDFin CFD model.

Eg : Now let us see an example of how UDF can be used.

Plots in cas e o f wi thout a UDF

In this example a UDF is used to define a variable velocity profile to the inlet. The geometry is a very simple, i.e.flow is simply from top and gets divided into 2 outlets streams at left and right bottom. The image shown below is arepresentation of a hexahedral mesh used for the study.

First we will study a constant inlet velocity profile without the use of UDF, so the graph shown below is of a constantvelocity profile i.e. 0.01 m/s in the negative Y axis.

After simulation the velocity contours display that the flow is taking some distance to develop the parabolic velocityprofile (as seen below). This can be seen more clearly using the velocity vectors wherein we see straight profile atthe inlet and after some time we see different color vectors in the same zone till the parabolic profile has beenachieved.

NowusingtheUDF,wecandefinea

parabolic inlet as in the plot below; ranging from 0 to 0.01 m/s in negative Y axis direction, and the velocity contoursof simulation has achieved the parabolic profile right at the very inlet; also proved using the velocity vector plot(shown below).

Thus

Plots in cas e o f wi th-UDF

Com parative ve loci ty pro fi les in the 2 cas es (wi thout & wi th UDF)

Thus

comparing both cases in detail, what we

Com parative contour p lo ts in the 2 cas es (wi thout & wi th UDF)

Com parative ve loci ty vector p lo ts in the 2 cas es (wi thout & wi th UDF)

comparing both cases in detail, what wesee is, without the use of UDF thevelocity profile at the inlet was a straightline and along the length of the pipe itstarted developing the actual velocityprofile. In order to achieve the actualvelocity profile right at the inlet, we haveto extend the inlet atleast till a lengthequal to or greater than the length theflow took to develop the profile. In thisapproach is where as with the use ofUDF we could have the real velocityprofile that is supposed to beat the inlet.

Sof tware Demo:

A below video shows a workedout demo in ANSYS FLUENT software.

References:1. A step-by-step UDFExample, ANSYS Fluent UDFtutorial, An airfoil in a freeshear layer.

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