Fluent users' manual

USER DEFINED FUNCTIONS, CUSTOM FIELD FUNCTIONS and MACROS MANUAL

I. Introduction

The aim of this manual is to allow Fluent users to define a gradient of temperature

as boundary condition on wall, inlet or outlet.
as initialisation on the interior of the domain.

This will be possible by using user defined fonctions to define boundary conditions and a custom field fonction to define initializing conditions.

Few indications will be given to help the user to define a macro with Fluent.

II. Gradient temperature as boundary conditions:

This is the most difficult thing to do with Fluent because you have to write few lines of a C++ program and to import this program in Fluent.
But , follow exactly these indications and you will be granted.

1. The C++ program

At first, you have to open a text editor which you can find in Unix.
Create a new document and write these few lines:

a.
DEFINE_PROFILE is a predefined fonction that you have to use when you want to change a boumdary condition in Fluent.
The arguments of this fonction are the following:

gradient_temp is, for example, the name of the user defined fonction when you will go under Fluent. You can give the name that you want.
thread is the argument that designed the cell zone you will applicate the fonction. (In Fluent, this will be for example a wall). You will define in Fluent what is exactly the argument thread.
nv is an integer that will represented the number of the cell in the cell zone.

b.
You have to program the instructions that this fonction will have to do.
face_t represents a face of the cell zone thread. So when you write "face_t f", you define a face named f.
real x[ND_ND] creates a real vector named x.

c.
begin_f_loop ( f,thread) { } end_f_loop (f,thread)
This is a fonction that will be applicated at all the faces f of the cell zone thread.

d.
F_CENTROID is a fonction that returns the coordonates (in the vector x) of the center of the face f (which belongs to the cell zone thread).

e.
F_PROFILE is the valor of the main fonction. You put here the expression of the temperature gradient. x[0] is the variable x in meter. Here, the expression of the temperature gradient is T=250*x +300. But, you can modify this expression.

When you have writen this program, you save it as, for example, gradient_temp.c

2. Importation and compilation in Fluent

At first, you have to copy the file "udf.h", that you will find on the server, in your own directory.

Second, you go, under Fluent, in the menu as described on the following picture:
Define>User-Defined>Fonctions>Interpreted...

When you click on interpreted..., a new window appears:

You put the name of your C++ file in the Source File Name field. (Here, gradtemp.c) and you click on Compile.

If the compilation has succeeded, you have created a user defined fonction. You have now to determinate the cell zone you want to applicate this fonction.

Go in the menu Define>Boundary Conditions:

Select a domain where you want to apply the temperature gradient, for example, wall-2 . Then, click on the set... bouton, a new window appears:

Replace the valor of the menu constant by your user defined fonction gradient_temp.
You have defined a temperature gradient on the wall-0 as boundary condition.
You can do the same thing on the wall-01.

3. Results

You can see the result of your fonction after solving few iterations and display some graphs.

III. Gradient temperature as initialization:

To do this, you will have to define a custom field fonction. Don't worry, it is easier than creating a user defined fonction.

1. Creation of the custom field fonction

At first, you have to run Fluent and to go in the menu Define>Custom Field Fonctions.

A new window appears where you can create a new fonction.

You put the expression of the temperature gradient by selecting the boutons on the left of the windows (For example, "1","2","3"... or "+","-"...or"log","ABS"...).

For the variables like X-velocity, Y-coordinate, temperature..., select them on the right of the windows in the menu Field Fonction and click on Select.

The expression is writen on the top left of the window.
When you are satisfied of the expression, give a name at your fonction in the New Function Name.field. So, click on Define and Close.

2. Use of the fonction

You have defined a new fonction and you can apply this as initial condition on the interior of the domain.
To do this, go in the menu Solve>Initialize>Patch...,

A new window appear;

Choose the option Use Field Fonction, Select on the left the variable Temperature.
Select on the middle your field fonction gradient_temperature and Select on the right the zones to patch fluid-5.
When everything is selected as shown upstairs, click on Patch and Close.
You have initialize the domain with a temperature gradient.

3. Results

You can see the temperature field obtained in the menu Display.

IV.Creation of a macro

You can create a lot of types of macros, but, they are useful when you have to do repetitive actions.

For example, we are going to learn how display the evolution of an unsteady calcul at each time step.

1. Create a macro

At first, you have to go in the menu Solve>Monitors>Command...

Then, a new menu appears:

Click on the bouton Define Macro...

Choose a name for the macro in the field Name, for example, macro-1. Click on the bouton OK and begin to define your macro by doing exactly what you will do narurally.
For example, go in the menu Display>Contour
Choose the variable you want to display, for example, temperature.
Select OK to display a new window with the graph of temperature.
So, click in the window named Monitor Commands on the bouton End Macro.

You have created a new macro named macro-1 which display the temperature field.

2. Use a macro

Stay in the window named Monitor Commands and choose a name in the first field Name for your command, for example, Affictemp.
Then, in the field Command, write the name of your macro , here, macro-1.
Then select the bouton On , the case is coched.
Now, you go in the field When to choose Time iteration (applicable for an unsteady calcul)