Introduction
- What is a User Defined Function?
- A UDF is a routine (programmed by the user) written in C which can be dynamically linked with the solver.
- Standard C functions
- v e.g., trigonometric, exponential, control blocks, do-loops, file i/o, etc.
- Pre-Defined Macros
- v Allows access to field variable, material property, and cell geometry data.
- Why build UDF’s?
- Standard interface cannot be programmed to anticipate all needs.
- Customization of boundary conditions, source terms, reaction rates, material properties, etc.
- Adjust functions (once per iteration)
- Execute on Demand functions
- Solution Initialization
UDF Basics
UDF’s assigns values (e.g., boundary data, source terms) to individual cells and cell faces in fluid and boundary zones.
- In a UDF, zones are referred to as threads.
- A looping macro is used to access individual cells belonging to a thread.
- e.g., a face-loop macro visits 563 faces on face zone 3 (velocity-inlet).
- v Position of each face is available to calculate and assign spatially varying properties.
- Thread and variable references are automatically passed to UDF when assigned to boundary in GUI.
- Values returned to the solver by UDFs must be in SI units.
Using UDFs in the Solvers
The basic steps for using UDFs in FLUENT are as follows:
STEP 1: Create a file containing the UDF source code
STEP 2: Start the solver and read in your case/data files
STEP 3: Interpret or Compile the UDF
STEP 4: Assign the UDF to the appropriate variable and zone in BC panel.
STEP 5: Set the UDF update frequency in the Iterate panel
STEP 6: Run the calculation
Example: Non-Uniform Inlet Velocity
A non-uniform inlet velocity is to be imposed on the 2D turbine vane shown below. The x-velocity variation is to be specified as:
u(y) = 20 [ 1 - (y/0.0745)2]
y = 0
Example: Source Code
The DEFINE_PROFILE macro allows the function inlet_x_velocity to be defined.
- All UDFs begin with a DEFINE_ macro.
- inlet_x_velocity will be identifiable in solver GUI.
thread and nv are dynamic references, input to the UDF to identify the zone and variable being defined, respectively.
The macro begin_f_loop loops over all faces, f, on thread.
The F_CENTROID macro assigns cell position vector to x[ ].
The F_PROFILE macro applies the velocity component to face f.
#include "udf.h"
DEFINE_PROFILE(inlet_x_velocity, thread, nv)
{
float x[3]; /* this will hold the position vector*/
float y;
face_t f;
begin_f_loop(f, thread)
{
F_CENTROID(x,f,thread);
y = x[1];
F_PROFILE(f, thread, nv)=20. *(1. - y*y/(.0745*.0745));
}
end_f_loop(f, thread)
}
Example: Interpreting the UDF
The UDF is saved as velprof.c
Define ¡User Defined
¡Functions ¡Interpreted…
Click Compile
The assembly language code will
scroll past window.
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