fluent udf 15.0 l01 introduction

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Lecture 1:

Introduction

User Defined Functions

in ANSYS Fluent


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Why use User Defined Functions (UDF

The Fluent solver is a general-purpose code. In order to customize the

Fluent solver, users can use their own C-codes called user-definedfunctions (UDFs)to accomplish:

Special boundary conditions

Customized or solution dependent material properties

New physical models

Reaction rates

Source terms Customized post-processing

Solving user-supplied partial differential equations

More .


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What are User Defined Functions?

Programs written in C

Similar mechanism used to develop models in Fluent Powerful and efficient

Extends the functionality of ANSYS Fluent

Initial and boundary conditions

Material properties

Material and energy transfer through source terms

Modify solution variables

Adding extra flow equations

And many more

Typically compiled to DLLs/shared object libraries

Loaded only when required


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Sinusoidal Wall Temperature Variation

T(x) = +

.

Heated wall T = T(x)

Adiabatic wall

inlet


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What Does It Take to Create a Model?

Ability to access and modify

Interact with the user through a GUI/TUI

Field variables

Geometrical and mesh data

Hooks to the solver at appropriate stages of the solution cycle

Convenience macros

Vector macros Looping macros

Parallel programming


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Basics of UDFs


in ANSYS Fluent


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CThe Foundation for UDFs

UDFs when compiled create C libraries that Fluent can use

Resources to learn C ANSYS Fluent UDF Manual (Appendix A. C Programming Basics)

E.g. The C-Programming Language, B. Kernighan & D. Ritchie

Programming practices

Simplicity, Clarity and Generality

Naming conventions

Macros and comments

Algorithms and data structures

Design principles

Testing


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User Access to the Fluent Solver

User-Defined Properties

User-Defined BCs

User Defined

INITIALIZE

Segregated PBCS

Exit Loop

Repeat

Check Convergence

Update Properties

Solve Turbulence Equation(s)

Solve Species

Solve Energy

Initialize Begin Loop

DBCS

Solve Other Transport Equations as required

Solver?

Solve Mass Continuity;

Update Velocity

Solve U-Momentum

Solve V-Momentum

Solve W-Momentum

Solve Mass

&

Momentum

Solve Mas

Momentum

Energy,

Species

User-

defined

ADJUST

Source terms

Source terms

SOLVERS


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Data Structures

Each type of UDF is passed different values and data structures

depending on its function.

Most UDFs will need solver data. This is stored in a hierarchy:

Domainsare collections of threads that make the whole mesh

Threads(or zones) are collections of cells or faces

Cellsand facesare made of Nodes


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Geometrical Entities

The parts of a mesh are:

Collection of cells and faces are stored in threads.


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Data Structures

The terms Thread and Zone are interchangeable. The bounda

and cell zones defined in the User Interface are stored internally

as threads.

Zoneshave an associated integer IDseen in the settings

panel

In a UDF, the corresponding threadcan be identified using

this ID

Threadsare an internal data type which stores information

about the mesh, models, properties, BCs all in one place

A cell has information about its bounding faces and a face has

access to its neighbouring cells.


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Introduction to the

C Programming Language


in ANSYS Fluent


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C ProgrammingBasic Syntax Rules

Each statement must be terminated with a semicolon ;

Comments can be inserted anywhere between /* and */ All variables must be explicitly declared (unlike in FORTRAN

Compound statements can be created by enclosing multiple

statements by braces: { }

Function definitions have the following format:

return-value-type function-name(parameter-lis{

function body}


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C Programming Data Types

Built-in data types: int, float, double, char, enum

Ansys Fluent: real, cxboolean

int niter, a; /* Declare niter and a as integefloat dx[10]; /* dx is an array of numbers indexed

from 0 to 9: dx[0],dx[1],,dx[9]

enum {X, Y, Z} /* X,Y,Z are enumeration constants

real a; /* real is a typedef that switches bfloat for single-precision anddouble for double-precision arith

cxboolean flag=TRUE; /* Pre-defined enumeration ofFALSE and TRUE*/


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C ProgrammingPointer

A pointer is a special kind of variable that contains an addressin me

content, of another variable

Pointers are declared using the *notation

We can make a pointer point to the address of pre-defined variable

int *ip; /* declares a pointer named ip that points to aninteger variable*/

int a=1;

int *ip; /* (*ip) contains an integer */

ip = &a;/* &a returns the address of variable a */Message(ip is a pointer with value %p\n,ip);Message(The integer a pointed to by ip has value = %d\n,*

ip is a pointer with value 0x40b2

The integer a pointed to by ip has value = 1

Output:

Message

Define Simila Recomm


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C ProgrammingPointers and Arrays

Pointers and arrays are closely related:

The name of an array without an index can be used as a pointthe array

BUT: An array name cannot be set to a new value. It is fixed tset when it is defined as an array.

int ar[10]; /* ar is an array of 10 integers */int *ip; /* (*ip) contains an integer */

ip = ar; /* ar is the address of its first integeso this is equivalent to ip = &(ar[0]

ar = ip; /* This will cause a compilation error.


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C ProgrammingOperators

Arithmetic Operators

Logical Operators

Assignments

= (assignment)

+, -, *, /,% (modulo reduction)++ (increment) /*i++ is post-increment (i=i+1) */-- (decrement) /* j- is post-decrement (j=j-1) */

, >=,


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C ProgrammingControl Statements

if-else statement: for loop:

while loop:

if ( logical-expression )

{ statements}else{ statements}

if ( x < 0 )

y = x/50;else{

x = -x;y = x/25;

}

for (begin ; end ; in

{statements}

while ( logical-expre{statements}

for ( k=0; k < 10; k++){statements}

while ( x < 5 ){ executed while the condition i

E.g.:

E.g.:


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C ProgrammingConditional Operato

If the condition/logical-expression is true, result1 is returned

else result2 is returned.

(logical-expression ? result1 : result2)

real y = 2;real x1 = (y < 0 ? 5 : 10);real x2 = (y >= 0 ? 5 : 10);Result: x1 = 10 and x2 = 5

E.g.:


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C ProgrammingUser-defined Data Ty

Structures are collection of data that are of different type.

Semicolon (;) at the end of structure declaration is needed!

Set and get values

struct {Element 1;

Element n;};

struct car_struct{char model_name[2int number_of_whreal top_speed;

};

struct car_struct alices_car, bobs_car;alices_car.top_speed = 120.0;if (bobs_car.top_speed > alices_car.top_speed)

E.g.:


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C ProgrammingUser-defined Data Ty

To simplify a declaration

If the example from previous slide is defined as follows

then the variable can be declared as

instead of

typedef ;

Combined example:typedefstruct car_struct{char model_name[25];

int number_of_wheels;real top_speed;

} car_model;

car_model alices_car; struct car_struct alic

Simplified:typedefstruct{char model_name[2

int number_of_whereal top_speed;

} car_model;


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C ProgrammingFluent Structures

The most common structure in Fluent is Thread.

Usually we use pointers to s

Pre-defined Marcos can be used to access structure elements.

Note operator: is the same as

typedefstruct thread_struct{int id;} Thread;

#define THREAD_ID(t) ((t)->id)

-> t->id (*t).id

Thread *ct;

E.g.:


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Introduction to UDF

Implementation Using C


in ANSYS Fluent


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Why UDFs are written in the C Language

Fluent is written in C because it is a very powerful language. It provides

level operations such as graphics and networking and low level capab

mathematical functions and memory operations.

It enables the linking of extra functionality using shared objects in Uni

Linked Libraries (DLLs) in windows systems.

This is a very convenient mechanism for linking in UDFs which allows a sconnection between the main program and the user functions

Users familiar with other procedural languages such as FORTRAN will

most of the ideas and syntax used in C


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Mesh Data Types (1) There are several Fluent-specific data types that are associated with mes

Some of the more commonly used ones are:

Node stores data associated with a grid point

face_t identifies a face within a face thread

cell_t identifies a cell within a cell thread

Thread stores data that is common to a collection of cells or faces

Domain stores data associated with a collection of face and cell threa

Each individual cell can be accessed by using a cell index of type cell_t an

(the zone which contains the cell)

Each individual face (boundary or internal) can be accessed by using a fa

face_t and the face thread (the zone which contains the face)


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Mesh Data Types (2)

Type Example Declaration

Domain *d; dis a pointer to a Domain structure

Thread *t; t is apointer to a Thread structurecell_t c; c iscell index, a simple integerface_t f; f is aface index, a simple integer

Node *node; node ispointer to a Node structure

Some mesh or solver data types have a capital letter. These are actual

structures in the C language and are usually passed between functions

these structures.

Examples of how these data types are defined are shown below:


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Mesh Data Types (3)

Every Zone is associated to a single ID available in the BC panel or the TUI

Given the ID, the thread associated with that zone

can be retrieved as:

Once we have the thread we can access its associated data

Similarly, a threads ID can be retrieved using:

/grid/modify-zones/list-zones

int t_id = 7;Thread *ft;ft = Lookup_Thread(domain, t_id);

t_id = THREAD_ID(ft);


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C macros and UDF Programming

C has very powerful macro definition capabilities. These are

extensively used in Fluent in many ways. Notable example incl

definitions of: Data structure looping macro

Geometry macros

Field data macros

Logic and status control macros

Safe arithmetic and trigonometry functions

Complete set of vector arithmetic operations

The definition of each UDF also uses a specific DEFINE_ macro


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Looping Macros in a UDF

thread_loop_c(t,d){} Loop over cell threads in a doma thread_loop_f(t,d){} Loop over face threads in a doma

begin_c_loop(c,t) Loop over the cells in a given thre

{}

end_c_loop(c,t)

begin_f_loop(f,t) Loop over the faces in a given thr

{}

end_f_loop(f,t)

Fluent provides a set of pre-defined macros to accomplish looping tasks


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Geometry Macros

Location of

C_NNODESC_NFACESF_NNODES

Faces

Nodes

C_CENTROID(pos,c,t) x, y, z coords of cell centroid

F_CENTROID(pos,f,t) x, y, z coords of face centroid

F_AREA(A,f,t) Area vector of a face; NV_MAG(A) Vector magnitude

C_VOLUME(c,t) Volume of a cell

C_VOLUME_2D(c,t) Volume of a 2D cell

pos and A are vectors which are discussed later.

Depth is 1m in 2D; 2

radians in axi-symmetric .

Node Coordinates:

NODE_X(nn) Node xcoord; (with Node *nn;)

NODE_Y(nn) Node ycoord;

NODE_Z(nn) Node zcoord;

M

m

s


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Cell-Face Connectivity (1)

The cells on either side of a face (f,ft) can be accessed using the mac

The macros THREAD_T0(ft)andTHREAD_T1(ft)only need theface thread, not the face index.

Boundary zones such as inlets will connect to a cell zone on one sidewith THREAD_T0(ft)but THREAD_T1(ft)will returnNULL, aspecial pointer which signifies there is no zone on the other side.

ct0 = THREAD_T0(ft);ct1 = THREAD_T1(ft);c0 = F_C0(f,ft);c1 = F_C1(f,ft);

C


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Cell-Face Connectivity (2)

A faces area can be obtained by

where F_AREA returns the face normal vectorAandNV_MAG(A)the magnitude of the vectorA.

The faces that bound a cell can be accessed using this loop :

real A[3], area;

F_AREA(A,f,ft);area = NV_MAG(A);

C0

A faces

points f

int nf;for(nf = 0; nf < C_NFACES(c,ct); nf++){

f = C_FACE(c,ct,nf);ft= C_FACE_THREAD(c,ct,nf);

}


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Macros used for Control

Data_Valid_P()

TRUE if data is available, FALSE if not.

FLUID_THREAD_P(t0)

TRUE if threat t0is a fluid thread, FALSE if not.

NULLP(ct)

TRUE if a pointer is NULL, FALSE if not. NNULLP(ct)

TRUE if a pointer is NOT NULL, FALSE if not.

if (!Data_Valid_P()) r

if(NNULLP(THREA{ ... Do things ocell thread.

}

f


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UDF Defining Macros

All UDFs that will be used must be defined using a specific

DEFINE_macro

Common examples:

Profiles : DEFINE_PROFILE

Source Terms : DEFINE_SOURCE

Properties : DEFINE_PROPERTY

User-defined Scalars : DEFINE_UDS_UNSTEADY , DEFINE_UDS_FLUXDEFINE_DIFFUSIVITY

Initialization : DEFINE_INIT

Global Functions : DEFINE_ADJUST

DEFINE_ON_DEMAND

DEFINE_RW_FILE

F

s

ddi i l f i


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Additional Information

Many additional macros are available to implement different physic

models including:

Combustion models Particle-based models

Turbulence models

Radiation models

etc.

It is also possible to develop additional numerical methods, particulwhen using User-Defined Scalars (see Lecture 4). Access to low-level

operations is possible, such as

Flux discretization

Variable reconstruction and clipping

O i f Fl M


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Overview of Fluent Macros

UDF macros are defined in the udf.h file, for instance

The udf.his a Fluent header file in the

path\ANSYS Inc\v150\fluent\fluent15.0.0\src

directory.

At the beginning of every UDF source code file, the header file MUST be incl

There are many more header files stored in the same directory which can be

browsed by users but most are already included automatically within udf.h.

#include udf.h

#define DEFINE_PROFILE(name, t, i) void name(Thread *t, int

#define DEFINE_PROPERTY(name,c,t) real name(cell_t c, Thread

fluent udf 15.0 l01 introduction

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