UDF Training- Petrodanesh

2011

ANSYS FLUENT GUIDE Advanced FLUENT User-Defined Functions You can access and manipulate almost every solver data and fully control the solution process in UDF-s written in C programming Language.

Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir) Pardad Petro Danesh (www.petrodanesh.ir) | Confidential 1/12/2011

Introduction

2

Table of Contents 1

Introduction ........................................... ............................................................................................ ......................................................................................... ........................................ 4

2

User Access to the FLUENT Solver.......................................... ........................................................................................... ......................................................... ........ 4

3

C Programming ............................................... ................................................................................................ ................................................................................. ................................ 5

4

Cfd Programming in FLUENT ............................................... ............................................................................................... ............................................................ ............ 7

5

UDF Basics ........................................... ............................................................................................ ............................................................................................ ........................................... 7

6

Using UDFs in the Solvers ......................................... .......................................................................................... ....................................................................... ...................... 8

7

Data structure overview.......................................... .......................................................................................... ........................................................................ ........................ 8 7.1

The Domain .......................................... .......................................................................................... ................................................................................... ................................... 8

7.2

The Threads .......................................... .......................................................................................... ................................................................................... ................................... 9

7.3

Cell and Face Datatypes ............................................ ............................................................................................ .............................................................. .............. 9

7.4

Some Additional info on Faces ............................................. ............................................................................................. ................................................... ... 9

8

Fluent UDF Data Structure Summary ........................................... ........................................................................................... .................................................. 10

9

Geometry Macros ................................................ ................................................................................................. ......................................................................... ........................ 11

10

Cell Field Variable Macros ............................................ ............................................................................................. ............................................................. ............ 12

11

Face Field Variable Macros .......................................... ........................................................................................... ............................................................. ............ 13

12

Looping and Thread Macros......................................... .......................................................................................... ............................................................. ............ 14

13

Macros .......................................... .......................................................................................... ............................................................................................. ............................................. 15

13.1

DEFINE Macros .............................................. .............................................................................................. ........................................................................ ........................ 15

13.2

Othe UDF Applications ............................................ ............................................................................................. .............................................................. ............. 16

14

User Defined Memories .............................................. .............................................................................................. .............................................................. .............. 17

15

User Defined Scalars.................................................. Scalars................................................................................................... ................................................................ ............... 18

16

Interpreted vs. Compiled UDF’s ............................................. ............................................................................................. ................................................... ... 19

17

UDF Technical Support ............................................... ................................................................................................ ............................................................... .............. 19

18

Example: Parabolic Inlet Velocity Profile ........................................... .................................................................................. ....................................... 20

18.1

Step 1: Prepare the Source Code ................................................ .......................................................................................... .......................................... 20

18.2

Step 3: Interpret or Compile the UDF .......................................... ................................................................................... ......................................... 20

18.3

Step 4: Activate the UDF ......................................... .......................................................................................... .............................................................. ............. 21

18.4

Steps 5 and 6: Run the Calculations ........................................... ...................................................................................... ........................................... 21

18.5

Numerical Solution of the Example............................................ ....................................................................................... ........................................... 21

19

Example: Checking Effect of Porous Zone on Momentum Equation ........................................ 22

19.1

Using Fluent Built-in Porous Zone ............................................. ......................................................................................... ............................................ 22

19.2

Using Momentum Source Terms.............................. Terms............................................................................... ............................................................. ............ 23

20

Example: Modeling Fuel Cell ............................................. .............................................................................................. ........................................................ ....... 24 Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)

Introduction

2

Table of Contents 1

Introduction ........................................... ............................................................................................ ......................................................................................... ........................................ 4

2

User Access to the FLUENT Solver.......................................... ........................................................................................... ......................................................... ........ 4

3

C Programming ............................................... ................................................................................................ ................................................................................. ................................ 5

4

Cfd Programming in FLUENT ............................................... ............................................................................................... ............................................................ ............ 7

5

UDF Basics ........................................... ............................................................................................ ............................................................................................ ........................................... 7

6

Using UDFs in the Solvers ......................................... .......................................................................................... ....................................................................... ...................... 8

7

Data structure overview.......................................... .......................................................................................... ........................................................................ ........................ 8 7.1

The Domain .......................................... .......................................................................................... ................................................................................... ................................... 8

7.2

The Threads .......................................... .......................................................................................... ................................................................................... ................................... 9

7.3

Cell and Face Datatypes ............................................ ............................................................................................ .............................................................. .............. 9

7.4

Some Additional info on Faces ............................................. ............................................................................................. ................................................... ... 9

8

Fluent UDF Data Structure Summary ........................................... ........................................................................................... .................................................. 10

9

Geometry Macros ................................................ ................................................................................................. ......................................................................... ........................ 11

10

Cell Field Variable Macros ............................................ ............................................................................................. ............................................................. ............ 12

11

Face Field Variable Macros .......................................... ........................................................................................... ............................................................. ............ 13

12

Looping and Thread Macros......................................... .......................................................................................... ............................................................. ............ 14

13

Macros .......................................... .......................................................................................... ............................................................................................. ............................................. 15

13.1

DEFINE Macros .............................................. .............................................................................................. ........................................................................ ........................ 15

13.2

Othe UDF Applications ............................................ ............................................................................................. .............................................................. ............. 16

14

User Defined Memories .............................................. .............................................................................................. .............................................................. .............. 17

15

User Defined Scalars.................................................. Scalars................................................................................................... ................................................................ ............... 18

16

Interpreted vs. Compiled UDF’s ............................................. ............................................................................................. ................................................... ... 19

17

UDF Technical Support ............................................... ................................................................................................ ............................................................... .............. 19

18

Example: Parabolic Inlet Velocity Profile ........................................... .................................................................................. ....................................... 20

18.1

Step 1: Prepare the Source Code ................................................ .......................................................................................... .......................................... 20

18.2

Step 3: Interpret or Compile the UDF .......................................... ................................................................................... ......................................... 20

18.3

Step 4: Activate the UDF ......................................... .......................................................................................... .............................................................. ............. 21

18.4

Steps 5 and 6: Run the Calculations ........................................... ...................................................................................... ........................................... 21

18.5

Numerical Solution of the Example............................................ ....................................................................................... ........................................... 21

19

Example: Checking Effect of Porous Zone on Momentum Equation ........................................ 22

19.1

Using Fluent Built-in Porous Zone ............................................. ......................................................................................... ............................................ 22

19.2

Using Momentum Source Terms.............................. Terms............................................................................... ............................................................. ............ 23

20

Example: Modeling Fuel Cell ............................................. .............................................................................................. ........................................................ ....... 24 Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)

Introduction

3

20.1

Preparation................................................ ................................................................................................ ............................................................................ ............................ 24

20.2

Mesh............................................... ............................................................................................... ....................................................................................... ....................................... 24

20.3

General Settings .......................................... .......................................................................................... .......................................................................... .......................... 24

20.4

Models ............................................. .............................................................................................. ...................................................................................... ..................................... 25

20.5

Materials .......................................... .......................................................................................... ..................................................................................... ..................................... 25

20.6

Boundary Conditions ............................................ ............................................................................................. ................................................................. ................ 26

20.6.1

Setting Velocity Inlet .............................................. .............................................................................................. ....................................................... ....... 29

20.6.2

Pressure Outlet............................................ ............................................................................................ .................................................................. .................. 29

20.7

Writing UDF ............................................. .............................................................................................. .............................................................................. ............................. 30

20.7.1 20.8 21

Compiling the UDF................................................ UDF................................................................................................ ......................................................... ......... 32

Checking Oxygen Mass Balance .............................................. ............................................................................................ .............................................. 37 Fluent Frequently Asked Questions ............................................. .......................................................................................... ............................................. 39

21.1

My UDF won't interpret or compile - what is wrong? ............................................ .......................................................... .............. 39

21.2

How to Set Environment Variables .......................................... ....................................................................................... ............................................. 39

21.2.1

On Windows 32 Bit ............................................... ............................................................................................... ......................................................... ......... 39

21.2.2

On Windows 64 Bit ............................................... ............................................................................................... ......................................................... ......... 39

Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)

Introduction

1 •

4

Introduction What is a User Defined Function? o

A UDF is a routine (programmed by the user) written in C which can be dynamically linked with the solver. 

Standard C functions •



Pre-Defined Macros •

•

e.g., trigonometric, exponential, control blocks, do-loops, file i/o, etc. Allows access to field variable, material property, and cell geometry data.

Why build UDF’s? o

Standard interface cannot be programmed to anticipate all needs. 

Customization of boundary conditions, source terms, reaction rates, material properties, etc.

2



Adjust functions (once per iteration)



Execute on Demand functions



Solution Initialization

User Access to the FLUENT Solver


C Programming

3

5

C Programming •

Basic syntax rules: o

Each statement must be terminated with a semicolon ;

o

Comments can be inserted anywhere between /* and */

o

Variables must be explicitly declared (unlike in FORTRAN)

o

Compound statements must be enclosed by {

o

Functions have the following format:

}

return-value-type function-name (parameter-list) { function body } o •

Macros are defined in the header files, they can be used just like functions

Built-in data types: int, float, double, enum, Boolean: int niter, a;

/* declaring ‘niter’ and ‘a’ as integers */

float dx[10];

/* ‘dx’ is a real array with 10 members, the array index always starts from dx[0] */

enum { X, Y, Z }; /* X, Y, Z are enumeration constants 0, 1, 2 */ •

pointer is a special kind of variable that contains the memory address, not content, of another variable

•

Pointers are declared using the * notation:

int *ip; •

/* ip is declared as a pointer to integer */

We can make a pointer point to the address of predefined variable as follows: int a=1; int *ip; ip = &a; /* &a returns the address of variable a */ printf(“content of address pointed to by ip = %d\n”, *ip);

•

Pointers are also used to point to the beginning of an array o

•

Thus pointers are used to address arrays in C

Array variables can be defined using the notation name[size], where name is the variable name and size is an integer which defines the number of elements in the array (from 0 to size-1)

•

Operators =

(assignment)

+, -, *, /, % (modulus) <, >, >=, <=, ==, != ++ increment; i++ is “pors-increment”; use the current value of I in the expression, then increment I by 1 (i=i+) -- decrement: j—post-decrement, use the current value of j, then decrement j by 1 (j=j-1) += addition assignment: Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)

C Programming

agg += single;

/* it means agg = agg + single;

*/

*= multiplication assignment, -= subtraction assignment, /= division assignment •

Basic control structures if ( … ) ; if ( … ) ; else ; if ( … ) ; else if ( … ) ; For Loops: for ( k=0; k; While loops: While ( … ) ; Conditional operator ( ? : ) ( condition ? operand a : operand b ) example: false real At = (rp_axi ? At*2*M_PI : At ); true


6

Cfd Programming in FLUENT

4

7

Cfd Programming in FLUENT We (as CFD programmers in FLUENT) want to know how FLUENT organizes data so that we

•

know: o

How to access mesh information for a particular cell zone or a fac e zone 

Cell centroids, cell volumes, the neighbors, etc.



Face centroids, face areas, face normal directions (vectors), etc.

o

How to access field data for cells (and faces): pressure, velocity, density, etc.

o

How to efficiently loop through these cells or faces in the codes

How to sypply customized source terms, boundary co nditions, and fluid properties, etc., to

•

the solver How to modify the behaviors or specific model parameters for various physical models as in

•

turbulence, reactions kinetics, multiphase, and dynamic mesh, etc. How to implement user’s own governing equations in the finite-volume framework of

•

FLUENT solver

5 •

UDF Basics UDF’s assigns values (e.g., boundary data, source terms) to individual cells and cell faces in fluid and boundary zones. o

In a UDF, zones are referred to as threads.

o

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). •

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

6 •

8

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

7

Data structure overview

7.1 The Domain •

Domain is the set of connectivity and hierarchy info for the entire data structure in a given problem for single phase flows. It includes: o

All fluid zones (‘fluid threads’)

o

All solid zones (‘solid threads’)

o

All boundary zones (‘boundary threads’)

•

Cell: cell is the computational unit, conservation equations are solved over each cell

•

Face: direction in the outward normal

•

Threads: represent the collection of cells or faces; a Thread represents a fluid or solid or boundary zone

•

Multiphase simulations (singlephase simulations use single domain only) o

Each phase has its own “Domain-structure”

o

Geometric and common property information are shared among ‘sub-domains’


Data structure overview

o

9

Multiphase UDF will be discussed later

7.2 The Threads •

A Thread is a sub-set of the Domain structure

•

Individual ‘fluid’, ‘solid’ and each ‘ boundary’ zones are identified as ‘ zones’ and their datatype is Thread

•

‘zone’ and ‘Thread’ terms are often used interchangeably

•

Some further details about Zone/Thread ID and Thread-datatype: o

Zones are identified at mesh level with an integer ID in the Define/Boundary

Condition panel o

Threads, a Fluent-specific datatype, store structured infor mation about the mesh, connectivity, models, property, etc. all in one place

o

Users identify zones through the ID’s

o

Zone/Thread-ID and Threads are correlated through UDF macro’s

7.3 Cell and Face Datatypes •

Control volumes of fluid and solid zones are also called ‘cells’ in FLUENT o

The data structure for the cell zones is typed as ‘cell_t’ (the cell thread index)

o

The data structure for the cell faces is typed as ‘face_t’ (the face thread index)

•

A fluid or solid zone is called a cell zone, which can be accessed by using cell threads

•

Boundary or internal faces can be accessed by using face threads

7.4 Some Additional info on Faces •

Each Control volume has a finite number of faces o

Faces on the boundary are also typed ‘ face_t’; their ensemble are listed as boundary face-

threads with the o

fluid & solid cell-threads under Define-

o

Boundary_Condition panel

o

Those faces which are inside the flow-domain and do not

o

share any external boundary are not accessible from GUI

o

(because you do not need them)

o

They can still be accessed from User-Defined-Functions


Fluent UDF Data Structure Summary

8

10

Fluent UDF Data Structure Summary •

The data structure for accessing a cell zone is typed as ‘cell_t’ (the cell thread index); the data structure for faces is typed as ‘face_t' (the face thread index)

•

Type

Example

Declaration

Domain

*d

d is a pointer to domain thread

Thread

*t

t is a pointer to thread

Cell_t

*c

c is cell thread index

Face_t

f

f is a face thread index

Node

*node

node is pointer to a node

Each thread (zone) has a unique integer ID available in the boundary condition panel (or can be listed by the list-zone TUI command: /grid/modify-zones/list-zones)

•

Given a correct ID, the Lookup_Thread macro can retrieve the thread pointer Int ID = 7; Thread *tf = Lookup_Thread (domain, ID);

•

Conversely, given a thread pointer tf, the zone ID can be retrieved ID = THREAD_ID(tf);

•

Once we have the correct pointer (for a specific zone), we can access the members belonging to the zone without any problem. Thread pointer provides the leading address of the thread (zone)


Geometry Macros

9

Geometry Macros •

C_NNODES(c, t)

Number of nodes in a cell

•

C_NFACES(c, t)

No. of faces in a cell

•

F_NNODES(f, t)

No. of nodes in face

•

C_CENTROID(x, c, t)

x, y, z - coords of cell centroid

•

F_CENTROID(x, f, t)

x, y, z – coords of face

centroid •

F_AREA(A, f, t)

Area vector of a face

•

NV_MAG(A)

Area-magnitude

•

C_VOLUME(c, t)

Volume of a cell

•

C_VOLUME_2D(c, t)

Volume of a 2D cell

(Depth is 1m in 2D; 2* π m in axi-symmetric solver) •

NODE_X(nn)

Node x-coord; (nn is a node pointer)

•

NODE_Y(nn)

Node y-coord;

•

NODE_Z(nn)

Node z-coord;

•

real flow_time();

returns actual time

•

int time_step;

returns time step number

•

RP_GET_Real(“physical-time-step”);

returns time step size


11

Cell Field Variable Macros

10 Cell Field Variable Macros C_R(c, t);

density

C_P(c,t);

pressure

C_U(c,t);

u-velocity

C_V(c,t);

v-velocity

C_W(c,t);

w-velocity

C_T(c,t);

temperature

C_H(c,t);

enthalpy

C_K(c,t);

turbulent KE

C_D(c,t);

tke dissipation

C_YI(c,t,i);

species mass fraction

C_UDSI(c,t,i);

UDS scalars

C_UDMI(c,t,i);

UDM scalars

C_DUDX(c,t);

velocity derivative

C_DUDY(c,t);

velocity derivative

C_DYDZ(c,t);

velocity derivative

C_DVDX(c,t);

velocity derivative

C_DVDY(c,t);

velocity derivative

C_DVDZ(c,t);

velocity derivative

C_DWDX(c,t);

velocity derivative

C_DWDY(c,t);

velocity derivative

C_DWDZ(c,t);

velocity derivative

C_MU_L(c,t);

laminar viscosity

C_MU_T(c,t);

turbulent viscosity

C_MU_EFF(c,t);

effective viscosity

C_K_L(c,t);

laminar thermal conductivity

C_K_T(c,t);

turbulent thermal conductivity

C_K_EFF(c,t);

effective thermal conductivity

C_CP(C c,t);

specific heat

C_RGAS(c,t);

gas constant

C_DIFF_L(c,t);

laminar species diffusivity

C_DIFF_EFF(c,t);

effective species diffusivity


12

Face Field Variable Macros

11 Face Field Variable Macros •

Face field variables are only available when using the segregated solver and generally, only at exterior boundaries. F_R(f, t);

density

F_P(f, t);

pressure

F_U(f, t);

u-velocity

F_V(f, t);

v-velocity

F_W(f, t);

w-velocity

F_T(f, t);

temperature

F_H(f, t);

enthalpy

F_K(f, t);

turbulent KE

F_D(f, t);

tke dissipation

F_YI(f, t);

species mass fraction

F_UDSI(f, t);

UDS scalars

F_UMI(f, t);

UDM scalars

F_FLUX(f, t);

Mass flux across face f , defined out of domain at boundaries.


13

Looping and Thread Macros

12 Looping and Thread Macros cell_t c;

defines a cell

face_t f;

defines a face

Thread *t;

pointer to a thread

Domain *d;

pointer to collection of all threads

Specialized variable types used for referencing

thread_loop_c(t, d) { ..........

loop that steps through all cell threads in domain

} thread_loop_f(t, d) { ……….

loop that steps through all face threads in domain

} begin_c_loop(c, t) { ………. } end_c_loop befing_f_loop(c, t) { ………. } C_face_loop(f, t, n) { ………. }

Loop that steps through all cells in a thread

Loop that steps through all faces in a thread

Loop that visits all faces of cell c in thread t

Thread *tf = Lookup_Thread(domain, ID);

return thread pointer of integer ID of zone

THREAD_ID(tf);

returns zone integer ID of thread pointer

Code enclosed in { } is executed in loop.


14

Macros

13 Macros •

•

Macros are pre-defined (Fluent) functions: o

Allows definition of UDF functionality and function name ( DEFINE_ macro)

o

Allow access to field variables, cell information, looping capabilities, etc.

Macros are defined in header files. o

The udf.h header file must be included in your source code. 

o

The header files must be accessible in your path. 

o

#include “udf.h” Typically stored in Fluent.Inc/src/directory.

Other “.h” header files may need to be included. 

Depends upon relevant variables and macros needed in your UDF, e.g., •

•

dpm.h for DPM variable access

A list of often used macros is provided in the UDF User’s Guide.

13.1 DEFINE Macros •

Any UDF you write must begin with a DEFINE_ macro: o

18 general purpose macros and 13 DPM-related macros (not listed);

DEFINE_ADJUST(name,domain);

general purpose UDF called every iteration

DEFINE_INIT(name,domain);

UDF used to initialize field variables

DEFINE_ON_DEMAND(name);

defines an ‘execute-on-demand’ function

DEFINE_RW_FILE(name,face,thread,index);

customize reads/writes to case/data files

DEFINE PROFILE(name,thread,index);

defines boundary profiles

DEFINE_SOURCE(name,cell,thread,ds,index);

defines source terms

DEFINE_HEAT_FLUX(name,face,thread,c0,t0,cid,cir);

defines heat flux

DEFINE_PROPERTY(name,cell,thread);

defines material properties

DEFINE_DIFFUSIVITY(name,cell,thread,index);

defines UDF and species diffusivities

DEFINE_UDS_FLUX(name,face,thread,index);

defines UDS flux terms

DEFINE_UDS_UNSTEADY(name,face,thread,index);

defines UDS transient terms

DEFINE_SR_RATE(name,face,thread,r,mw,yi,rr);

defines surface reaction rates

DEFINE_VR_RATE(name,cell,thread,r,mw,yi,rr,rr_t);

defines vol. reaction rates

DEFINE_SCAT_PHASE_FUNC(name,cell,face); DEFINE_DELTAT(name, domain);

defines scattering phase function for DOM

defines variable time step size for unsteady problems


15

Macros

DEFINE_TURBULENT_VISCOSIT(name, cell, thread);

16

defines procedure for calculating turbulent viscosity

DEFINE_TURB_PREMIX_SOURCE(name, cell, thread, turbflamespeed, source);

defines turbulent

flame speed DEFINE_NOX_RATE(name, cell, thread, nox);

defines NOx production and destruction rates

13.2 Othe UDF Applications •

In addition to defining boundary values, source terms and material properties, UDFs can be used for: o

Initialization 

o

Adjust 

o

Executes once per initialization Executes every iteration

Wall Heat Flux 

Defines fluid-side diffusive and radiative wall heat fluxes in terms of heat transfer coefficients



Applies to all walls

o

User Defined Surface and Volumetric Reactions

o

Read-Write to/from case and data files 

o

Read order and Write order must be same.

Execute-on-Demand capability 

Not accessible during solve


/User Defined Memories

14 User Defined Memories •

User-allocated memory o

Up to 500 field variables can be defined.

o

Can be accessed by UDFs: 

C_UDMI(cell, thread, index);



F_UDMI(face, thread, index);

o

Can be accessed for post-processing.

o

Information is stored in data file.


17

User Defined Scalars

18

15 User Defined Scalars •

FLUNET can solve (up to 50) generic transport equations for User Defined Scalars:

∂φk ∂  ∂φ k  +  Fi φ k − Γ k  = Sφ ∂t ∂xi  ∂xi 

k

•

k

= 1,..., N scalars

User Specifies: Define  Models  User-Defined Scalars… o

Number of User-Defined Scalars

o

Flux Function F 

DEFINE_UDS_FLUX(name, face, thread, index)



DEFINE_UDS_UNSTEADY(name, cell, thread, index)



‘case’ statement can be used to associate multiple f lux and transient functions with each UDS.

o

Example 

•

Can be used to determine magnetic and/or electric field in a fluid zone.

User must also specify: o

Source terms

o

Diffusivity, 

‘case’ statement needed to define UDF diffusivities for each UDS.

o

Boundary Conditions for each UDS. 

Specified Flux or Specified Value.

•

Define as constant or with UDF.


Interpreted vs. Compiled UDF’s

19

16 Interpreted vs. Compiled UDF’s •

Functions can either be read in and interpreted at run time or compiled and grouped into a shared library that is linked with the standard FLUENT executable.

•

Interpreted vs. compiled code o

o

Interpreter 

Interpreter is a large program that sits in the computer’s memory.



Executes code on a “line by line” basis instantaneously



Advantages – does not need a separate compiler



Disadvantage – slow and takes up space in memory

Compiler (refer to User’s Guide for i nstructions) 

Code is translated “once” into machine l anguage (object modules).



Efficient way to run UDF’s. Uses Makefiles



Creates “shared libraries” which are linked with the rest of the solver



Overcomes interpreter limitations e.g. mixed mode arithmetic, structure references etc.

17 UDF Technical Support •

Because UDF’s can be very complicated, Fluent Inc. does not assume responsibility for the accuracy or stability of solutions obtained using UDFs that are user-generated. o

Support will be limited to guidance related to communication between a UDF and the FLUENT solver.

o

Other aspects of the UDF development process that include conceptual function design, implementation (writing C code), compilation and debugging of C source code, execution of the UDF, and function design verification will remain the responsibility of the UDF author. 

A consulting option is available for complex projects.


Example: Parabolic Inlet Velocity Profile

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18 Example: Parabolic Inlet Velocity Profile •

We would like to impose a parabolic inlet velocity to the 2D elbow shown.

•

The x velocity is to be specified as

  y 2  u(y ) = 201−      0.0745 

y

=0

18.1 Step 1: Prepare the Source Code •

The DEFINE_PROFILE macro allows the function inlet_x_velocity to be defined. o

All UDFs begin with a DEFINE_ macro.

o

inlet_x_velocity will be identifiable in solver GUI.

o

thread and nv are arguments of the DEFINE_PROFILE macro, which are used to identify the zone and variable being defined, respectively.

o

The macro begin_f_loop loops over all faces f , pointed by thread.

•

The F_CENTROID macro assigns cell position vector to x[].

•

The F_PROFILE macro applies the velocity component to face f .

18.2 Step 3: Interpret or Compile the UDF Define  User-Defined  Functions  Interpreted…

•

• •

Add the UDF source code to the Source File Name list. Click Interpret. The assembly language code will display in the FLUENT console.

Define  User-Defined  Functions  Compiled…

Add the UDF source code to the Source files list. Click Build to create UDF library. • Click Load to load the library. • You can also unload a library if needed. • Define  User-Defined  Functions  Manage… •


Example: Parabolic Inlet Velocity Profile

18.3 Step 4: Activate the UDF •

Open the boundary condition panel for the surface to which you would like to apply the UDF.

•

Switch from Constant to the UDF function in the X_Velocity drop-down list.

18.4 Steps 5 and 6: Run the Calculations •

You can change the UDF Profile Update Interval in the Iterate panel (default value is 1). o

This setting controls how often (either iterations or time steps if unsteady) the UDF profile is updated.

•

Run the calculation as usual.

18.5 Numerical Solution of the Example •

The figure at the right shows the velocity field through the 2D elbow.

•

The bottom figure shows the velocity vectors at the inlet. Notice the imposed parabolic profile.


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Example: Checking Effect of Porous Zone on Momentum Equation

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19 Example: Checking Effect of Porous Zone on Momentum Equation Many industrial applications such as filters, catalyst beds and packing, involve modeling the flow through porous media. This tutorial illustrates how to set up and solve a problem involving gas flow through porous media. The problem solved here involves gas flow through a channel using FLUENT in two different ways to validate it. In the first one we use porous zone parameters and in the second one we write a UDF includes momentum source terms for porous zone pressure drop.

19.1 Using Fluent Built-in Porous Zone 

File  Read  Case Read a mesh file



Define  Boundary Conditions



Select fluid zone then Click Set to enter the fluid zone parameters.



Check the “ Porous Zone” to enable porous zone



On Porous zone, set the Viscous Resistance equal to 5.68e6 (1/m ) in both directions.



Change the Fluid Porosity from 1 to 0.4 at the end of this Porous Zone Tab.



For inlet condition, set the Velocity inlet equal to 1 (m/s) on the inlet velocity magnitude

2

normal to boundary. 

To solve the case, go to: Solve  Iterate…

Number of Iterations = 1000



After convergence we can display results as follows.

Figure 19-1: Static pressure in the channel (Pascal).


Example: Checking Effect of Porous Zone on Momentum Equation

19.2 Using Momentum Source Terms

#include "udf.h" #define por_gdl

0.4

#define i_permeability 5.68e6 #define urf

0.1

// Inverse Permeability (1/m^2)

// under relaxation factor for stability of momentum source term

real s1=0., s2=0.; DEFINE_SOURCE(xmom_src,c,t,dS,eqn) { real source, mu, u; mu = C_MU_L(c,t); u = C_U(c,t); source = -(mu*u*i_permeability); dS[eqn] = -(mu*i_permeability); s1 = s1*(1-urf) + source*urf; return s1; } DEFINE_SOURCE(ymom_src,c,t,dS,eqn) { real source, mu, v; mu = C_MU_L(c,t); v = C_V(c,t); source = -(mu*v*i_permeability); dS[eqn] = -(mu*i_permeability); s2 = s2*(1-urf) + source*urf; return s2; }

Figure 19-2: Static pressure in the channel (Pascal).


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Example: Modeling Fuel Cell

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20 Example: Modeling Fuel Cell 20.1 Preparation Use FLUENT Launcher to start 2D version of ANSYS FLUENT.

Note: The Display Options are enabled by default. Therefore, once you read in the mesh, it will be displayed in the embedded graphics windows.

20.2 Mesh 1. Read the mesh file File  Read  Mesh 2. Check the mesh 

General  Check

ANSYS FLUENT will perform various checks on the mesh and report the progress in the ANSYS FLUENT console window. Ensure that the minimum volume reported is a positive number. 3. Scale the mesh 

General  Scale…

(a) Select mm (millimeter) from the Mesh Was Created In drop-down list in the Scaling group box. (b) Click Scale to scale the mesh. (c) Close the Scale Mesh dialog box. 4. Check the mesh. 

General  Check

Note: It is a good idea to check the mesh after you manipulate it (i.e., scale, convert to polyhedral, merge, separate, fuse, add zones, or smooth and swap). This will ensure that the quality of the mesh has not been compromised.

20.3 General Settings 

Define  Models  Solver

1. Retain the default settings for the solver. Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)


20.4 Models 

Define  Models  Species  Transport & Reaction…

(a) Enable Species Transport (b) Click OK to Close the Species Model box.

(c) FLUENT will notify that “Available material properties or method have changed. Please

confirm the property values before continuing”. Click OK to close the dialog box.

20.5 Materials 

Define  Materials…

1. Revise the properties for the mixture materials. 

Define  Material  Mixture


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(a) Click the Edit… button to the right of the Mixture Species drop-down list to open Species dialog box.

You can add or remove species from the mixture material as necessary using the Species dialog box. i.

Retain the default selections from the Selected Species selection list.

ii.

Click OK to close the Species dialog box.

(b) Click Change/Create to accept the material property settings. (c) Close the Create/Edit Materials dialog box.

20.6 Boundary Conditions 

Define  Operating Conditions

Because the effect of gravity can be neglected, Retain setting in Operating Conditions and Click OK to Close the dialog box.





Define  Boundary Conditions

(a) Set Boundary condition for gdl fluid zone, by clicking on the Set button

(b) Check Porous zone radio button to enable porous zone. (c) Set value for Viscous Resistance in both directions. These parameters are inverse of

permeability (1/m^2). Fluent uses these values for calculating momentum source term due to the porous zone.

(d) Set the porosity of the porous zone. (e) Click Ok to save data.


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(f) Set Boundary condition for cl fluid zone, by clicking on the Set button

(g) Check the porous zone button to enable the cl porous zone. (h) Set viscous resistance variables for both directions.

(i) Now you should set porosity for cl porous zone that is equal to 0.1, here.


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20.6.1 Setting Velocity Inlet (a) By selecting inlet at Boundary Condition dialog box and Clicking on set button, you will see the Velocity inlet dialog box, which can be used to specify inlet conditions such as inlet velocity, temperature, species mass fraction, etc. (b) Set Velocity magnitude equal to 0.03 (m/s), that is entering the domain normal to the boundary face.

(c) Click the Species Tab to set mass fraction of species. Fluent will automatically calculate the mass fraction of the last specie, that is equal to 1 minus sum of the mass fraction of the other species.

20.6.2 Pressure Outlet (a) Click on the outlet to set outlet conditions. Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)


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(b) Select pressure outlet on the right side of the boundary conditions dialog box, to specify pressure at the outlet of the fuel cell. (c) Click set button to enter Pressure Outlet dialog box. (d) To prevent solution damage by reverse flows, we should specify outlet conditions near to real condition. (e) For this, we have chosen inlet species mass fraction for the outlet condition. (f) Leave the other parameters by default and cli ck OK.

20.7 Writing UDF #include "udf.h" #define por_cl 0.1 #define por_gdl 0.4 #define i_permeability 5.68e6 //5.681e10 // (1/m^2) #define R UNIVERSAL_GAS_CONSTANT // 8.3143 kJ/kmol.K this is known within Fluent #define zita_cat 2.e3 #define V_oc 1.207 #define j_ref_cat 20. #define alpha_cat 2. #define lambda_cat 1. #define phi_sol 0. #define phi_mem 0. #define C_ref_o2 1. #define F 9.65e4 // C/kmol #define h_cl 0.02e-3 // Cathalyst layer thickness [m] #define L 71.12e-3 // Channel Length [m] #define diff_o2 #define diff_h2o #define diff_n2 #define i_h2o #define i_o2 #define i_n2

3.2348e-5 3.89e-5 2.88e-5

/* mass diffusivity of oxygen */ /* mass diffusivity of water */ /* mass diffusivity of nitrogen */

0 1 2

#define mw_h2o #define mw_o2

18. 32.

// kg/kmol // kg/kmol



#define mw_n2

28.

// kg/kmol

enum{ current }; real R_cat(cell_t c, Thread *t) { // this routine returns Reraction_Rate_cat as "R_cat" real eta_cat, C_o2, T, xyz[ND_ND], x ,y, z, Reraction_Rate_cat, i, i0_over_C_ref; C_o2 = C_YI(c,t,i_o2); T = C_T(c,t); //eta_cat = phi_sol-phi_mem-V_oc; i=C_UDMI(c,t,current); i0_over_C_ref=0.8*1e4; //[A/m^2] Ref. Khakbaz-Kermani eta_cat = R*T / (alpha_cat*F) * log(i/C_o2/i0_over_C_ref); //Butler-Volmer Ref. Khakbaz-Kermani C_CENTROID(xyz,c,t); x=xyz[0]; y=xyz[1]; //z=xyz[2]; // Option 0 // Reraction_Rate_cat = 10000.0; // Option 1 Reraction_Rate_cat = 100000. *(1.0 - x/L); // Option 2 from Fluent FC manual // Reraction_Rate_cat = (zita_cat*j_ref_cat)*pow(C_o2/C_ref_o2,lambda_cat)*(exp(alpha_cat*F*eta_cat/(R*T))); // Option 3 // Reraction_Rate_cat = (zita_cat*j_ref_cat)*pow(C_o2/C_ref_o2,lambda_cat)*(exp(alpha_cat*F*eta_cat/(R*T))); return Reraction_Rate_cat; } DEFINE_SOURCE(O2_src,c,t,dS,eqn) { real source; source = -mw_o2*R_cat(c,t)/(4.*F) *por_cl;// (kg/kmol)()/(C/kmol) //Message("%f, %f, %f, %f, %f\n",mw_o2,R_cat(c,t),por_cl, source, F); C_UDMI(c,t,current)= -4.*F /mw_o2 *source *h_cl; // this is local current density dS[eqn] = 0.0; return source; } DEFINE_SOURCE(H2O_src,c,t,dS,eqn) { real source; source = +mw_h2o*R_cat(c,t)/(2.*F) *por_cl; Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)

31


dS[eqn] = 0.0; return source; } DEFINE_SOURCE(MASS_src,c,t,dS,eqn) { real source; source = ( -mw_o2/(4.*F) + mw_h2o/(2.*F) )*R_cat(c,t) *por_cl; dS[eqn] = 0.0; return source; } DEFINE_DIFFUSIVITY(mix_diff, c, t, i) { real diff; switch (i) { case 0: diff = diff_h2o*por_cl*sqrt(por_cl); break; case 1: diff = diff_o2*por_cl*sqrt(por_cl); break; case 2: diff = diff_n2*por_cl*sqrt(por_cl); break; default: diff = 0.0; } return diff; }

20.7.1 Compiling the UDF  Define  User-Defined  Functions  Compiled



Add the UDF source files and headers if needed.


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33



Build the UDF and then click on Load to load the Library that is named libudf .



Note that the UDF source files should be in the directory that contains your case and data files. Click OK to build the library.



Now the fluent creates library libudf.lib, object libudf.exp and other required files.



You can load the library if fluent reveal no error and the message “ Done” have seen at the end of the commend window.



By loading the library, fluent command window show all defined functions. Here it shows “O2_src, H2O_src, MASS_src, …”. Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)




34

If you have build and load a udf before, be sure to unload it in the Manage dialog box. Else an error will appear that an libudf is in use.



Now you should hook functions to its specified equation.



On the cl fluid zone, check Source Terms.



On the Source Terms Tab, click Edit in front of Mass equation.



Then add a source and select MASS_src and click ok to add the source term on solution.



Repeat this procedure for the other equations, such as H2O, O2, … .



We have used a UDM in the UDF, so we should specify a UDM for it.



Go to: Define  User-Defined  Memory… then set the number of UDM-s. Saeed Sadeghi ([email protected]) Pardad Petro Danesh (www.petrodanesh.ir)




35

Fluent enables the energy equation by enabling Species Transport equation. In this example, we don’t need the energy equation. To prevent solving this equation go to Solve

Solution

Controls and click on Energy in Equations box.



To have a good initial guess for better and time saving convergence, we can specify the conditions of inlet to the all domain.



So, Solve

Initialize

Solution Initialize, Compute from inlet and click on Init to specify its

condition to all domains.



You can change Residual criteria and Residual window settings. Solve  Monitors  Residual.



To prevent printing on command window, uncheck print. And to show residual on the residual window, check the plot box.





On iterate window, Solve

Iterate, specify the number iterations and start iteration by

clicking on iterate.

Figure 20-1: Scaled Residuals

Figure 20-2: Mass fraction of H 2O on top and bottom walls


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20.8 Checking Oxygen Mass Balance To check Oxygen mass balance, we should calculate the rate of oxygen mass enters the domain at the inlet (kg/s), the rate of oxygen mass consumes at the catalyst layer (kg/s), and the rate of oxygen mass exits the domain from outlet. So, we should add a macro to the end of previous UDF as follows: DEFINE_ON_DEMAND(data) { real A[ND_ND]; real sum; int inlet_ID = 7; int outlet_ID = 6; int cl_ID = 2; Domain *d = Get_Domain(1); Thread *t_inlet = Lookup_Thread (d,inlet_ID); Thread *t_outlet = Lookup_Thread (d,outlet_ID); Thread *t_cl = Lookup_Thread (d,cl_ID); face_t f; cell_t c; /////////////////////////// sum = 0.; begin_f_loop(f,t_inlet) { F_AREA(A,f,t_inlet); sum += F_FLUX(f,t_inlet) *F_YI(f,t_inlet,i_o2)*1.e6; } end_f_loop(f,t_inlet); Message("inlet = %f *e-6(kg/s)\n", sum); /////////////////////////// sum =0.; begin_f_loop(f,t_outlet) { F_AREA(A,f,t_outlet); sum += F_FLUX(f,t_outlet) *F_YI(f,t_outlet,i_o2)*1.e6; } end_f_loop(f,t_outlet); Message("outlet= %f *e-6(kg/s)\n", sum); /////////////////////////// sum =0.; begin_c_loop(c,t_cl) { sum += -mw_o2*R_cat(c,t_cl)/(4.*F) *por_cl*C_VOLUME(c,t_cl)*1.e6; } end_c_loop(f,t_cl); Message("porous= %f *e-6(kg/s)\n", sum); }



•

38

The macro “ DEFINE_ON_DEMAN” executes when we want. For example at the beginning, intermediate and at the end of solution, it can be executed.

•

The array A is defined in ND_ND dimension to reserve area.

•

ND_ND is a Fluent macro that is equal to 2 in 2-dimensioanl and 3 in 3-dimensional case.

•

sum is defined to summarize value of cells in a group cells ( Threads)

•

inlet_ID, outlet_ID and cl_ID are the number of zone that can be found in Boundary Conditions dialog box.

•

Get_Domain(1) specifies the domain to the 1 to d which is defined as Domain pointer

•

Lookup_Thread (d,ID) is defined to find the Thread which its ID number is equal to inlet_D, outlet_ID and cl_ID.

•

With face_t and cell_t we can define variables that reserves face and cell data.

•

begin_f_loop(f,t_inlet), loops throw all faces of the Thread t_inlet that its ID is 7.

•

F_AREA(A,f,t_inlet), returns area of the face f of thread t_inlet.

•

F_FLUX(f,t_inlet), returns the mass flow rate the face f of the thread t_inlet.

•

F_YI(f,t_inlet,i_o2), returns mass fraction of the specie that its ID is i_o2 of the face f of the thread t_inlet.

•

Message(“…\n”,…), is used to write mass flow rate of Oxygen on the fluent command window. Result is written below: inlet = -5.721122 *e-6(kg/s) outlet= 5.153047 *e-6(kg/s) porous= -0.589597 *e-6(kg/s)


UDF Training- Petrodanesh

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