Ssadeghi.ir Advanced User Defined Function

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ANSYS FLUENT GUIDEAdvanced 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.

2012

Saeed Sadeghi ([email protected])

http://www.ssadeghi.ir

4/23/2012


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20BIntroduction

Table of Contents1 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.................................................................................................................. 18

16 Interpreted vs. Compiled UDFs ................................................................................................ 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........................................................................................... 23

20 Example: Modeling Fuel Cell ..................................................................................................... 24


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30BIntroduction

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

20.8 Checking Oxygen Mass Balance ............................................................................................ 37

21 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


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40BIntroduction

1 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

e.g., trigonometric, exponential, control blocks, do-loops, file i/o, etc.

Pre-Defined Macros

Allows access to field variable, material property, and cell geometry data.

Why build UDFs?

o 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

2 User Access to the FLUENT Solver


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52BC Programming

3 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 fromdx[0] */

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

pointeris a special kind of variable that contains the memory address, not content, of

another variable

Pointersare 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);

Pointersare also used to point to the beginning of an array

o

Thus pointers are used to address arrays in C

Arrayvariables 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)

, >=,


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62BC 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


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84BUsing UDFs in the Solvers

6 Using UDFs in the Solvers

The basic steps for using UDFs in FLUENTare 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 Iteratepanel.

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


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106BFluent UDF Data Structure Summary

8 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 dis a pointer to domain thread

Thread *t tis a pointer to thread

Cell_t *c cis cell thread index

Face_t f fis a face thread index

Node *node nodeis 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)


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117BGeometry 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* 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


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128BCell Field Variable Macros

10Cell 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


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139BFace Field Variable Macros

11Face 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.


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1410BLooping and Thread Macros

12Looping 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

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)

{

. Loop that steps through all cells in a thread

}

end_c_loop

befing_f_loop(c, t)

{

. Loop that steps through all faces in a thread

}

C_face_loop(f, t, n)

{

. Loop that visits all faces of cell cin 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.

Specialized variable types

used for referencing


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1511BMacros

13Macros

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.

#include udf.h

o The header files must be accessible in your path.

Typically stored in Fluent.Inc/src/directory.

o 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 Users 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); defines scattering phase function for DOM

DEFINE_DELTAT(name, domain); defines variable time step size for unsteady problems


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1611BMacros

DEFINE_TURBULENT_VISCOSIT(name, cell, thread); 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

Executes once per initialization

o

Adjust

Executes every iteration

o 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

Read order and Write order must be same.

o

Execute-on-Demand capability Not accessible during solve


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17/12BUser Defined Memories

14User 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.


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1813BUser Defined Scalars

15User Defined Scalars

FLUNET can solve (up to 50) generic transport equations

for User Defined Scalars:

1,...,k

k k

i k k scalars

i i

F S k N t x x

+ = =

User Specifies:

DefineModelsUser-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 flux 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.


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2016BExample: Parabolic Inlet Velocity Profile

18Example: 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

2

( ) 20 10.0745

yu y

=

18.1Step 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

DefineUser-DefinedFunctions

Interpreted


Compiled

Add the UDF source code to the Source

File Name list.

Click Interpret.

The assembly language code will

display in the FLUENT console.

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.


Manage

0y =


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2116BExample: 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.

18.5Numerical 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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2217BExample: Checking Effect of Porous Zone on Momentum Equation

19Example: 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

FileReadCase

Read a mesh file

DefineBoundary 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 Resistanceequal to 5.68e6(1/m2) in both directions.

Change the Fluid Porosityfrom 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

normal to boundary.

To solve the case, go to: SolveIterate Number of Iterations = 1000

After convergence we can display results as follows.

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


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2317BExample: 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 // Inverse Permeability (1/m^2)

#define urf 0.1 // 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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2418BExample: Modeling Fuel Cell

20Example: Modeling Fuel Cell

20.1Preparation

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

FileReadMesh

2.

Check the mesh

GeneralCheck

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

GeneralScale

(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. GeneralCheck

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.

20.3General Settings

DefineModelsSolver

1. Retain the default settings for the solver.


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20.4

Models

DefineModelsSpeciesTransport & Reaction

(a) Enable Species Transport

(b)

Click OK to Close the Species Model box.

(c)

FLUENT will notify thatAvailable material properties or method have changed. Please

confirm the property values before continuing. Click OKto close the dialog box.

20.5Materials

DefineMaterials

1.

Revise the properties for the mixture materials.

DefineMaterialMixture


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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.

20.6Boundary Conditions

DefineOperating Conditions

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

Close the dialog box.


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

(g)

Check the porous zonebutton to enable the clporous zone.

(h)

Set viscous resistance variables for both directions.

(i) Now you should set porosity for clporous 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.


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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 click OK.

20.7Writing 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 3.2348e-5 /* mass diffusivity of oxygen */

#define diff_h2o 3.89e-5 /* mass diffusivity of water */

#define diff_n2 2.88e-5 /* mass diffusivity of nitrogen */

#define i_h2o 0

#define i_o2 1

#define i_n2 2

#define mw_h2o 18. // kg/kmol

#define mw_o2 32. // kg/kmol


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#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;


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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

DefineUser-DefinedFunctionsCompiled

Add the UDF source files and headers if needed.


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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 Donehave 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, .


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If you have build and load a udf before, be sure to unload it in the Managedialog 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 TermsTab, 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: DefineUser-DefinedMemory then set the number of UDM-s.


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Fluent enables the energy equation by enabling Species Transport equation. In this example,

we dont need the energy equation. To prevent solving this equation go to Solve Solution

Controlsand 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 inletand click on Initto specify its

condition to all domains.

You can change Residual criteria and Residual window settings. SolveMonitors

Residual.

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

residual window, check the plot box.


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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 H2O 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);

}


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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 Ais defined in ND_ND dimension to reserve area.

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

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

inlet_ID, outlet_IDand cl_IDare 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)


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3919BFluent Frequently Asked Questions

21Fluent Frequently Asked Questions

21.1My UDF won't interpret or compile - what is wrong?

The answer depends upon what exactly has happened.

If Fluent complains about not being able to find a compiler, then check to make sure that the

compiler is properly installed. On Windows machines, it is possible to install the Visual C++

compiler without fully setting up the command line compiler (which Fluent needs to be able

to find). During the installation process, you need to select the "register environment

variables" option. Failure to do so will likely lead to complaints about things being "not

recognized as an internal or external command, operable program or batch file" or missing

DLL's. It is theoretically possible to fix this issue by setting the appropriate environment

variables, but keep in mind that even when nmakecan be found there still may be DLL

issues. The easy path is probably reinstallation of Visual Studio (taking special care to make

sure that the command line interface is set up properly), but the reinstallation path is always

perilous. If you have long term experience using Windows you should probably know the

risks, and if you don't you should consult an expert.

If you are interpreting, keep in mind that not everything that is supported for compiled UDF's

is supported for interpreted UDF's. This is true both for the UDF interface and the C

language. If you are doing something pretty involved and it fails inexplicably, try compiling to

see if that makes a difference.

There is also the possibility of coding errors. Keep in mind that your source code gets run

through the C preprocessor (to change the Fluent macros into C code), so unintended

interactions are very possible.

21.2How to Set Environment Variables

21.2.1 On Windows 32 Bit

If you are using Windows 32 bit, its better to install Visual Studio 2005(Just install Visual

C++, deselect other components)

Then, Go to this address: C:\Program Files\Microsoft Visual Studio\VC98\bin\

Double click VCVARS32.BAT

Then go the FLUENTinstall address: C:\fluent.inc\ntbin\ntx86\

And double click setenv.exe

21.2.2 On Windows 64 Bit

If you are using Window 64 Bit, its better to install Visual Studio 2008 (Just install Visual

C++, deselect other components)

Then, Go to this address: C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\bin\

Double click vcvars32.BAT

Go to the FLUENTinstall add: C:\Program Files\ANSYS Inc\v121\fluent\ntbin\win64\

And double click setenv.exe

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Ssadeghi.ir Advanced User Defined Function