udf training- petrodanesh

Pardad Petro Danesh (www.petrodanesh.ir) | Confidential

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.

2011

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

1/12/2011


2 Introduction

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.................................................................................................................. 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........................................................................................... 23

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


3 Introduction

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


4 Introduction

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 UDF’s? 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


5 C 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 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:


6 C Programming

agg += single; /* it means agg = agg + single; */

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

• Basic control structures

if ( … ) <statement>;

if ( … ) <statement>;

else <statement>;

if ( … ) <statement>; else if ( … ) <statement>;

For Loops:

for ( k=0; k<NUM; k++ )

<statement>;

While loops:

While ( … )

<statement>;

Conditional operator ( ? : )

( condition ? operand a : operand b )

example:

real At = (rp_axi ? At*2*M_PI : At );

true

false


7 Cfd Programming in FLUENT

4 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 face 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 conditions, 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

.


8 Using UDFs in the Solvers

6 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’


9 Data structure overview

o 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 information 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


10 Fluent 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 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)


11 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


12 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


13 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.


14 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

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 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.

Specialized variable types used for referencing


15 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 #include “udf.h”

be included in your source code.

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 User’s Guide.

13.1 DEFINE Macros

• Any UDF you write musto 18 general purpose macros and 13 DPM-related macros (not listed);

begin with a DEFINE_ macro:

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


16 Macros

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


17 /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.


18 User Defined Scalars

15 User Defined Scalars

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

1,...,k

k ki k k scalars

i i

F S k Nt x x φφ φφ

∂ ∂∂+ −Γ = = ∂ ∂ ∂

• 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 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.


19 Interpreted vs. Compiled UDF’s

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 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

o Compiler (refer to User’s Guide for instructions) Code is translated “once” into machine language (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.


20 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

2

( ) 20 10.0745

yu y = −

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…

Define User-Defined Functions 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.

Define User-Defined Functions Manage…

0y =



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.


22 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/m2) 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

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).



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).


24 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.



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



(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.



(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.



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.



(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.

20.7 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 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



#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;



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.



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, …”.



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.



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 H2O on top and bottom walls



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); }



• 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)


39 Fluent Frequently Asked Questions

21 Fluent Frequently Asked Questions

21.1 My 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 nmake can 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.

21.2 How to Set Environment Variables

21.2.1 On Windows 32 Bit • If you are using Windows 32 bit, it’s 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 FLUENT install 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, it’s 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 FLUENT install add: “C:\Program Files\ANSYS Inc\v121\fluent\ntbin\win64\”

• And double click “setenv.exe”

udf training- petrodanesh

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