Tutorial to set up a case for

chtMultiRegionFoam in OpenFOAM

2.0.0

Arpit Singhal

University of Luxembourg

March 3, 2014

The OpenFOAM-solver chtMultiRegionFoam is meant to be used for heat-

transfer between a solid and a fluid originally. As it does work with different

regions of different properties, the setup is therefore different from the other

OpenFOAM cases.

This tutorial is written for setting up a basic case for chtMultiRegionFoam

(cMRF) in an Openfoam.

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Contents 1

Contents

1 Introduction 2

2 Basic Workflow 3

3 chtMRF Case Setup 4

3.1 Geometry and Mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.2 Creating the Regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2.1 Declaring the Regions . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2.2 Defining the Region by Zones . . . . . . . . . . . . . . . . . . . . 6

3.3 Splitting the Mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.4 Necessary Files and Folders . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.4.1 Files setup by the user . . . . . . . . . . . . . . . . . . . . . . . . 9

3.4.2 Files setup by OpenFOAM utilities . . . . . . . . . . . . . . . . . 15

4 Running the case 18

5 Using scripting 18

6 Appendice 21

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1 Introduction 2

1 Introduction

What is meant by ”multiregion multi physics modeling”? It is inherently-coupled physics

on disparate continua (e.g. fluid, solid, different solids). In multiregion multi physics

separate governing equations for each continuum/region are solved, as shown in 1.1

seperate governing equations will be solved for Region 1 and Region 2 depending upon

their phase and Γ represents a region interface. A region can be defined as coherent

continum of the same phase.

Figure 1.1: Example

Generally two different approaches to solving such problems are distinguished:

• Monolithic: use same primitive variables, cast governing equations in terms of

these variables, solve a single coupled matrix equation system

• Partitioned: separate governing equations, solve separate matrix equation systems,

couple at the boundary interface, sub-iterate until coupled convergence is reached

Here, we focus on partitioned approaches using OpenFOAM’s multiregion function-

ality (conjugate heat/mass transfer). For a multiregion partitioned solver the working

steps are as follows:

1. Define multiple meshes, one for each ”region”

2. Create field variables on each mesh

3. Solve separate governing equations on each mesh

4. Multiregion coupling at the boundary interface between regions

5. Subiterate until fully-coupled solution is reached

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2 Basic Workflow 3

2 Basic Workflow

The basic work flow for a case setup is explained in fig. 2.1.

Create a Mesh of the full domain

Define regions inside the domainby selecting cells (using cellSet)

Create cell zones from the created cellSets

Split mesh into regionsaccording to defined zones

Basic work flow of a chtMultiRegion case

Run case

Prepare a general OpenFoam Case

Create patches and fields for all regions

changeDict

Define coupled patches

Define boundary conditions

Define region properties

Figure 2.1: The case visualization in 2D

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3 chtMRF Case Setup 4

3 chtMRF Case Setup

In this case we have four different regions (Air1, Air2, Solid1 and Solid2). The

case is a simple set up with warm solid parts (Solid1 and Solid2), which are subjected

to fluid parts (Air1 and Air2). In this case we are not taking into account any airflow

(i.e. there is no inlet or outlet airflow) but the buoyancy effects are considered, while

solving the case. There will be heat transfer taking place between the warm solid parts

and the fluid parts.

3.1 Geometry and Mesh

A graphical representation of the geometry can be seen in Fig. 3.1.

The geometry is defined and then meshed using the OpenFOAM blockMesh tool. After

Air2_to_Air1

Air1_to_Solid1

Air2_to_Solid2Solid1_to_Solid2

minX

maxY

maxX

minY

x

yz

Air1_to_Solid2

Figure 3.1: The case visualization in 2D

running the blockMesh utility by typing

$ blockMesh

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3.2 Creating the Regions 5

the mesh is created as depicted in fig.3.2.

Figure 3.2: Mesh of the full domain

3.2 Creating the Regions

The regions are created in the domain depending upon their phases and they are created

on the basis of the zones defined.

3.2.1 Declaring the Regions

The regions and their property type are given in table 3.1.

Table 3.1: Table for region properties

Region Type

Air1 fluidSolid1 solidAir2 fluid

Solid2 solid

Every region has several patches for which boundary or coupling conditions have to

be specified. Thus, a patch can be of the following two types:

• boundary patch

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3.2 Creating the Regions 6

• coupling patch

A boundary patch is a regular type patch for which the user may define any possible

boundary condition available in OpenFOAM. Coupling patches are those patches where

the solutions of the different regions are coupled. A coupling patch belongs to a so-

called coupled patch pair. Such a pair consists of coinciding patches, one associated with

each region. Table 3.2 lists all patches, the region they belong to and their type for the

presented case.

Table 3.2: Table of patches

Patch Regions Type

minX Air1, Solid1 boundary patchmaxX Air2, Solid2 boundary patchminY Solid1, Solid2 boundary patchmaxY Air1, Air2 boundary patchminZ Air1, Air2, Solid1, Solid2 boundary patchmaxZ Air1, Air2, Solid1, Solid2 boundary patch

Solid1 to Solid2 Solid1, Solid2 coupling patchAir1 to Solid1 Solid1, Air1 coupling patchAir1 to Solid2 Air1, Solid2 coupling patchAir1 to Air2 Air1, Air2 coupling patch

Air2 to Solid2 Air2, Solid2 coupling patch

3.2.2 Defining the Region by Zones

In the presented case the following regions must be created: Air1, Air2, Solid1 and

Solid2. In order to create these regions the domain is divided into zones. To do so a

subset of cells within the domain is selected to form a so-called cellSet. A cellSet is a

random selection of cells from the domain, whereas a zone is a coherent subset of cells

which finally can be used to define a region. According to the cellSets four zones are

defined which mark the different regions.

In order to define cellSets and cellZones a OpenFOAM commandline utility called setSet

(topoSet in newer versions of OpenFOAM) is used. This tool requires a dictionary-file as

input. Thus, within the case folder a file ending with .setSet must exist, which contains

the settings used to define the different cellSets/cellZones/regions in the domain. The

following command starts the utility with the dictionary file that contains the commands

to be executed.

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3.3 Splitting the Mesh 7

$ s e t S e t −batch makeCel lSets . s e t S e t

An example of the dictionary file of the presented case is attached for simplicity (Listing

1).

Listing 1: Extract from makeCellSets.setSet

1 c e l l S e t So l i d1 new boxToCell (0 0 0 ) (10 0 .3 1)

c e l lZoneSe t So l i d1 new setToCel lZone So l id1

c e l l S e t So l id2 new boxToCell (10 0 0 ) (20 0 .5 1)

5 c e l lZoneSe t So l id2 new setToCel lZone So l i d2

c e l l S e t Air1 new boxToCell (0 0 .3 0 ) (10 1 1)

c e l lZoneSe t Air1 new setToCel lZone Air1

10 c e l l S e t Air2 new boxToCell (10 0 .5 0 ) (20 1 1)

c e l lZoneSe t Air2 new setToCel lZone Air2

The first two lines used in the .setSet file are briefly explained as:

c e l l S e t So l i d1 new boxToCell (0 0 0) (10 0 .3 1)

This creates a new cellSet from a selection of cells. The new action shows it will

be a new set. The name of the cellSet is Solid1 and the source for the cellSet-function

is the boxToCell function. The numbers in brackets are parameters to the boxToCell-

function: All cells contained within the rectangular box spanning between the points

with coordinates (0 0 0) and (10 0.3 1) are selected for the cellSet Solid1. The line

c e l lZoneSe t So l i d1 new setToCel lZone So l id1

builds a cellZoneSet from an existing cellSet (here using Solid1). Thus, within the

original domain a new zone has been created as depicted in fig.3.3.

This can be repeated in order to define the desired zones representing regions within

the domain.

3.3 Splitting the Mesh

After the user has defined all necessary regions by creating zones for them as described in

the previous section the mesh of the domain has to be split into several disjoint meshes.

Note that the originally created mesh of the full domain will be used within the regions.

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3.4 Necessary Files and Folders 8

Figure 3.3: Zone created by cellZone from the entire domain

Thus, proper grid resolution for the regional meshes must already be accounted for when

creating the mesh of the full domain.

$ sp l i tMeshRegions −c e l l Z o n e s −ove rwr i t e

The splitted mesh can be checked/visualized in paraview using the command as shown

in the listing3.3 for Air1

$ paraFoam −touch −r eg i on Air1

Then in paraview, *.OpenFoam file should be loaded and can be visualized. The splitted

mesh is shown in fig. 3.4.

3.4 Necessary Files and Folders

A typical OpenFOAM case directory consists of the following three folders:

• 0

• constant

• system

This general case structure is also kept for multiregion cases. The final correct setup of

a multiregion case is shown in fig.3.5. Note that for each region a subdirectory containing

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3.4 Necessary Files and Folders 9

Figure 3.4: Splitted mesh

the information for the particular region exists. Some of the files are manually created

by the user while others are created by the OpenFOAM utilities. It is explained in the

further sections in detail.

In the following sections details about the individual directories will be given.

3.4.1 Files setup by the user

When starting a new multiregion case the directories and their content highlighted in

fig. 3.6 must be created manually by the user according to the problem definition.

0 directory: First, manually bring in the necessary field files as usual. For a chtMulti-

RegionFoam case it is necessary to have files for:epsilon, k, p, p rgh, T, U, Ychar and Ypmma.

These files are identical to what one would find in any other chtMultiRegionFoam case

/0-directory.

For the solid regions:T, Ychar and Ypmma are necessary, whereas for the fluid regions:epsilon, g, k, p, p rgh, T and U

are the required files.

constant directory: As is any standard OpenFOAM case, the constant folder must

contain a standard polymesh directory, including a standard blockMeshDict-file, which

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3.4 Necessary Files and Folders 10

Figure 3.5: Final case structure of a multiregion case before running the solver

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3.4 Necessary Files and Folders 11

Files for all desired fieldshave to be created inside0 directory by the user.

Folders for each region mustbe created by the user. Theyshould contain default files for any fluid and solid.

Defines the geometry and mesh the full domain.

Defines desired regions and their property.

Folders for each region mustbe created by the user. Theyshould contain default filesfor any fluid and solid. Theuser must add and edit thechangeDictionaryDict

Figure 3.6: Case structure as prepared by the user before running scripts

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3.4 Necessary Files and Folders 12

created by splitMesh:For each region a folderis created inside the0 folder and all originally field files from the 0folder are copied into theregion folders

created by splitMesh:The mesh is splitted intothe different regions andfor every region a separate mesh is defined.

created by changeDict:This file defines all thepatches of a particularregion.

modified by changeDict:boundary, initial andcoupling conditions for allfields are modified accordingto the changeDictionaryDict file of the region

Figure 3.7: Files and directories created by OpenFOAM utilities

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3.4 Necessary Files and Folders 13

defines the full domain and its mesh.

In contrast to a standard case, the files defining the other properties have to go into the

different regional folders, i.e. transportProperties and thermophysicalProperties

within the folders for fluid regions and solidThermophysicalProperties within the

folders for the solid regions as shown in listing3 and listing4 .

Within the constant-folder it is necessary to produce all the region folders. Addition-

ally within the constant folder it also is necessary to build (or copy from elsewhere) a

file called regionProperties. This file assigns the physical phase to each region: Either

fluid or solid. An example for this file can be seen in the listing 2.

Listing 2: Extract from regionProperties

// ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ //

fluidRegionNames ( a i r 1 a i r 2 ) ;

sol idRegionNames ( s o l i d 1 s o l i d 2 ) ;

// ∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗ //

Inside the solver fluidRegions and solidRegions are treated differently by solving

different governing equations for each phase.

For the fluid regions the default constant files should be included from any chtMulti-

RegionFoam case/constant file. For the fluid regions, the fluid region should contain a

thermophysicalProperties file containing the properties of the fluid in the fluid region.

Like the Air1 thermophysicalProperties file contains:

Listing 3: Extract from Air1 thermophysicalProperties file

mixture

{s p e c i e

{nMoles 1 ;

molWeight 2 8 . 9 ;

}thermodynamics

{

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3.4 Necessary Files and Folders 14

Cp 1000 ;

Hf 0 ;

}t r anspo r t

{mu 1.8 e−05;

Pr 0 . 7 ;

}}

Similarly for solid regions a solidThermophysicalProperties must exist with the

folder of the region, an example of such a solidThermophysicalProperties is given

for Solid1:

Listing 4: Extract from Solid1 solidThermophysicalProperties file

constSol idThermoCoef f s

{//− thermo p r o p e r t i e s

rho rho [ 1 −3 0 0 0 0 0 ] 8000 ;

Cp Cp [ 0 2 −2 −1 0 0 0 ] 450 ;

K K [ 1 1 −3 −1 0 0 0 ] 80 ;

//− r a d i a t i o n p r o p e r t i e s

kappa kappa [ 0 −1 0 0 0 0 0 ] 0 ;

sigmaS sigmaS [ 0 −1 0 0 0 0 0 ] 0 ;

e m i s s i v i t y e m i s s i v i t y [ 0 0 0 0 0 0 0 ] 0 ;

//− chemica l p r o p e r t i e s

Hf Hf [ 0 2 −2 0 0 0 0 ] 0 ;

}.

.

.

. . . . . .

. .

}

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3.4 Necessary Files and Folders 15

system directory: In the system directory, once again set up the folders for the regions.

In each region folder there should be a changeDictionaryDict file, which contains

details about the necessary fields in the region like T, U, etc.

Get a working controlDict file, for example from any tutorial into this folder. Af-

terwards get a dummy fvSchemes file. This one is the same as any other fvSchemes,

except for the different functions containing no values between the curly brackets. Fur-

theron one has to get a fvSolution which only defines the outer correctors into this

folder. It is optional to get a decomposeParDict file in case one opts for running parallel

computations. For all of the different regional folders: Get a decomposeParDict file

and get full fvSchemes and fvSolution files into the folders. For the latter ones, keep

in mind that they will be different for the fluids and for the solids.

3.4.2 Files setup by OpenFOAM utilities

Most of the necessary case files and folder (This is highlighted in 3.7 for the different

regions are created by automated generation using scripts and OpenFOAM utilities.

The most important OpenFOAM utilities for a multiregion case are:

splitMesh creates the polyMesh directories and their content within the constant/regionXYZ/

folders;

additionally it creates 0/regionXYZ/ directories for all regions and copies all the

field files existing in the 0 directory into the 0/regionXYZ/ directories

changeDictionary uses changeDictionaryDict files located in system/regionXYZ/ fold-

ers to create initial, boundary and coupling conditions for all fields existing in

0/regionXYZ/ directory for all regions

0 directory: During execution of splitMesh the user created field files are copied to

the region subdirectories. As a next step unnecessary fields are removed for some regions

(see extract given in Listing 5) and only those being part of the governing equations are

kept (for example solids are assumed to be stationary, thus no velocity field is required).

Listing 5: Extract from Allrun script

# remove f l u i d f i e l d s from s o l i d r e g i o n s

f o r i in s o l i d 1 s o l i d 2

do

rm −f 0∗/ $ i /{mut , alphat , ep s i l on , k , p ,U, p rgh}

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3.4 Necessary Files and Folders 16

done

# remove s o l i d f i e l d s from f l u i d r e g i o n s

f o r i in a i r 1 a i r 2

do

rm −f 0∗/ $ i /{Ychar ,Ypmma}done

Now the initial, boundary and coupling conditions for all fields in every region have to

be specified appropriately. In order to do so the commandline utility changeDictionary

is used. For example for the region Air1 the following line must be executed:

$ changeDict ionary −r eg i on Air1 > l og . changeDict ionary . Air1 2>&1

The utility expects a file called changeDictionaryDict to exist within the folder system/Air/.

Initial and coupling condition of the regions are defined with in changeDictionaryDict.

Boundary conditions for the boundaries on the outside of the complete simulation do-

main and boundary conditions or so-called coupling conditions for any of the coupling

patches between the regions are built using the following scheme: For example for region

Air1 in the changeDictionaryDict file contains the following code for the T field:

T

{i n t e r n a l F i e l d uniform 300 ;

boundaryField

{” .∗”

{type zeroGradient ;

}

” a i r 1 t o .∗”

{type compres s ib l e : : turbulentTemperatureCoupledBaff leMixed ;

neighbourFieldName T;

. . .

}

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3.4 Necessary Files and Folders 17

}}

In this example the boundary patches are all treated the same (using a wildcard ".*")

and are given the boundary conditions of type zeroGradient. For the coupling patch

there is special kind of boundary condition required (here

compressible::turbulentTemperatureCoupledBaffleMixed).

The keyword neighbourFieldName indicates that the T field of air1 is coupled to the

T field of the other regions.

Similarly for all other regions, the initial, boundary and coupling conditions must be

taken care of.

In each of the region folders a file called cellToRegion is created during execution of

the changeDictionaryDict command. The content of these files are like any other

field file but not associated to a specific field. The BC type is either zeroGradient or

calculated. Possibly other parameters would work as well, but only these have been

tested. In any case every coupling patches to other regions have to be defined of type

calculated whereas boundary patches (to outside of domain) are of type zeroGradient.

for Air1 the entry used is shown by listing 6

Listing 6: Extract from cellToRegion

boundaryField

{maxY

{type zeroGradient ;

}minX

{type zeroGradient ;

}minZ

{type zeroGradient ;

}maxZ

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4 Running the case 18

{type zeroGradient ;

}a i r 1 t o a i r 2

{type c a l c u l a t e d ;

va lue uniform 0 ;

}a i r 1 t o s o l i d 2

{type c a l c u l a t e d ;

va lue uniform 0 ;

}a i r 1 t o s o l i d 1

{type c a l c u l a t e d ;

va lue uniform 0 ;

}}

constant directory: In the constant folder, the splitMesh utility creates a seperate

mesh for each region. This can be seen in the fig. 3.7

4 Running the case

After following each step as defined in the above tutorial. The case can be executed by

using the command in the terminal as depicted in the listing. 4

$ chtMultiRegionFoam

5 Using scripting

An Allrun can be explained as a script file which contains all the commands used to

execute the case, for example in this case the Allrun used can be seen as in Listing

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5 Using scripting 19

shown in 5.1. So just executing the Allrun will now run the case, instead of typing each

command seperately in the terminal.

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5 Using scripting 20

Figure 5.1: Allrun script

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6 Appendice 21

6 Appendice

Listing 7: Extract from Allrun

#!/ bin / sh

cd ${0%/∗} | | e x i t 1 # run from t h i s d i r e c t o r y

RUNPAR=”YES” ;

# Source t u t o r i a l run f u n c t i o n s

. $WM PROJECT DIR/ bin / t o o l s /RunFunctions

# −−− (A) −− Def in ing geometry and mesh o f f u l l domain

runAppl i cat ion blockMesh

# −−− (B) −− Creat ing the r e g i o n s in two s t ep s :

# 1) d e f i n i n g c e l l S e t s

# 2) c r e a t i n g c e l l Z o n e s which d e f i n e the r e g i o n s

runAppl i cat ion s e t S e t −batch makeCel lSets . s e t S e t

# −−− (C) −− S p l i t t i n g the mesh accord ing to the de f ined r e g i o n s

runAppl i cat ion spl i tMeshRegions −c e l l Z o n e s −ove rwr i t e

e x i t

# −−− (D) −− Remove unnecessary f i e l d f i l e s from r e g i o n s

#

# −−−− (D. 1 ) − Remove f l u i d f i e l d s from s o l i d r e g i o n s

f o r i in s o l i d 1 s o l i d 2

do

rm −f 0∗/ $ i /{mut , alphat , ep s i l on , k , p ,U, p rgh}done

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6 Appendice 22

# −−−− (D. 2 ) − Remove s o l i d f i e l d s from f l u i d r e g i o n s

f o r i in a i r 1 a i r 2

do

rm −f 0∗/ $ i /{Ychar ,Ypmma}done

# −−− (E) −− Def ine i n i t i a l , boundary and coup l ing c o n d i t i o n s

# f o r a l l f i e l d s

# o f a l l r e g i o n s

#

# loop over r e g i o n s

f o r i in a i r 1 a i r 2 s o l i d 1 s o l i d 2

do

changeDict ionary −r eg i on $ i > l og . changeDict ionary . $ i 2>&1

done

# −−− (F) −− RUN case

# choose between PARALLEL or SEQUENTIAL RUN

i f [ ”$RUNPAR” = ”YES” ] ;

then

echo ’ −−− Running in p a r a l l e l mode ’ ;

# Decompose

f o r i in a i r 1 a i r 2 s o l i d 1 s o l i d 2

do

decomposePar −r eg i on $ i > l og . decomposePar . $ i 2>&1

done

# Run

mpirun . openmpi −np 4 chtMultiRegionFoam −p a r a l l e l > l og

# Reconstruct

f o r i in a i r 1 a i r 2 s o l i d 1 s o l i d 2

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6 Appendice 23

do

recons t ruc tPar −r eg i on $ i > l og . r e cons t ruc tPar . $ i 2 >&1

done

e l s e

echo ’ −−− Running in s e q u e n t i a l mode ’ ;

#−− Run on s i n g l e p ro c e s s o r

chtMultiRegionFoam > l og

f i

echo

echo ” c r e a t i n g f i l e s f o r paraview post−p r o c e s s i n g ”

echo

f o r i in s o l i d 1 s o l i d 2 a i r 1 a i r 2

do

paraFoam −touch −r eg i on $ i

done

# −−−−−−−−−−−−−−−−−− end−of− f i l e

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