Mesh conversion

5.5 Mesh conversion

The user can generate meshes using other packages and convert them into the format that OpenFOAM uses. There are numerous mesh conversion utilities listed in Table 3.6. Some of the more popular mesh converters are listed below and their use is presented in this section.

fluentMeshToFoam
reads a Fluent.msh mesh file, working for both 2-D and 3-D cases;
starToFoam
reads STAR-CD/PROSTAR mesh files.
gambitToFoam
reads a GAMBIT.neu neutral file;
ideasToFoam
reads an I-DEAS mesh written in ANSYS.ans format;
cfx4ToFoam
reads a CFX mesh written in .geo format;

5.5.1 fluentMeshToFoam

Fluent writes mesh data to a single file with a .msh extension. The file must be written in ASCII format, which is not the default option in Fluent. It is possible to convert single-stream Fluent meshes, including the 2 dimensional geometries. In OpenFOAM, 2 dimensional geometries are currently treated by defining a mesh in 3 dimensions, where the front and back plane are defined as the empty boundary patch type. When reading a 2 dimensional Fluent mesh, the converter automatically extrudes the mesh in the third direction and adds the empty patch, naming it frontAndBackPlanes.

The following features should also be observed.

  • The OpenFOAM converter will attempt to capture the Fluent boundary condition definition as much as possible; however, since there is no clear, direct correspondence between the OpenFOAM and Fluent boundary conditions, the user should check the boundary conditions before running a case.
  • Creation of axi-symmetric meshes from a 2 dimensional mesh is currently not supported but can be implemented on request.
  • Multiple material meshes are not permitted. If multiple fluid materials exist, they will be converted into a single OpenFOAM mesh; if a solid region is detected, the converter will attempt to filter it out.
  • Fluent allows the user to define a patch which is internal to the mesh, i.e. consists of the faces with cells on both sides. Such patches are not allowed in OpenFOAM and the converter will attempt to filter them out.
  • There is currently no support for embedded interfaces and refinement trees.

The procedure of converting a Fluent.msh file is first to create a new OpenFOAM case by creating the necessary directories/files: the case directory containing a controlDict file in a system subdirectory. Then at a command prompt the user should execute:


    fluentMeshToFoam <meshFile>
where <meshFile> is the name of the .msh file, including the full or relative path.

5.5.2 starToFoam

This section describes how to convert a mesh generated on the STAR-CD code into a form that can be read by OpenFOAM mesh classes. The mesh can be generated by any of the packages supplied with STAR-CD, i.e.PROSTAR, SAMM, ProAM and their derivatives. The converter accepts any single-stream mesh including integral and arbitrary couple matching and all cell types are supported. The features that the converter does not support are:

  • multi-stream mesh specification;
  • baffles, i.e. zero-thickness walls inserted into the domain;
  • partial boundaries, where an uncovered part of a couple match is considered to be a boundary face;
  • sliding interfaces.

For multi-stream meshes, mesh conversion can be achieved by writing each individual stream as a separate mesh and reassemble them in OpenFOAM.

OpenFOAM adopts a policy of only accepting input meshes that conform to the fairly stringent validity criteria specified in section 5.1. It will simply not run using invalid meshes and cannot convert a mesh that is itself invalid. The following sections describe steps that must be taken when generating a mesh using a mesh generating package supplied with STAR-CD to ensure that it can be converted to OpenFOAM format. To avoid repetition in the remainder of the section, the mesh generation tools supplied with STAR-CD will be referred to by the collective name STAR-CD.

5.5.2.1 General advice on conversion

We strongly recommend that the user run the STAR-CD mesh checking tools before attempting a starToFoam conversion and, after conversion, the checkMesh utility should be run on the newly converted mesh. Alternatively, starToFoam may itself issue warnings containing PROSTAR commands that will enable the user to take a closer look at cells with problems. Problematic cells and matches should be checked and fixed before attempting to use the mesh with OpenFOAM. Remember that an invalid mesh will not run with OpenFOAM, but it may run in another environment that does not impose the validity criteria.

Some problems of tolerance matching can be overcome by the use of a matching tolerance in the converter. However, there is a limit to its effectiveness and an apparent need to increase the matching tolerance from its default level indicates that the original mesh suffers from inaccuracies.

5.5.2.2 Eliminating extraneous data

When mesh generation in is completed, remove any extraneous vertices and compress the cells boundary and vertex numbering, assuming that fluid cells have been created and all other cells are discarded. This is done with the following PROSTAR commands:


    CSET NEWS FLUID
    CSET INVE
The CSET should be empty. If this is not the case, examine the cells in CSET and adjust the model. If the cells are genuinely not desired, they can be removed using the PROSTAR command:


    CDEL CSET
Similarly, vertices will need to be discarded as well:


    CSET NEWS FLUID
    VSET NEWS CSET
    VSET INVE
Before discarding these unwanted vertices, the unwanted boundary faces have to be collected before purging:


    CSET NEWS FLUID
    VSET NEWS CSET
    BSET NEWS VSET ALL
    BSET INVE
If the BSET is not empty, the unwanted boundary faces can be deleted using:


    BDEL BSET

At this time, the model should contain only the fluid cells and the supporting vertices, as well as the defined boundary faces. All boundary faces should be fully supported by the vertices of the cells, if this is not the case, carry on cleaning the geometry until everything is clean.

5.5.2.3 Removing default boundary conditions

By default, STAR-CD assigns wall boundaries to any boundary faces not explicitly associated with a boundary region. The remaining boundary faces are collected into a default boundary region, with the assigned boundary type 0. OpenFOAM deliberately does not have a concept of a default boundary condition for undefined boundary faces since it invites human error, e.g. there is no means of checking that we meant to give all the unassociated faces the default condition.

Therefore all boundaries for each OpenFOAM mesh must be specified for a mesh to be successfully converted. The default boundary needs to be transformed into a real one using the procedure described below:

  1. Plot the geometry with Wire Surface option.
  2. Define an extra boundary region with the same parameters as the default region 0 and add all visible faces into the new region, say 10, by selecting a zone option in the boundary tool and drawing a polygon around the entire screen draw of the model. This can be done by issuing the following commands in PROSTAR:

           RDEF 10 WALL
           BZON 10 ALL
      
  3. We shall remove all previously defined boundary types from the set. Go through the boundary regions:

          BSET NEWS REGI 1
          BSET NEWS REGI 2
           3, 4, 
      
    Collect the vertices associated with the boundary set and then the boundary faces associated with the vertices (there will be twice as many of them as in the original set).

          BSET NEWS REGI 1
          VSET NEWS BSET
          BSET NEWS VSET ALL
          BSET DELE REGI 1
          REPL
      
    This should give the faces of boundary Region 10 which have been defined on top of boundary Region 1. Delete them with BDEL BSET. Repeat these for all regions.

5.5.2.4 Renumbering the model

Renumber and check the model using the commands:


    CSET NEW FLUID
    CCOM CSET

    VSET NEWS CSET
    VSET INVE (Should be empty!)
    VSET INVE
    VCOM VSET

    BSET NEWS VSET ALL
    BSET INVE (Should be empty also!)
    BSET INVE
    BCOM BSET

    CHECK ALL
    GEOM
Internal PROSTAR checking is performed by the last two commands, which may reveal some other unforeseeable error(s). Also, take note of the scaling factor because PROSTAR only applies the factor for STAR-CD and not the geometry. If the factor is not 1, use the scalePoints utility in OpenFOAM.

5.5.2.5 Writing out the mesh data

Once the mesh is completed, place all the integral matches of the model into the couple type 1. All other types will be used to indicate arbitrary matches.


    CPSET NEWS TYPE INTEGRAL
    CPMOD CPSET 1
The components of the computational grid must then be written to their own files. This is done using PROSTAR for boundaries by issuing the command


    BWRITE
by default, this writes to a .23 file (versions prior to 3.0) or a .bnd file (versions 3.0 and higher). For cells, the command


    CWRITE
outputs the cells to a .14 or .cel file and for vertices, the command


    VWRITE
outputs to file a .15 or .vrt file. The current default setting writes the files in ASCII format. If couples are present, an additional couple file with the extension .cpl needs to be written out by typing:


    CPWRITE

After outputting to the three files, exit PROSTAR or close the files. Look through the panels and take note of all STAR-CD sub-models, material and fluid properties used – the material properties and mathematical model will need to be set up by creating and editing OpenFOAM dictionary files.

The procedure of converting the PROSTAR files is first to create a new OpenFOAM case by creating the necessary directories. The PROSTAR files must be stored within the same directory and the user must change the file extensions: from .23, .14 and .15 (below STAR-CD version 3.0), or .pcs, .cls and .vtx (STAR-CD version 3.0 and above); to .bnd, .cel and .vrt respectively.

5.5.2.6 Problems with the .vrt file

The .vrt file is written in columns of data of specified width, rather than free format. A typical line of data might be as follows, giving a vertex number followed by the coordinates:


    19422      -0.105988957    -0.413711881E-02 0.000000000E+00
If the ordinates are written in scientific notation and are negative, there may be no space between values, e.g.:


    19423      -0.953953117E-01-0.338810333E-02 0.000000000E+00
The starToFoam converter reads the data using spaces to delimit the ordinate values and will therefore object when reading the previous example. Therefore, OpenFOAM includes a simple script, foamCorrectVrt to insert a space between values where necessary, i.e. it would convert the previous example to:


    19423      -0.953953117E-01 -0.338810333E-02 0.000000000E+00
The foamCorrectVrt script should therefore be executed if necessary before running the starToFoam converter, by typing:


    foamCorrectVrt <file>.vrt

5.5.2.7 Converting the mesh to OpenFOAM format

The translator utility starToFoam can now be run to create the boundaries, cells and points files necessary for a OpenFOAM run:


     starToFoam <meshFilePrefix>
where <meshFilePrefix> is the name of the the prefix of the mesh files, including the full or relative path. After the utility has finished running, OpenFOAM boundary types should be specified by editing the boundary file by hand.

5.5.3 gambitToFoam

GAMBIT writes mesh data to a single file with a .neu extension. The procedure of converting a GAMBIT.neu file is first to create a new OpenFOAM case, then at a command prompt, the user should execute:


    gambitToFoam <meshFile>
where <meshFile> is the name of the .neu file, including the full or relative path.

The GAMBIT file format does not provide information about type of the boundary patch, e.g. wall, symmetry plane, cyclic. Therefore all the patches have been created as type patch. Please reset after mesh conversion as necessary.

5.5.4 ideasToFoam

OpenFOAM can convert a mesh generated by I-DEAS but written out in ANSYS format as a .ans file. The procedure of converting the .ans file is first to create a new OpenFOAM case, then at a command prompt, the user should execute:


    ideasToFoam <meshFile>
where <meshFile> is the name of the .ans file, including the full or relative path.

5.5.5 cfx4ToFoam

CFX writes mesh data to a single file with a .geo extension. The mesh format in CFX is block-structured, i.e. the mesh is specified as a set of blocks with glueing information and the vertex locations. OpenFOAM will convert the mesh and capture the CFX boundary condition as best as possible. The 3 dimensional ‘patch’ definition in CFX, containing information about the porous, solid regions etc. is ignored with all regions being converted into a single OpenFOAM mesh. CFX supports the concept of a ‘default’ patch, where each external face without a defined boundary condition is treated as a wall. These faces are collected by the converter and put into a defaultFaces patch in the OpenFOAM mesh and given the type wall; of course, the patch type can be subsequently changed.

Like, OpenFOAM 2 dimensional geometries in CFX are created as 3 dimensional meshes of 1 cell thickness. If a user wishes to run a 2 dimensional case on a mesh created by CFX, the boundary condition on the front and back planes should be set to empty; the user should ensure that the boundary conditions on all other faces in the plane of the calculation are set correctly. Currently there is no facility for creating an axi-symmetric geometry from a 2 dimensional CFX mesh.

The procedure of converting a CFX.geo file is first to create a new OpenFOAM case, then at a command prompt, the user should execute:


    cfx4ToFoam <meshFile>
where <meshFile> is the name of the .geo file, including the full or relative path.
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