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Chapter 5. Reading and Manipulating

Grids

FLUENT can import many different types of grids from various sources.You can modify the grid by translating or scaling node coordinates, par-titioning the cells for parallel processing, reordering the cells in the do-main to decrease bandwidth, and merging or separating zones. You canalso obtain diagnostic information on the grid, including memory usageand simplex, topological, and domain information. You can find out thenumber of nodes, faces, and cells in the grid, determine the minimum andmaximum cell volumes in the domain, and check for the proper numbersof nodes and faces per cell. These and other capabilities are describedin the following sections.

• Section 5.1: Grid Topologies

• Section 5.2: Grid Requirements and Considerations

• Section 5.3: Grid Import

• Section 5.4: Non-Conformal Grids

• Section 5.5: Checking the Grid

• Section 5.6: Reporting Grid Statistics

• Section 5.7: Modifying the Grid

See Chapter 23 for information about adapting the grid based on solutiondata and related functions, and Section 28.4 for details on partitioningthe grid for parallel processing.

c© Fluent Inc. November 28, 2001 5-1

Reading and Manipulating Grids

5.1 Grid Topologies

Since FLUENT is an unstructured solver, it uses internal data structuresto assign an order to the cells, faces, and grid points in a mesh and tomaintain contact between adjacent cells. It does not, therefore, requirei,j,k indexing to locate neighboring cells. This gives you the flexibility touse the grid topology that is best for your problem, since the solverdoes not force an overall structure or topology on the grid. In 2D,quadrilateral and triangular cells are accepted, and in 3D, hexahedral,tetrahedral, pyramid, and wedge cells can be used. (Figure 5.1.1 depictseach of these cell types.) Both single-block and multi-block structuredmeshes are acceptable, as well as hybrid meshes containing quadrilateraland triangular cells or hexahedral, tetrahedral, pyramid, and wedge cells.In addition, FLUENT also accepts grids with hanging nodes (i.e., nodeson edges and faces that are not vertices of all the cells sharing thoseedges or faces). See Section 23.2.2 for details. Grids with non-conformalboundaries (i.e., grids with multiple subdomains in which the grid nodelocations at the internal subdomain boundaries are not identical) arealso acceptable. See Section 5.4 for details.

Some examples of grids that are valid for FLUENT are presented in Sec-tion 5.1.1. Section 5.1.2 explains how to choose the grid type that is bestsuited for your problem.

5.1.1 Examples of Acceptable Grid Topologies

As mentioned above, FLUENT can solve problems on a wide variety ofgrids. Figures 5.1.2–5.1.12 show examples of grids that are valid for FLU-ENT. O-type grids, grids with zero-thickness walls, C-type grids, confor-mal block-structured grids, multiblock structured grids, non-conformalgrids, and unstructured triangular, tetrahedral, quadrilateral, and hexa-hedral grids are all acceptable. Note that while FLUENT does not requirea cyclic branch cut in an O-type grid, it will accept a grid that containsone.

5-2 c© Fluent Inc. November 28, 2001

5.1 Grid Topologies

Tetrahedron Hexahedron

Prism/Wedge Pyramid

Triangle Quadrilateral

2D Cell Types

3D Cell Types

Figure 5.1.1: Cell Types



Figure 5.1.2: Structured Quadrilateral Grid for an Airfoil

Figure 5.1.3: Unstructured Quadrilateral Grid


5.1 Grid Topologies

Figure 5.1.4: Multiblock Structured Quadrilateral Grid

Figure 5.1.5: O-Type Structured Quadrilateral Grid



Figure 5.1.6: Parachute Modeled With Zero-Thickness Wall

Branch Cut

Figure 5.1.7: C-Type Structured Quadrilateral Grid


5.1 Grid Topologies

Figure 5.1.8: 3D Multiblock Structured Grid

Figure 5.1.9: Unstructured Triangular Grid for an Airfoil



Figure 5.1.10: Unstructured Tetrahedral Grid

Figure 5.1.11: Hybrid Triangular/Quadrilateral Grid with HangingNodes


5.1 Grid Topologies

Figure 5.1.12: Non-Conformal Hybrid Grid for a Rotor-Stator Geometry



5.1.2 Choosing the Appropriate Grid Type

FLUENT can use grids comprised of triangular or quadrilateral cells (ora combination of the two) in 2D, and tetrahedral, hexahedral, pyramid,or wedge cells (or a combination of these) in 3D. The choice of whichmesh type to use will depend on your application. When choosing yourmesh type, you should consider the following issues:

• Setup time

• Computational expense

• Numerical diffusion

To clarify the trade-offs inherent in your choice of mesh type, these issuesare discussed further.

Setup Time

Many flow problems solved in engineering practice involve complex ge-ometries. The creation of structured or block-structured grids (consist-ing of quadrilateral or hexahedral elements) for such problems can beextremely time-consuming, if not impossible. Setup time for complexgeometries is, therefore, the major motivation for using unstructuredgrids employing triangular or tetrahedral cells. If your geometry is rela-tively simple, however, there may be no clear saving in setup time witheither approach.

If you already have a mesh created for a structured code such as FLUENT4, it would clearly save you time to use this mesh in FLUENT rather thanregenerate it. This might be a strong motivation for using quadrilateralor hexahedral cells in your FLUENT simulation. Note that FLUENT hasa range of filters that allow you to import structured meshes from othercodes, including FLUENT 4 (see Section 5.3).

Computational Expense

When geometries are complex or the range of length scales of the flow islarge, a triangular/tetrahedral mesh can often be created with far fewer


5.1 Grid Topologies

cells than the equivalent mesh consisting of quadrilateral/hexahedralelements. This is because a triangular/tetrahedral mesh allows cellsto be clustered in selected regions of the flow domain, whereas struc-tured quadrilateral/hexahedral meshes will generally force cells to beplaced in regions where they are not needed. Unstructured quadrilat-eral/hexahedral meshes offer many of the advantages of triangular/tetra-hedral meshes for moderately-complex geometries.

One characteristic of quadrilateral/hexahedral elements that might makethem more economical in some situations is that they permit a muchlarger aspect ratio than triangular/tetrahedral cells. A large aspect ratioin a triangular/tetrahedral cell will invariably affect the skewness of thecell, which is undesirable as it may impede accuracy and convergence.Therefore, if you have a relatively simple geometry in which the flowconforms well to the shape of the geometry, such as a long thin duct, youcan use a mesh of high-aspect-ratio quadrilateral/hexahedral cells. Themesh is likely to have far fewer cells than if you use triangular/tetrahedralcells.

Numerical Diffusion

A dominant source of error in multidimensional situations is numeri-cal diffusion, also termed false diffusion. (The term “false diffusion” isused because the diffusion is not a real phenomenon, yet its effect ona flow calculation is analogous to that of increasing the real diffusioncoefficient.)

The following points can be made about numerical diffusion:

• Numerical diffusion is most noticeable when the real diffusion issmall, that is, when the situation is convection-dominated.

• All practical numerical schemes for solving fluid flow contain afinite amount of numerical diffusion. This is because numericaldiffusion arises from truncation errors that are a consequence ofrepresenting the fluid flow equations in discrete form.

• The second-order discretization scheme used in FLUENT can helpreduce the effects of numerical diffusion on the solution.



• The amount of numerical diffusion is inversely related to the reso-lution of the mesh. Therefore, one way of dealing with numericaldiffusion is to refine the mesh.

• Numerical diffusion is minimized when the flow is aligned with themesh.

The last point is the most relevant to the choice of the grid. It isclear that if you use a triangular/tetrahedral mesh the flow can neverbe aligned with the grid. On the other hand, if you use a quadrilat-eral/hexahedral mesh this situation might occur, but not for complexflows. It is only in a simple flow, such as the flow through a long duct,in which you can rely on a quadrilateral/hexahedral mesh to minimizenumerical diffusion. In such situations, there might be some advan-tage to using a quadrilateral/hexahedral mesh, since you will be able toget a better solution with fewer cells than if you were using a triangu-lar/tetrahedral mesh.

5.2 Grid Requirements and Considerations

This section contains information about special geometry/grid require-ments and general comments on mesh quality.

5.2.1 Geometry/Grid Requirements

You should be aware of the following geometry setup and grid construc-tion requirements at the beginning of your problem setup:

• Axisymmetric geometries must be defined such that the axis ofrotation is the x axis of the Cartesian coordinates used to definethe geometry (Figure 5.2.1).

• FLUENT allows you to set up periodic boundaries using either con-formal or non-conformal periodic zones. For conformal periodicboundaries, the periodic zones must have identical grids. The con-formal periodic boundaries can be created in GAMBIT or TGridwhen you are generating the volume mesh. (See the GAMBIT Mod-eling Guide or the TGrid User’s Guide for more information about



y

x CLFigure 5.2.1: Setup of Axisymmetric Geometries with the x Axis as theCenterline

creating periodic boundaries in GAMBIT or TGrid.) Alternatively,you can create the conformal periodic boundaries in FLUENT usingthe make-periodic text command (see Section 5.7.5 for details).

Although GAMBIT and TGrid can produce true periodic bound-aries, most CAD packages do not. If your mesh was created insuch a package, you can create the periodic boundaries using thenon-conformal periodic option in FLUENT (see Section 5.7.5 fordetails). This option, however, is recommended only for periodiczones that are planar.

5.2.2 Mesh Quality

The quality of the mesh plays a significant role in the accuracy andstability of the numerical computation. The attributes associated withmesh quality are node point distribution, smoothness, and skewness.



Node Density and Clustering

Since you are discretely defining a continuous domain, the degree towhich the salient features of the flow (such as shear layers, separatedregions, shock waves, boundary layers, and mixing zones) are resolveddepends on the density and distribution of nodes in the mesh. In manycases, poor resolution in critical regions can dramatically alter the flowcharacteristics. For example, the prediction of separation due to an ad-verse pressure gradient depends heavily on the resolution of the boundarylayer upstream of the point of separation.

Resolution of the boundary layer (i.e., mesh spacing near walls) alsoplays a significant role in the accuracy of the computed wall shear stressand heat transfer coefficient. This is particularly true in laminar flowswhere the grid adjacent to the wall should obey

yp

√u∞νx

≤ 1 (5.2-1)

where yp = distance to the wall from the adjacent cell centroidu∞ = free-stream velocityν = kinematic viscosity of the fluidx = distance along the wall from the starting point

of the boundary layer

Equation 5.2-1 is based upon the Blasius solution for laminar flow overa flat plate at zero incidence [203].

Proper resolution of the mesh for turbulent flows is also very important.Due to the strong interaction of the mean flow and turbulence, the nu-merical results for turbulent flows tend to be more susceptible to griddependency than those for laminar flows. In the near-wall region, dif-ferent mesh resolutions are required depending on the near-wall modelbeing used. See Section 10.9 for detailed guidelines.

In general, no flow passage should be represented by fewer than 5 cells.Most cases will require many more cells to adequately resolve the pas-sage. In regions of large gradients, as in shear layers or mixing zones, thegrid should be fine enough to minimize the change in the flow variables



from cell to cell. Unfortunately, it is usually very difficult to determinein advance the locations of important flow features. Moreover, the gridresolution in most complicated three-dimensional flow fields will be con-strained by CPU time and computer resource limitations (i.e., memoryand disk space). Although accuracy increases with larger grids, the CPUand memory requirements to compute the solution and postprocess theresults also increase. Solution-adaptive grid refinement can be used toincrease and/or decrease grid density based on the evolving flow field,and thus provides the potential for more economical use of grid points(and, hence, reduced time and resource requirements). See Chapter 23for more information on solution adaption.

Smoothness

Rapid changes in cell volume between adjacent cells translate into largertruncation errors. Truncation error is the difference between the partialderivatives in the governing equations and their discrete approximations.FLUENT provides the capability to improve the smoothness by refiningthe mesh based on the change in cell volume or the gradient of cellvolume. See Sections 23.4 and 23.7 for more information on refining thegrid based on change in cell volume.

Cell Shape

The shape of the cell (including its skewness and aspect ratio) also has asignificant impact on the accuracy of the numerical solution. Skewnesscan be defined as the difference between the cell’s shape and the shape ofan equilateral cell of equivalent volume. Highly skewed cells can decreaseaccuracy and destabilize the solution. For example, optimal quadrilat-eral meshes will have vertex angles close to 90 degrees, while triangularmeshes should preferably have angles of close to 60 degrees and have allangles less than 90 degrees.

Aspect ratio is a measure of the stretching of the cell. As discussed inSection 5.1.2, for highly anisotropic flows, extreme aspect ratios mayyield accurate results with fewer cells. However, a general rule of thumbis to avoid aspect ratios in excess of 5:1.



Flow-Field Dependency

The effect of resolution, smoothness and cell shape on the accuracy andstability of the solution process is strongly dependent on the flow fieldbeing simulated. For example, very skewed cells can be tolerated inbenign flow regions, but can be very damaging in regions with strong flowgradients. Since the locations of strong flow gradients generally cannotbe determined a priori, you should strive to achieve a high-quality meshover the entire flow domain.

5.3 Grid Import

Since FLUENT can handle a number of different grid topologies, there aremany sources from which you can obtain a grid to be used in your simu-lation. You can generate a grid using GAMBIT, TGrid, GeoMesh, preBFC,ICEMCFD, I-DEAS, NASTRAN, PATRAN, ARIES, ANSYS, or other pre-processors, or use the grid contained in a FLUENT/UNS, RAMPANT,or FLUENT 4 case file. You can also prepare multiple mesh files andcombine them to create a single mesh.

5.3.1 GAMBIT Grid Files

You can use GAMBIT to create 2D and 3D structured/unstructured/hybridgrids. To create any of these meshes for FLUENT, follow the proceduredescribed in the GAMBIT Modeling Guide, and export your mesh in FLU-ENT 5/6 format. All such meshes can be imported directly into FLUENTusing the File/Read/Case... menu item, as described in Section 3.2.

5.3.2 GeoMesh Grid Files

You can use GeoMesh to create complete 2D quadrilateral or triangulargrids, 3D hexahedral grids, and triangular surface grids for 3D tetra-hedral grids. To create any of these meshes for FLUENT, follow theprocedure described in the GeoMesh User’s Guide. To complete the gen-eration of a 3D tetrahedral mesh, you must read the surface mesh intoTGrid and generate the volume mesh there. All other meshes, however,can be imported directly into FLUENT using the File/Read/Case... menuitem, as described in Section 3.2.


5.3 Grid Import

5.3.3 TGrid Grid Files

You can use TGrid to create 2D and 3D unstructured triangular/tetrahe-dral grids from boundary or surface grids. Follow the meshing proce-dure described in the TGrid User’s Guide, and save your mesh using theFile/Write/Mesh... menu item. To import the grid into FLUENT, use theFile/Read/Case... menu item, as described in Section 3.2.

5.3.4 preBFC Grid Files

You can use preBFC to create two different types of grids for FLUENT:structured quadrilateral/hexahedral and unstructured triangular/tetrahe-dral. The procedure for generating and importing each is outlined below.

Structured Grid Files

To generate a structured 2D or 3D grid, follow the procedure describedin the preBFC User’s Guide (Chapters 6 and 7). The resulting grid willcontain quadrilateral (2D) or hexahedral (3D) elements. Remember tospecify no more than 70 wall zones and no more than 35 inlet zones.

To import the grid, use the File/Import/preBFC Structured Mesh... menuitem, as described in Section 3.2.4.

To manually convert a file in preBFC format to a mesh file suitable forFLUENT, enter the following command:

utility fl42seg input-filename output-filename

The output file produced can be read into FLUENT using theFile/Read/Case... menu item, as described in Section 3.2.

Unstructured Triangular and Tetrahedral Grid Files

To generate an unstructured 2D grid, follow the procedure described inthe preBFC User’s Guide (Chapter 8), and save your mesh file in theRAMPANT format using the MESH-RAMPANT/TGRID command. Note thatthe current FLUENT format is the same as the RAMPANT format. The



resulting grid will contain triangular elements. To import the grid, usethe File/Read/Case... menu item, as described in Section 3.2.

To generate a 3D unstructured tetrahedral grid, follow the procedure de-scribed in Chapter 8 of the preBFC User’s Guide for generating a surfacemesh. You will then read the surface mesh into TGrid, and complete thegrid generation there. See Section 5.3.3 for information about TGrid gridfiles.

5.3.5 ICEMCFD Grid Files

You can use ICEMCFD to create structured grids in FLUENT 4 formatand unstructured grids in RAMPANT format. To import a FLUENT 4grid, follow the instructions in Section 5.3.8. To import a RAMPANTgrid, use the File/Read/Case... menu item, as described in Section 3.2.Note that the current FLUENT format is the same as the RAMPANTformat; it is not the same as the FLUENT 4 format. After reading atriangular or tetrahedral ICEMCFD volume mesh, you should performsmoothing and swapping (as described in Section 23.11) to improve itsquality.

5.3.6 Grid Files from Third-Party CAD Packages

FLUENT can import grid files from a number of third-party CAD pack-ages, including I-DEAS, NASTRAN, PATRAN, and ANSYS.

I-DEAS Universal Files

There are three different ways in which you can import an I-DEAS Uni-versal file into FLUENT:

1. You can generate an I-DEAS surface or volume mesh containingtriangular, quadrilateral, tetrahedral, wedge and/or hexahedral el-ements, and import it into TGrid using the commands described inthe TGrid User’s Guide and adhering to the restrictions describedin Appendix B of the TGrid User’s Guide. In TGrid, complete thegrid generation (if necessary) and then follow the instructions inSection 5.3.3.


5.3 Grid Import

2. You can generate an I-DEAS volume mesh with linear triangular,quadrilateral, tetrahedral, wedge, or hexahedral elements, and im-port it directly using the File/Import/IDEAS Universal... menu item,as described in Section 3.2.6.

3. You can generate an I-DEAS volume mesh with linear triangular,quadrilateral, tetrahedral, wedge, or hexahedral elements, and thenuse the filter fe2ram to convert the Universal file to the format usedby FLUENT. To convert an input file in I-DEAS Universal format toan output file in FLUENT format, follow the instructions on page5-24. After the output file has been written, you can read it intoFLUENT using the File/Read/Case... menu item, as described inSection 3.2.

Recognized I-DEAS Datasets

The following Universal file datasets are recognized by the FLUENT gridimport utility:

Node Coordinates dataset number 15, 781, 2411

Elements dataset number 71, 780, 2412

Permanent Groups dataset number 752, 2417, 2429, 2430, 2432, 2435

For 2D volume grids, the elements must exist in a constant z plane.

Note that mesh area/mesh volume datasets are not recognized. This im-plies that writing multiple mesh areas/mesh volumes to a single Universalfile may confuse FLUENT.

Grouping Nodes to Create Face Zones

Nodes are grouped in I-DEAS using the Group command to create bound-ary face zones. In FLUENT, boundary conditions are applied to eachzone. Faces that contain the nodes in a group are gathered into a singlezone. It is important not to group nodes of internal faces with nodes ofboundary faces.



One technique is to generate groups automatically based on curves ormesh areas—i.e., every curve or mesh area will be a different zone inFLUENT. You may also create the groups manually, generating groupsconsisting of all nodes related to a given curve (2D) or mesh area (3D).

Grouping Elements to Create Cell Zones

Elements in I-DEAS are grouped using the Group command to create themultiple cell zones. All elements grouped together are placed in a singlecell zone in FLUENT. If the elements are not grouped, FLUENT will placeall the cells into a single zone.

Deleting Duplicate Nodes

I-DEAS may generate duplicate or coincident nodes in the process of cre-ating elements. These nodes must be removed in I-DEAS before writingthe universal file for import into FLUENT.

NASTRAN Files

There are three different ways in which you can import a NASTRAN fileinto FLUENT:

1. You can generate a NASTRAN surface or volume mesh containingtriangular, quadrilateral, tetrahedral, wedge, and/or hexahedralelements, and import it into TGrid using the commands describedin the TGrid User’s Guide and adhering to the restrictions describedin Appendix B of the TGrid User’s Guide. In TGrid, complete thegrid generation (if necessary) and then follow the instructions inSection 5.3.3.

2. You can generate a NASTRAN volume mesh with linear triangu-lar, quadrilateral, tetrahedral, wedge, or hexahedral elements, andimport it directly using the File/Import/NASTRAN... menu item,as described in Section 3.2.7.

3. You can generate a NASTRAN volume mesh with linear triangular,quadrilateral, tetrahedral, wedge, or hexahedral elements, and then


5.3 Grid Import

use the filter fe2ram to convert the NASTRAN file to the formatused by FLUENT. To convert an input file in NASTRAN format toan output file in FLUENT format, follow the instructions on page5-24. After the output file has been written, you can read it intoFLUENT using the File/Read/Case... menu item, as described inSection 3.2.

After reading a triangular or tetrahedral NASTRAN volume mesh usingmethod 2 or 3 above, you should perform smoothing and swapping (asdescribed in Section 23.11) to improve its quality.

Recognized NASTRAN Bulk Data Entries

The following NASTRAN file datasets are recognized by the FLUENTgrid import utility:

GRID single-precision node coordinates

GRID* double-precision node coordinates

CBAR line elements

CTETRA, CTRIA3 tetrahedral and triangular elements

CHEXA, CQUAD4, CPENTA hexahedral, quadrilateral, and wedgeelements


Deleting Duplicate Nodes

NASTRAN may generate duplicate or coincident nodes in the process ofcreating elements. These nodes must be removed in NASTRAN beforewriting the file for import into FLUENT.



PATRAN Neutral Files

There are three different ways in which you can import a PATRAN Neutralfile into FLUENT.

1. You can generate a PATRAN surface or volume mesh containingtriangular, quadrilateral, tetrahedral, wedge, and/or hexahedralelements, and import it into TGrid using the commands describedin the TGrid User’s Guide and adhering to the restrictions describedin Appendix B of the TGrid User’s Guide. In TGrid, complete thegrid generation (if necessary) and then follow the instructions inSection 5.3.3.

2. You can generate a PATRAN volume mesh with linear triangular,quadrilateral, tetrahedral, wedge, or hexahedral elements (group-ing nodes with the same component-group name) and import itdirectly to FLUENT by selecting the File/Import/ PATRAN... menuitem, as described in Section 3.2.8.

3. You can generate a PATRAN volume mesh with linear triangular,quadrilateral, tetrahedral, wedge, or hexahedral elements and thenuse the filter fe2ram to convert the Neutral file into the format usedby FLUENT. To convert an input file in PATRAN Neutral format toan output file in FLUENT format, follow the instructions on page5-24. After the output file has been written, you can read it intoFLUENT using the File/Read/Case... menu item, as described inSection 3.2.

After reading a triangular or tetrahedral PATRAN volume mesh usingmethod 2 or 3 above, you should perform smoothing and swapping (asdescribed in Section 23.11) to improve its quality.

Recognized PATRAN Datasets

The following PATRAN Neutral file packet types are recognized by theFLUENT grid import utility:

Node Data Packet Type 01


5.3 Grid Import

Element Data Packet Type 02

Distributed Load Data Packet Type 06

Node Temperature Data Packet Type 10

Name Components Packet Type 21

File Header Packet Type 25


Grouping Elements to Create Cell Zones

Elements are grouped in PATRAN using the Named Component commandto create the multiple cell zones. All elements grouped together areplaced in a single cell zone in FLUENT. If the elements are not grouped,FLUENT will place all the cells into a single zone.

ANSYS Files

There are three different ways in which you can import an ANSYS fileinto FLUENT.

1. You can generate a surface or volume mesh containing triangu-lar, quadrilateral, tetrahedral, wedge, or hexahedral elements usingANSYS or ARIES, and import it into TGrid using the commandsdescribed in the TGrid User’s Guide and adhering to the restric-tions described in Appendix B of the TGrid User’s Guide. In TGrid,complete the grid generation (if necessary) and then follow the in-structions in Section 5.3.3.

2. You can generate an ANSYS volume mesh with linear triangular,quadrilateral, tetrahedral, wedge, or hexahedral elements and im-port it directly to FLUENT using the File/Import/ANSYS... menuitem, as described in Section 3.2.5.

3. You can generate an ANSYS volume mesh with linear triangular,quadrilateral, tetrahedral, wedge, or hexahedral elements and then



use the filter fe2ram to convert the ANSYS file into the format usedby FLUENT. To convert an input file in ANSYS 5.4 or 5.5 format toan output file in FLUENT format, follow the instructions on page5-24. After the output file has been written, you can read it intoFLUENT using the File/Read/Case... menu item, as described inSection 3.2.

After reading a triangular or tetrahedral volume mesh using method 2or 3 above, you should perform smoothing and swapping (as describedin Section 23.11) to improve its quality.

Recognized ANSYS 5.4 and 5.5 Datasets

FLUENT can import mesh files from ANSYS 5.4 and 5.5 (.cdb files),retaining original boundary names. The following ANSYS file datasetsare recognized by the FLUENT grid import utility:

NBLOCK node block data

EBLOCK element block data

CMBLOCK element/node grouping

The elements must be STIF63 linear shell elements. In addition, if el-ement data without an explicit element ID is used, the filter assumessequential numbering of the elements when creating the zones.

Using the fe2ram Filter to Convert Files

If you choose to convert the CAD file manually before reading it intoFLUENT, you can enter the following command:

utility fe2ram [dimension] format [zoning] input-file output-file

where items enclosed in square brackets are optional. (Do not type thesquare brackets.)


5.3 Grid Import

dimension indicates the dimension of the dataset. Replace dimension by-d2 to indicate that the grid is 2-dimensional. For a 3D grid, do notenter anything for dimension, because 3D is the default.

format indicates the format of the file you wish to convert. Replace for-mat by -tANSYS for an ANSYS file, -tIDEAS for an I-DEAS file, -tNASTRANfor a NASTRAN file, or -tPATRAN for a PATRAN file. To check if conver-sion from any other CAD packages has been added, typeutility fe2ram -cl -help.

zoning indicates how zones were identified in the CAD package. Replacezoning by -zID for a grid that was zoned by property IDs, or -zNONEto ignore all zone groupings. For a grid zoned by group, do not enteranything for zoning, because zoning by groups is the default.

input-file and output-file are the names of the original file and the file towhich you want to write the converted grid information, respectively.

For example, if you wanted to convert the 2D I-DEAS volume mesh filesample.unv to an output file called sample.grd, you would enter thefollowing command:

utility fe2ram -d2 -tIDEAS sample.unv sample.grd

5.3.7 FLUENT/UNS and RAMPANT Case Files

If you have a FLUENT/UNS 3 or 4 case file or a RAMPANT 2, 3, or 4case file and you want to run a FLUENT simulation using the same grid,you can read it into FLUENT using the File/Read/Case... menu item, asdescribed in Section 3.4.

5.3.8 FLUENT 4 Case Files

If you have a FLUENT 4 case file and you want to run a FLUENT sim-ulation using the same grid, you can import it into FLUENT using theFile/Import/FLUENT 4 Case... menu item, as described in Section 3.5.FLUENT will read grid information and zone types from the FLUENT 4case file.

Note that FLUENT 4 may interpret some pressure boundaries differently!



from the current release of FLUENT. Check the conversion informationprinted out by FLUENT to see if you need to modify any boundary types.

To manually convert an input file in FLUENT 4 format to an output filein the current FLUENT format, enter the following command:

utility fl42seg input-filename output-filename

After the output file has been written, you can read it into FLUENTusing the File/Read/Case... menu item, as described in Section 3.2.

5.3.9 FIDAP Neutral Files

If you have a FIDAP Neutral file and you want to run a FLUENT simula-tion using the same grid, you can import it using the File/Import/FIDAP...menu item, as described in Section 3.6. FLUENT will read grid informa-tion and zone types from the FIDAP file.

To manually convert an input file in FIDAP format to an output file inFLUENT format, enter the following command:

utility fe2ram [dimension] -tFIDAP7 input-file output-file

where the item in square brackets is optional. (Do not type the squarebrackets.) For a 2D file, replace dimension with -d2. For a 3D file, donot enter anything for dimension, because 3D is the default.

After the output file has been written, you can read it into FLUENTusing the File/Read/Case... menu item, as described in Section 3.2.


5.3 Grid Import

5.3.10 Reading Multiple Mesh Files

There may be some cases in which you will need to read multiple meshfiles (subdomains) to form your computational domain. Some examplesare listed below:

• If you plan to solve on a multiblock mesh, you can generate eachblock of the mesh in the grid generator and save it to a separategrid file.

• For very complicated geometries, it may be more efficient to savethe mesh for each part as a separate grid file.

Note that you do not need to ensure that the grid node locations areidentical at the boundaries where two separate meshes meet; FLUENTcan handle non-conformal grid interfaces. See Section 5.4 for detailsabout non-conformal grid boundaries.

The procedure for reading multiple grid files is as follows:

1. Generate the grid for the whole domain in the grid generator, andsave each cell zone (or block or part) to a separate grid file forFLUENT.

If one (or more) of the grids you wish to import is structured (e.g.,!a FLUENT 4 grid file), you will need to first convert it to FLUENTformat using the fl42seg filter described in Section 5.3.8.

2. Before you start the solver, use either TGrid or the tmerge filterto combine the grids into one grid file. The TGrid method is moreconvenient, but the tmerge method allows you to rotate, scale,and/or translate the grids before they are merged.

• To use TGrid, follow the procedure below:

(a) Read all of the grid files into TGrid. When TGrid reads thegrid files, it will automatically merge them into a singlegrid.

(b) Save the merged grid file.



See the TGrid User’s Guide for information about reading andwriting files in TGrid.

• To use the tmerge filter, follow the procedure below (beforestarting FLUENT):

(a) For 3D problems, type utility tmerge -3d. For 2Dproblems, type utility tmerge -2d.

(b) When prompted, specify the names of the input files (theseparate grid files) and the name of the output file inwhich to save the complete grid. For each input file, youcan specify scaling factors, translation distances, or a ro-tation angle. In the example below, no scaling, transla-tion, or rotation is performed.


5.3 Grid Import

user@mymachine:> utility tmerge -2d

Starting /Fluent.Inc/utility/tmerge2.1/ultra/tmerge_2d.2.1.7

Append 2D grid files.tmerge2D Fluent Inc, Version 2.0.16

Enter name of grid file (ENTER to continue) : my1.msh

x,y scaling factor, eg. 1 1 : 1 1

x,y translation, eg. 0 1 : 0 0

rotation angle (deg), eg. 45 : 0

Enter name of grid file (ENTER to continue) : my2.msh

x,y scaling factor, eg. 1 1 : 1 1

x,y translation, eg. 0 1 : 0 0

rotation angle (deg), eg. 45 : 0

Enter name of grid file (ENTER to continue) : <ENTER>

Enter name of output file : final.msh

Reading...node zone: id 1, ib 1, ie 1677, typ 1node zone: id 2, ib 1678, ie 2169, typ 2

.

.

.done.Writing...492 nodes, id 1, ib 1678, ie 2169, type 2.1677 nodes, id 2, ib 1, ie 1677, type 1.

.

.

.done.Appending done.



In the above example, where no scaling, translation, or rota-tion is requested, you could simplify the inputs to the follow-ing:

utility tmerge -2d -cl -p my1.msh my2.msh final.msh

3. Read the combined grid file into the solver in the usual manner(using the File/Read/Case... menu item).

For a conformal mesh, if you do not want a boundary between the ad-jacent cell zones, you can use the Fuse Face Zones panel to fuse the“overlapping” boundaries (see Section 5.7.7). The matching faces willbe moved to a new zone with a boundary type of interior. If all faces oneither or both of the original zones have been moved to the new zone,the original zone(s) will be discarded.

If you are planning to use sliding meshes, or if you have non-conformal!boundaries between adjacent cell zones, you should not fuse the overlap-ping zones. You must instead change the type of the two overlappingzones to interface (as described in Section 5.4).


5.4 Non-Conformal Grids


In FLUENT it is possible to use a grid composed of cell zones with non-conformal boundaries. That is, the grid node locations do not need tobe identical at the boundaries where two subdomains meet. FLUENThandles such meshes using the same technique that is used in the slidingmesh model, although in this situation the meshes do not slide.

5.4.1 Non-Conformal Grid Calculations

To compute the flux across the non-conformal boundary, FLUENT mustfirst compute the intersection between the “interface” zones that com-prise the boundary. The resulting intersection produces an interior zonewhere the two interface zones overlap (see Figure 5.4.1). If one of theinterface zones extends beyond the other (as shown in Figure 5.4.2),FLUENT will create one or two additional wall zones for the portion(s)of the boundary where the two interface zones do not overlap.

interface zone 1

interface zone 2

interior zone

Figure 5.4.1: Completely Overlapping Grid Interface Intersection

interface zone 2

interface zone 1interior zone

wall zone 1 wall zone 2

Figure 5.4.2: Partially Overlapping Grid Interface Intersection



Principally, fluxes across the grid interface are computed using the facesresulting from the intersection of the two interface zones, rather thanfrom the interface zone faces themselves. In the example shown in Fig-ure 5.4.3, the interface zones are composed of faces A-B and B-C, andfaces D-E and E-F. The intersection of these zones produces the facesa-d, d-b, b-e, and e-c. Faces produced in the region where the two cellzones overlap (d-b, b-e, and e-c) are grouped to form an interior zone,while the remaining face (a-d) forms a wall zone. To compute the fluxacross the interface into cell IV, for example, face D-E is ignored andfaces d-b and b-e are used instead, bringing information into cell IVfrom cells I and III, respectively.

A B C

D E F

a b e cd

cell zone 1

cell zone 2

interfacezone 2

interfacezone 1

I II

III

IV VI

V

Figure 5.4.3: Two-Dimensional Non-Conformal Grid Interface



5.4.2 Requirements and Limitations of Non-Conformal Grids

Please note the following requirements and limitations when using non-conformal grids:

• The grid interface can be any shape (including a non-planar sur-face, in 3D), provided that the two interface boundaries are basedon the same geometry. If there are sharp features (e.g., 90-degreeangles) or curvature in the mesh, it is especially important thatboth sides of the interface closely follow that feature.

For example, consider the case of two concentric circles that definetwo fluid zones with a circular, non-conformal interface betweenthem, as shown in Figure 5.4.4. Because the node spacing on theinterface edge of the outer fluid zone is coarse compared to theradius of curvature, the interface does not closely follow the feature(in this case, the circular edge.)

In general, the maximum tolerance between two interfaces should!not be larger than their adjacent cell size at that location; i.e., nocell should be completely enclosed between two interfaces.

Large tolerances betweeninterfaces should be avoided

Figure 5.4.4: A Circular Non-Conformal Interface



• A face zone cannot share a non-conformal interface with more thanone other face zone. This is best illustrated by an example, asshown in Figure 5.4.5. Each volume in the figure is meshed sepa-rately and does not match node-to-node at the interface. To createthe non-conformal interface, you will need to work with three sur-faces: one side of the box (rectangle 1) and an end cap from eachpipe (circle 1 and circle 2). To create interfaces between rectangle 1and the two circles, there needs to be one-to-one mapping betweeneach end cap and the side of the box (see Figure 5.4.6).

rectangle 1

circle 2

circle 1

Figure 5.4.5: Box with Two Pipes Intersecting One Side

• If you create a single grid with multiple cell zones separated bya non-conformal boundary, you must be sure that each cell zonehas a distinct face zone on the non-conformal boundary. The facezones for two adjacent cell zones will have the same position andshape, but one will correspond to one cell zone and one to theother. (Note that it is also possible to create a separate grid filefor each of the cell zones, and then merge them as described inSection 5.3.10.)

• All periodic zones must be correctly oriented (either rotational ortranslational) before you create the non-conformal interface. Peri-odic non-conformal interfaces must overlap exactly; i.e., they need



Allowed: Divide rectangle 1 into two surfaces

interface pairings are:

rectangle 1a ⇔ circle 2rectangle 1b ⇔ circle 1

Not allowed: Keeping things as they are

rectangle 1 ⇔ circle 2rectangle 1 ⇔ circle 1

Not Valid:

rectangle 1a rectangle 1b

circle 2 circle 1

rectangle 1

circle 2 circle 1

Figure 5.4.6: One-to-One Mapping

to have the same rotational or translational extent and, in addi-tion, have the same axial extent. This is not true for interfaces ingeneral, where a wall zone is created for non-overlapping regions.

• For 3D cases, if the interface is periodic, only one pair of periodicboundaries can neighbor the interface.

See also Section 5.4.4 for information about using non-conformal FLU-ENT/UNS and RAMPANT cases.

5.4.3 Using a Non-Conformal Grid in FLUENT

If your multiple-zone grid includes non-conformal boundaries, you mustfollow the procedure below (after ensuring that your grid meets all therequirements listed in Section 5.4.2) to ensure that FLUENT can obtaina solution on your mesh:



1. Read the grid into FLUENT. (If you have multiple grid files thathave not yet been merged, first follow the instructions in Sec-tion 5.3.10 to merge them into a single grid.)

2. After reading in the grid, change the type of each pair of zones thatcomprises the non-conformal boundary to interface (as described inSection 6.1.3).

Define −→Boundary Conditions...

3. Define the non-conformal grid interfaces in the Grid Interfaces panel(Figure 5.4.7).

Define −→Grid Interfaces...

Figure 5.4.7: The Grid Interfaces Panel

(a) Enter a name for the interface in the Grid Interface field.



(b) Specify the two interface zones that comprise the grid inter-face by selecting one in the Interface Zone 1 list and one inthe Interface Zone 2 list.

If one of your interface zones is much smaller than the other,!you should specify the smaller zone as Interface Zone 1 toimprove the accuracy of the intersection calculation.

(c) Set the Interface Type, if appropriate. There are two options:

• Enable Periodic for periodic problems.

• Enable Coupled if the interface lies between a solid zoneand a fluid zone.

(d) Click on Create to create a new grid interface. For all types ofinterfaces, FLUENT will create boundary zones for the inter-face (e.g., wall-9, wall-10), which will appear under Bound-ary Zone 1 and Boundary Zone 2. If you have enabled theCoupled option, FLUENT will also create wall interface zones(e.g., wall-4, wall-4-shadow), which will appear under In-terface Wall Zone 1 and Interface Wall Zone 2.

(e) If the two interface zones did not overlap entirely, check theboundary zone type of the zone(s) created for the non-overlap-ping portion(s) of the boundary. If the zone type is not cor-rect, you can use the Boundary Conditions panel to changeit.

(f) If you have any Coupled-type interfaces, define boundary con-ditions (if relevant) by updating the interface wall zones inthe Boundary Conditions panel.

Define −→Boundary Conditions...

If you create an incorrect grid interface, you can select it in theGrid Interface list and click on the Delete button to delete it. (Anyboundary zones or wall interface zones that were created when theinterface was created will also be deleted.)

You may then proceed with the problem setup as usual.



5.4.4 Starting From a FLUENT/UNS or RAMPANT Case

FLUENT/UNS and RAMPANT case files with non-conformal interfacescan be read and used by FLUENT without any changes. However, youmay want to recompute the grid interface to take advantage of FLU-ENT’s improved intersection calculation. You cannot simply delete theoriginal grid interface and recompute it. Instead, you must use thedefine/grid-interfaces/recreate text command.

define −→ grid-interfaces −→recreate

When you select this command, FLUENT will recreate all grid interfacesin the domain. You can then continue the problem setup or calculationas usual.

If you have a FLUENT/UNS or RAMPANT data file for the non-conformal!case, you must read it in before you use the recreate command.

5.5 Checking the Grid

The grid checking capability in FLUENT provides domain extents, vol-ume statistics, grid topology and periodic boundary information, verifi-cation of simplex counters, and (for axisymmetric cases) node positionverification with respect to the x axis. You can obtain this informationby selecting the Check menu item in the Grid pull-down menu.

Grid −→Check

It is generally a good idea to check your grid right after reading it into!the solver, in order to detect any grid trouble before you get started withthe problem setup.

5.5.1 Grid Check Information

The information that FLUENT generates when you use the Check itemwill appear in the console window. Sample output is shown below:


5.5 Checking the Grid

Grid Check

Domain Extents:x-coordinate: min (m) = 0.000000e+00, max (m) = 6.400001e+01y-coordinate: min (m) = -4.538534e+00, max (m) = 6.400000e+01

Volume statistics:minimum volume (m3): 2.782193e-01maximum volume (m3): 3.926232e+00

total volume (m3): 1.682930e+03Face area statistics:minimum face area (m2): 8.015718e-01maximum face area (m2): 4.118252e+00

Checking number of nodes per cell.Checking number of faces per cell.Checking thread pointers.Checking number of cells per face.Checking face cells.Checking face handedness.Checking element type consistency.Checking boundary types:Checking face pairs.Checking periodic boundaries.Checking node count.Checking nosolve cell count.Checking nosolve face count.Checking face children.Checking cell children.Checking storage.Done.

The domain extents list the minimum and maximum x, y, and z coor-dinates in meters. The volume statistics include minimum, maximum,and total cell volume in m3. A negative value for the minimum volumeindicates that one or more cells have improper connectivity. Cells witha negative volume can often be identified using the Iso-Value Adaptioncapability to mark them for adaption and view them in the graphics win-dow. For more information on creating and viewing isovalue adaptionregisters, see Section 23.5. You must eliminate these negative volumesbefore continuing the flow solution process.



The topological information to be verified begins with the number offaces and nodes per cell. A triangular cell (2D) should have 3 facesand 3 nodes, a tetrahedral cell (3D) should have 4 faces and 4 nodes, aquadrilateral cell (2D) should have 4 faces and 4 nodes, and a hexahedralcell (3D) should have 6 faces and 8 nodes.

Next, the face handedness for each zone is checked. The zones shouldcontain all right-handed faces. Usually a grid with negative volumes willalso have left-handed faces. Again, you cannot obtain a flow solutionuntil you eliminate these connectivity problems.

The last topological verification is checking the element-type consistency.If a mesh does not contain mixed elements (quadrilaterals and trianglesor hexahedra and tetrahedra), FLUENT will determine that it does notneed to keep track of the element types. By doing so, it can eliminatesome unnecessary work.

For axisymmetric cases, the number of nodes below the x axis is listed.Nodes below the x axis are forbidden for axisymmetric cases, since theaxisymmetric cell volumes are created by rotating the 2D cell volumeabout the x axis; thus nodes below the x axis would create negativevolumes.

For solution domains with rotationally periodic boundaries, the mini-mum, maximum, average, and prescribed periodic angles are computed.A common mistake is to specify the angle incorrectly. For domains withtranslationally periodic boundaries, the boundary information is checkedto ensure that the boundaries are truly periodic.

Finally, the simplex counters are verified. The actual numbers of nodes,faces, and cells the solver has constructed are compared to the valuesspecified in the corresponding header declarations in the grid file. Anydiscrepancies are reported.

5.5.2 Repairing Duplicate Shadow Nodes

If the Grid/Check report includes the following message:

WARNING: node on face thread 2 has multiple shadows.


5.6 Reporting Grid Statistics

you can repair the grid using the following text commands:

• For coupled-type walls:

grid −→ modify-zones −→repair-duplicate-shadows

No further inputs are required.

• For periodic-type walls:

grid −→ modify-zones −→repair-periodic

If the interface is rotational periodic, you will be prompted for therotation angle.


There are several methods for reporting information about the grid afterit has been read into FLUENT. You can report the amount of memoryused by the current problem, the grid size, and statistics about the gridpartitions. Zone-by-zone counts of cells and faces can also be reported.

Information about grid statistics is provided in the following sections:

• Section 5.6.1: Grid Size

• Section 5.6.2: Memory Usage

• Section 5.6.3: Grid Zone Information

• Section 5.6.4: Partition Statistics

5.6.1 Grid Size

You can print out the numbers of nodes, faces, cells, and partitions inthe grid by selecting the Grid/Info/Size menu item.

Grid −→ Info −→Size

A partition is a piece of a grid that has been segregated for parallelprocessing (see Chapter 28).

A sample of the resulting output follows:



Grid Size

Level Cells Faces Nodes Partitions0 7917 12247 4468 1

2 cell zones, 11 face zones.

If you are interested in how the cells and faces are divided among thedifferent zones, you can use the Grid/Info/Zones menu item, as describedin Section 5.6.3.

If you are using the coupled explicit solver, the grid information will beprinted for each grid level. The grid levels result from creating coarse gridlevels for the FAS multigrid convergence acceleration (see Section 22.5.4).A sample of the resulting output is shown below:

Grid Size

Level Cells Faces Nodes Partitions0 7917 12247 4468 11 1347 3658 0 12 392 1217 0 13 133 475 0 14 50 197 0 15 17 78 0 1

2 cell zones, 11 face zones.

5.6.2 Memory Usage

During a FLUENT session you may want to check the amount of memoryused and allocated in the present analysis. FLUENT has a feature thatwill report the following information: the numbers of nodes, faces, cells,edges, and object pointers (generic pointers for various grid and graph-ics utilities) that are used and allocated; the amount of array memory(scratch memory used for surfaces) used and allocated; and the amountof memory used by the solver process.

You can obtain this information by selecting the Grid/Info/Memory Usagemenu item.



Grid −→ Info −→Memory Usage

The memory information will be different for UNIX and Windows sys-tems.

UNIX Systems

On UNIX systems, the process memory information includes the follow-ing:

• Process Static memory is essentially the size of the code itself.

• Process Dynamic memory is the allocated heap memory used tostore the grid and solution variables

• Process Total memory is the sum of static and dynamic memory.

Windows Systems

On Windows systems, the process memory information includes the fol-lowing:

• Process Physical memory is the allocated heap memory currentlyresident in RAM.

• Process Virtual memory is the allocated heap memory currentlyswapped to the Windows system page file.

• Process Total memory is the sum of physical and virtual memory.

Note the following:

• The memory information does not include the static (code) mem-ory.

• In the serial version of FLUENT, the heap memory value includesstorage for the solver (grid and solution variables), and Cortex (GUIand graphics memory), since Cortex and the solver are containedin the same process.



• In the parallel version, Cortex runs in its own process, so the heapmemory value includes storage for the grid and solution variablesonly.

On Windows systems, you can also get more information on the FLUENTprocess (or processes) by using the Task Manager (see your Windows doc-umentation for details). For the serial version, the process image namewill be something like fl542s.exe. For the parallel version, examplesof process image names are as follows: cx332.exe (Cortex), fl542.exe(solver host), and fl smpi542.exe (one solver node).

5.6.3 Grid Zone Information

You can print information in the console window about the nodes, faces,and cells in each zone using the Grid/Info/Zones menu item.

Grid −→ Info −→Zones

The grid zone information includes the total number of nodes and, foreach face and cell zone, the number of faces or cells, the cell (and, in 3D,face) type (triangular, quadrilateral, etc.), the boundary condition type,and the zone ID. Sample output is shown below:

Zone sizes on domain 1:21280 hexahedral cells, zone 4.

532 quadrilateral velocity-inlet faces, zone 1.532 quadrilateral pressure-outlet faces, zone 2.

1040 quadrilateral symmetry faces, zone 3.1040 quadrilateral symmetry faces, zone 7.

61708 quadrilateral interior faces, zone 5.1120 quadrilateral wall faces, zone 6.

23493 nodes.

5.6.4 Partition Statistics

You can print grid partition statistics in the console window by selectingthe Grid/Info/Partitions menu item.

Grid −→ Info −→Partitions


5.7 Modifying the Grid

The statistics include the numbers of cells, faces, interfaces, and neigh-bors of each partition. See Section 28.4.5 for further details, includingsample output.


There are several ways in which you can modify or manipulate the gridafter it has been read into FLUENT. You can scale or translate the grid,merge or separate zones, create or slit periodic zones, and fuse bound-aries. In addition, you can reorder the cells in the domain to decreasebandwidth. Smoothing and diagonal swapping, which can be used to im-prove the mesh, are described in Section 23.11. Methods for partitioninggrids to be used in a parallel solver are discussed in Section 28.4.

Whenever you modify the grid, you should be sure to save a new case!file (and a data file, if data exist). If you have old data files that youwould like to be able to read in again, be sure to retain the original casefile as well, as the data in the old data files may not correspond to thenew case file.

Information about grid manipulation is provided in the following sec-tions:

• Section 5.7.1: Scaling the Grid

• Section 5.7.2: Translating the Grid

• Section 5.7.3: Merging Zones

• Section 5.7.4: Separating Zones

• Section 5.7.5: Creating Periodic Zones

• Section 5.7.6: Slitting Periodic Zones

• Section 5.7.7: Fusing Face Zones

• Section 5.7.8: Slitting Face Zones

• Section 5.7.9: Extruding Face Zones

• Section 5.7.10: Reordering the Domain and Zones



5.7.1 Scaling the Grid

Internally, FLUENT stores the computational grid in meters, the SI unitof length. When grid information is read into the solver, it is assumedthat the grid was generated in units of meters. If your grid was createdusing a different unit of length (inches, feet, centimeters, etc.), you mustscale the grid to meters. To do this, you can select from a list of com-mon units to convert the grid or you can supply your own custom scalefactors. Each node coordinate will be multiplied by the correspondingscale factor.

Scaling can also be used to change the physical size of the grid. Forinstance, you could stretch the grid in the x direction by assigning ascale factor of 2 in the x direction and 1 in the y and z directions.This would double the extent of the grid in the x direction. However,you should use anisotropic scaling with caution, since it will change theaspect ratios of the cells in your grid.

If you plan to scale the grid in any way, you should do so before you!initialize the flow or begin calculations. Any data that exist when youscale the grid will be invalid.

You will use the Scale Grid panel (Figure 5.7.1) to scale the grid from astandard unit of measurement or to apply custom scaling factors.

Grid −→Scale...

Using the Scale Grid Panel

The procedure for scaling the grid is as follows:

1. Indicate the units in which you created the grid by choosing the ap-propriate abbreviation for centimeters, millimeters, inches, or feetin the Grid Was Created In drop-down list. The Scale Factors willautomatically be set to the correct values (e.g., 0.0254 meters/inchor 0.3048 meters/foot). If you created your grid using units otherthan those in the list, you can enter the Scale Factors (e.g., thenumber of meters per yard) manually.



Figure 5.7.1: The Scale Grid Panel

2. Click on the Scale button. The Domain Extents will be updatedto show the correct range in meters. If you prefer to use youroriginal unit of length during the FLUENT session, you can followthe procedure described below to change the unit.

Changing the Unit of Length

As mentioned above in step 2, when you scale the grid you do not changethe units; you just convert the original dimensions of your grid pointsfrom your original units to meters by multiplying each node coordinateby the specified Scale Factors. If you want to work in your original units,instead of in meters, you can click on the Change Length Units button.This is a shortcut for changing the length unit in the Set Units panel(see Section 4.4). When you click on the Change Length Units button,the Domain Extents will be updated to show the range in your originalunits. This unit will be used for all future inputs of length quantities.



Unscaling the Grid

If you use the wrong scale factor, accidentally click the Scale buttontwice, or wish to undo the scaling for any other reason, you can clickon the UnScale button. “Unscaling” simply divides each of the nodecoordinates by the specified Scale Factors. (Selecting m in the Grid WasCreated In list and clicking on Scale will not unscale the grid.)

Changing the Physical Size of the Grid

You can also use the Scale Grid panel to change the physical size of thegrid. For example, if your grid is 5 feet by 8 feet, and you want to modelthe same geometry with dimensions twice as big (10 × 16), you can enter2 for the X and Y Scale Factors and click on Scale. The Domain Extentswill be updated to show the new range.

5.7.2 Translating the Grid

You can “move” the grid by applying prescribed offsets to the Cartesiancoordinates of all the nodes in the grid. This would be necessary for arotating problem if the grid were set up with the axis of rotation notpassing through the origin, or for an axisymmetric problem if the gridwere set up with the axis of rotation not coinciding with the x axis. Itis also useful if, for example, you want to move the origin to a particularpoint on an object (such as the leading edge of a flat plate) to make anXY plot have the desired distances on the x axis.

You can translate grid points in FLUENT using the Translate Grid panel(Figure 5.7.2).

Grid −→Translate...

Using the Translate Grid Panel

The procedure for translating the grid is as follows:

1. Enter the desired translations in the x, y, and (for 3D) z directions(i.e., the desired delta in the axes origin) in the X, Y, and Z fields



under Translation Offsets. You can specify positive or negative realnumbers in the current unit of length.

2. Click on the Translate button. The Domain Extents will be updatedto display the new extents of the translated grid. (Note that theDomain Extents are purely informational; you cannot edit themmanually.)

Figure 5.7.2: The Translate Grid Panel

5.7.3 Merging Zones

To simplify the solution process, you may want to merge zones. Mergingzones involves combining multiple zones of similar type into a single zone.Setting boundary conditions and postprocessing may be easier after youhave merged similar zones.

Zone merging is performed in the Merge Zones panel (Figure 5.7.3).

Grid −→Merge...



Figure 5.7.3: The Merge Zones Panel

When to Merge Zones

FLUENT allows you to merge zones of similar type into a single zone.This is not necessary unless the number of zones becomes prohibitive toefficient setup or postprocessing of the numerical analysis. For example,setting the same boundary condition parameters for a large number ofzones can be time-consuming and may introduce inconsistencies. In ad-dition, the postprocessing of the data often involves surfaces generatedusing the zones. A large number of zones often translates into a largenumber of surfaces that must be selected for the various display options,such as color contouring. Fortunately, surfaces can also be merged (seeSection 24.11), minimizing the negative impact of a large number ofzones on postprocessing efficiency.

Although merging zones can be helpful, there may be cases where youwill want to retain a larger number of zones. Since the merging processis not fully reversible, a larger number of zones provides more flexibilityin imposing boundary conditions. Although a large number of zones canmake selection of surfaces for display tedious, it can also provide morechoices for rendering the grid and the flow-field solution. For instance, itcan be difficult to render an internal flow-field solution. If the outer do-main is composed of several zones, the grids of subsets of these zones can



be plotted along with the solution to provide the relationship betweenthe geometry and solution field.

Using the Merge Zones Panel

The procedure for merging multiple zones of the same type into a singlezone is as follows:

1. Select the zone type in the Multiple Types list. This list containsall the zone types for which there are multiple zones. When youchoose a type from this list, the corresponding zones will appearin the Zones of Type list.

2. Select two or more zones in the Zones of Type list.

3. Click on the Merge button to merge the selected zones.

Remember to save a new case file (and a data file, if data exist).!

5.7.4 Separating Zones

There are several methods available in FLUENT that allow you to sepa-rate a single face or cell zone into multiple zones of the same type. If yourgrid contains a zone that you want to break up into smaller portions,you can make use of these features. For example, if you created a singlewall zone when generating the grid for a duct, but you want to specifydifferent temperatures on specific portions of the wall, you will need tobreak that wall zone into two or more wall zones. If you plan to solve aproblem using the sliding mesh model or multiple reference frames, butyou forgot to create different fluid zones for the regions moving at dif-ferent speeds, you will need to separate the fluid zone into two or morefluid zones.

After performing any of these separations, you should save a new case!file. If data exist, they are automatically assigned to the proper zoneswhen separation occurs, so you should also write a new data file.

There are four ways to separate face zones and two ways to separatecell zones. The face separation methods will be described first, followed



by the cell separation tools. Slitting (decoupling) of periodic zones isdiscussed in Section 5.7.6.

Note that all of the separation methods allow you to report the result ofthe separation before you commit to performing it.

Separating Face Zones

Methods for Separating Face Zones

For geometries with sharp corners, it is often easy to separate face zonesbased on significant angle. Faces with normal vectors that differ by anangle greater than or equal to the specified significant angle will be placedin different zones. For example, if your grid consists of a cube, and all 6sides of the cube are in a single wall zone, you would specify a significantangle of 89◦. Since the normal vector for each cube side differs by 90◦

from the normals of its adjacent sides, each of the 6 sides will be placedin a different wall zone.

If you have a small face zone and would like to put each face in the zoneinto its own zone, you can do so by separating the faces based on face.

You can also separate face zones based on the marks stored in adap-tion registers. For example, you can mark cells for adaption based ontheir location in the domain (region adaption), their boundary close-ness (boundary adaption), isovalues of some variable, or any of the otheradaption methods discussed in Chapter 23. When you specify whichregister is to be used for the separation of the face zone, all faces of cellsthat are marked will be placed into a new face zone. (Use the ManageAdaption Registers panel to determine the ID of the register you wish touse.)

Finally, you can separate face zones based on contiguous regions. Forexample, when you use coupled wall boundary conditions you need thefaces on the zone to have a consistent orientation. Consistent orienta-tion can only be guaranteed on contiguous regions, so you may need toseparate face zones to allow proper boundary condition specification.



Inputs for Separating Face Zones

To break up a face zone based on angle, face, adaption mark, or region,use the Separate Face Zones panel (Figure 5.7.4).

Grid −→ Separate −→Faces...

Figure 5.7.4: The Separate Face Zones Panel

If you are planning to separate face zones, you should do so before!performing any adaptions using the (default) hanging node adaptionmethod. Face zones that contain hanging nodes cannot be separated.

The steps for separating faces are as follows:

1. Select the separation method (Angle, Face, Mark, or Region) underOptions.

2. Specify the face zone to be separated in the Zones list.

3. If you are separating by face or region, skip to the next step. Oth-erwise, do one of the following:

• If you are separating faces by angle, specify the significantangle in the Angle field.



• If you are separating faces by mark, select the adaption reg-ister to be used in the Registers list.

4. (optional) To check what the result of the separation will be beforeyou actually separate the face zone, click on the Report button. Thereport will look like this:

Zone not separated.45 faces in contiguous region 030 faces in contiguous region 111 faces in contiguous region 214 faces in contiguous region 3Separates zone 4 into 4 zone(s).

5. To separate the face zone, click on the Separate button. FLUENTwill print the following information:

45 faces in contiguous region 030 faces in contiguous region 111 faces in contiguous region 214 faces in contiguous region 3Separates zone 4 into 4 zone(s).Updating zone information ...created zone wall-4:001created zone wall-4:002created zone wall-4:010

done.

When you separate the face zone by adaption mark, you may sometimes!find that a face of a corner cell will be placed in the wrong face zone. Youcan usually correct this problem by performing an additional separation,based on angle, to move the offending face to a new zone. You can thenmerge this new zone with the zone in which you want the face to beplaced, as described in Section 5.7.3.



Separating Cell Zones

Methods for Separating Cell Zones

If you have two or more enclosed cell regions sharing internal boundaries(as shown in Figure 5.7.5), but all of the cells are contained in a single cellzone, you can separate the cells into distinct zones using the separation-by-region method. Note that if the shared internal boundary is of typeinterior, you must change it to another double-sided face zone type (fan,radiator, etc.) prior to performing the separation.

zone 1 zone 2

Figure 5.7.5: Cell Zone Separation Based on Region

You can also separate cell zones based on the marks stored in adaptionregisters. You can mark cells for adaption using any of the adaptionmethods discussed in Chapter 23 (e.g., you can mark cells with a certainisovalue range or cells inside or outside a specified region). When youspecify which register is to be used for the separation of the cell zone,cells that are marked will be placed into a new cell zone. (Use the ManageAdaption Registers panel to determine the ID of the register you wish touse.)

Inputs for Separating Cell Zones

To break up a cell zone based on region or adaption mark, use the Sep-arate Cell Zones panel (Figure 5.7.6).

Grid −→ Separate −→Cells...

If you are planning to separate cell zones, you should do so before!



Figure 5.7.6: The Separate Cell Zones Panel

performing any adaptions using the (default) hanging node adaptionmethod. Cell zones that contain hanging nodes cannot be separated.

The steps for separating cells are as follows:

1. Select the separation method (Mark or Region) under Options.

2. Specify the cell zone to be separated in the Zones list.

3. If you are separating cells by mark, select the adaption register tobe used in the Registers list.

4. (optional) To check what the result of the separation will be beforeyou actually separate the cell zone, click on the Report button. Thereport will look like this:

Zone not separated.Separates zone 14 into two zones, with 1275 and 32 cells.

5. To separate the cell zone, click on the Separate button. FLUENTwill print the following information:



Separates zone 14 into two zones, with 1275 and 32 cells.No faces marked on thread, 2No faces marked on thread, 3No faces marked on thread, 1No faces marked on thread, 5No faces marked on thread, 7No faces marked on thread, 8No faces marked on thread, 9No faces marked on thread, 61Separates zone 62 into two zones, with 1763 and 58 faces.All faces marked on thread, 4No faces marked on thread, 66Moved 20 faces from face zone 4 to zone 6Updating zone information ...Moved 32 cells from cell zone 14 to zone 10created zone interior-4created zone interior-6created zone fluid-14:010

done.

As shown in the example above, separation of a cell zone will often resultin the separation of face zones as well. When you separate by mark, facesof cells that are moved to a new zone will be placed in a new face zone.When you separate by region, faces of cells that are moved to a new zonewill not necessarily be placed in a new face zone.

If you find that any faces are placed incorrectly, see the comment above,at the end of the inputs for face zone separation.

5.7.5 Creating Periodic Zones

FLUENT allows you to set up periodic boundaries using either conformalor non-conformal periodic zones. You can assign periodicity to your gridby coupling a pair of face zones. If the two zones have identical node andface distributions, you can create a conformal periodic zone. If the twozones are not identical at the boundaries within a specified tolerance,



then you can create a non-conformal periodic zone. (See Section 5.4 formore information about non-conformal grids.)

Remember to save a new case file (and a data file, if data exist) after!creating or slitting a periodic boundary.

Creating Conformal Periodic Zones

To create conformal periodic boundaries, you will use the make-periodictext command.

grid −→ modify-zones −→make-periodic

You will need to specify the two face zones that will comprise the periodicpair (you can enter their full names or just their IDs), and indicatewhether they are rotationally or translationally periodic. The order inwhich you specify the periodic zone and its matching shadow zone is notsignificant.

/grid/modify-zones> mp

Periodic zone [()] 1

Shadow zone [()] 4Rotational periodic? (if no, translational) [yes] nCreate periodic zones? [yes] yes

computed translation deltas: -2.000000 -2.000000all 10 faces matched for zones 1 and 4.

zone 4 deletedCreated periodic zones.

When you create a conformal periodic boundary, the solver will check tosee if the faces on the selected zones “match” (i.e., whether or not thenodes on corresponding faces are coincident). The matching tolerancefor a face is a fraction of the minimum edge length of the face. If theperiodic boundary creation fails, you can change the matching toleranceusing the matching-tolerance command, but it should not exceed 0.5or you may match up the periodic zones incorrectly and corrupt the grid.



grid −→ modify-zones −→matching-tolerance

Creating Non-Conformal Periodic Zones

To create non-conformal periodic boundaries, you need to change thenon-conformal periodic zones to interface zones. You will then need to setup the origin as well as the axes of the adjacent cell zone. For example, ifinterface-15 and interface-2 are the two non-conformal periodic zones, thenon-conformal periodic boundaries are set up using the make-periodiccommand in the define/grid-interfaces text menu.

define −→ grid-interfaces −→make-periodic

For example:

/define/grid-interfaces> make-periodic

Periodic zone [()] interface-15

Shadow zone [()] interface-2Rotational periodic? (if no, translational) [yes] yes

Rotation angle (deg) [0] 40.0Create periodic zone? [yes] yes

grid-interface name [] fan-periodic

5.7.6 Slitting Periodic Zones

If you wish to decouple the zones in a periodic pair, you can use theslit-periodic command.

grid −→ modify-zones −→slit-periodic

You will specify the periodic zone’s name or ID, and the solver willdecouple the two zones in the pair (the periodic zone and its shadow)and change them to two symmetry zones:



/grid/modify-zones> sp

periodic zone [()] periodic-1

Separated periodic zone.

5.7.7 Fusing Face Zones

The face fusing utility is a convenient feature that can be used to fuseboundaries (and merge duplicate nodes and faces) created by assemblingmultiple mesh regions. When the domain is divided into subdomains andthe grid is generated separately for each subdomain, you will combine thesubdomains into a single file before reading the grid into the solver. (SeeSection 5.3.10 for details.) This situation could arise if you generate eachblock of a multiblock grid separately and save it to a separate grid file.Another possible scenario is that you decided, during grid generation,to save the mesh for each part of a complicated geometry as a separatepart file. (Note that the grid node locations need not be identical at theboundaries where two subdomains meet; see Section 5.4 for details.)

The Fuse Face Zones panel (Figure 5.7.7) allows you to merge the dupli-cate nodes and delete these artificial internal boundaries.

Grid −→Fuse...

The boundaries on which the duplicate nodes lie are assigned zone IDnumbers (just like any other boundary) when the grid files are combined,as described in Section 5.3.10. You need to keep track of the zone IDnumbers when tmerge or TGrid reports its progress or, after the completegrid is read in, display all boundary grid zones and use the mouse-probebutton to determine the zone names (see Section 25.3 for informationabout the mouse button functions).

Inputs for Fusing Face Zones

The steps for fusing face zones are as follows:

1. Select the zones to be fused in the Zones list.

2. Click on the Fuse button to fuse the selected zones.



Figure 5.7.7: The Fuse Face Zones Panel

If all of the appropriate faces do not get fused using the default Tolerance,you should increase it and attempt to fuse the zones again. (This toler-ance is the same as the matching tolerance discussed in Section 5.7.5.)The Tolerance should not exceed 0.5, or you may fuse the wrong nodes.

Remember to save a new case file (and a data file, if data exist) after!fusing faces.

Fusing Zones on Branch Cuts

Meshes imported from structured mesh generators or solvers (such asFLUENT 4) can often be O-type or C-type grids with a reentrant branchcut where coincident duplicate nodes lie on a periodic boundary. SinceFLUENT uses an unstructured grid representation, there is no reason toretain this artificial internal boundary. (Of course, you can preserve theseperiodic boundaries and the solution algorithm will solve the problemwith periodic boundary conditions.)

To fuse this periodic zone with itself, you must first slit the periodiczone, as described in Section 5.7.6. This will create two symmetry zonesthat you can fuse using the procedure above.

Note that if you need to fuse portions of a non-periodic zone with itself,



you must use the text command fuse-face-zones.

grid −→ modify-zones −→fuse-face-zones

This command will prompt you for the name or ID of each zone tobe fused. (You will enter the same zone twice.) To change the nodetolerance, use the matching-tolerance command.

5.7.8 Slitting Face Zones

The face-zone slitting feature has two uses:

• You can slit a single boundary zone of any double-sided type (i.e.,any boundary zone that has cells on both sides of it) into twodistinct zones.

• You can slit a coupled wall zone into two distinct, uncoupled wallzones.

When you slit a face zone, the solver will duplicate all faces and nodes,except those nodes that are located at the ends (2D) or edges (3D) ofthe zone. One set of nodes and faces will belong to one of the resultingboundary zones and the other set will belong to the other zone. Theonly ill effect of the shared nodes at each end is that you may see someinaccuracies at those points when you graphically display solution datawith a slit boundary. (Note that if you adapt the boundary after slitting,you will not be able to fuse the boundaries back together again.)

Normally, you will not need to manually slit a face zone. Two-sided wallsare automatically slit, but remain coupled. (This coupling refers only tothe grid; it is independent of thermal coupling.) The adaption processtreats these coupled walls just like periodic boundaries; adapting on onewall causes the same adaption on the shadow. If you want to adapt onewall independent of its shadow, you should slit the coupled wall to obtaintwo distinct walls.

You should not confuse “slitting” a face zone with “separating” a face!zone. When you slit a face zone, additional faces and nodes are createdand placed in a new zone. When you separate a face zone, a new zone



will be created, but no new faces or nodes are created; the existing facesand nodes are simply redistributed among the zones.

Inputs for Slitting Face Zones

If you wish to slit a face zone, you can use the slit-face-zone com-mand.

grid −→ modify-zones −→slit-face-zone

You will specify the face zone’s name or ID, and the solver will replacethe zone with two zones:

/grid/modify-zones> slfz

face zone id/name [] wall-4

zone 4 deletedface zone 4 createdface zone 10 created

Remember to save a new case file (and a data file, if data exist) after!slitting faces.

5.7.9 Extruding Face Zones

The ability to extrude a boundary face zone allows you to extend thesolution domain without having to exit the solver. A typical applica-tion of the extrusion capability is to extend the solution domain whenrecirculating flow is impinging on a flow outlet. The current extrusioncapability creates prismatic or hexahedral layers based on the shape ofthe face and normal vectors computed by averaging the face normals tothe face zone’s nodes. You can define the extrusion process by specifyinga list of displacements (in SI units) or by specifying a total distance (inSI units) and parametric coordinates.

Specifying Extrusion by Displacement Distances

You can specify the extrusion by entering a list of displacement distances(in SI units) using the extrude-face-zone-delta command.



grid −→ modify-zones −→extrude-face-zone-delta

You will be prompted for the boundary face zone ID or name and a listof displacement distances.

Specifying Extrusion by Parametric Coordinates

You can specify the extrusion by specifying a distance (in SI units) andparametric coordinates using the extrude-face-zone-para command

grid −→ modify-zones −→extrude-face-zone-para

You will be prompted for the boundary face zone ID or name, a totaldistance, and a list of parametric coordinates. The list of parametriccoordinates should begin with 0.0 and end with 1.0. For example, thefollowing list of parametric coordinates would create two equally spacedextrusion layers: 0.0, 0.5, 1.0.

5.7.10 Reordering the Domain and Zones

Reordering the domain can improve the computational performance ofthe solver by rearranging the nodes, faces, and cells in memory. TheGrid/Reorder submenu contains commands for reordering the domain andzones, and also for printing the bandwidth of the present grid partitions.The domain can be reordered to increase memory access efficiency, andthe zones can be reordered for user-interface convenience. The band-width provides insight into the distribution of the cells in the zones andin memory.

To reorder the domain, select the Domain menu item.

Grid −→ Reorder −→Domain

To reorder the zones, select the Zones menu item.

Grid −→ Reorder −→Zones

Finally, you can print the bandwidth of the present grid partitions byselecting the Print Bandwidth menu item. This command prints the semi-bandwidth and maximum memory distance for each grid partition.

Grid −→ Reorder −→Print Bandwidth



Remember to save a new case file (and a data file, if data exist) after!reordering.

About Reordering

The Reverse Cuthill-McKee algorithm [44] is used in the reordering pro-cess to construct a “level tree” initiated from a “seed cell” in the domain.First a cell (called the seed cell) is selected using the algorithm of Gibbs,Poole, and Stockmeyer [74]. Each cell is then assigned to a level basedon its distance from the seed cell. These level assignments form the leveltree. In general, the faces and cells are reordered so that neighboringcells are near each other in the zone and in memory. Since most of thecomputational loops are over faces, you would like the two cells in mem-ory cache at the same time to reduce cache and/or disk swapping—i.e.,you want the cells near each other in memory to reduce the cost of mem-ory access. The present scheme reorders the faces and cells in the zone,and the nodes, faces, and cells in memory.

You may also choose to reorder the zones. The zones are reorderedby first sorting on zone type and then on zone ID. Zone reordering isperformed simply for user-interface convenience.

A typical output produced using the domain reordering is shown below:

>> Reordering domain using Reverse Cuthill-McKee method:zones, cells, faces, done.

Bandwidth reduction = 809/21 = 38.52Done.

If you print the bandwidth, you will see a report similar to the following:

Maximum cell distance = 21

The bandwidth is the maximum difference between neighboring cells inthe zone—i.e., if you numbered each cell in the zone list sequentially andcompared the differences between these indices.


chapter 5. reading and manipulating grids - enea 5. reading and manipulating grids ... to assign an...

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