1. Meshing Techniques in Ansys ICEM CFD 5.0
Abstract
Creating an adequate discretization of the geometry is an integral part of CFD and FEA, and this apparently simple task has proved to be rather challenging. Advanced meshing includes model diagnosis, geometry cleanup, and feature detection in a common environment. ANSYS ICEM CFD traditionally provides structured and unstructured grid generation for more than hundred CFD codes. CAD surface models to be analyzed by the Finite Element Method are often characterized by general n-sided, strongly curved boundaries, as well as by holes. Hybrid meshing, i.e. composition of different types of grids as well as automatic volume filling, requires an "outside-inside" approach. In addition, prescribed curves and points within the structure need to be taken into account. A recursive meshing scheme based on the "looping" algorithm satisfying the listed constraints will be presented. Finally, multiple automatic and semi-automatic pathways from CAD geometry to high quality meshes will be discussed.
Christian Nienhüser
ANSYS Germany

2. CADFEM CZ User’s Meeting
Contents
New Graphic User Interface
Geometry import, clean up
Meshing Tools
Surface meshing
Patch based
Patch independent
Volume meshing
Tetrahedral
Hexahedral
Hybrid
Outlook
Final remarks
Leading-edge
All-purpose meshing
Advanced meshing
October 2004
CADFEM CZ User’s Meeting

3. Complete CAE Pre-processing
What is ANSYS ICEM CFD?
Direct CAD
CATIA, Pro/E, I-DEAS, UG, SE, SW…
3rd Party CAD
IGES, ACIS, Parasolid…
Faceted Data
STL, Nastran, Patran, VRML…
ICEM CFD
Complete CAE Pre-processing
Create/Edit geometry
Generate mesh
Edit Mesh
Apply Poperties, B.C.’s
Export to Analysis
Over 100 supported solver formats.
October 2004
CADFEM CZ User’s Meeting

4. CADFEM CZ User’s Meeting
Development Process
ICEM CFD 4.2
ICEM CFD 4.CFX
AI*Environment 1.0
AI*Environment 2.0
ICEM CFD 4.3
AI*Environment / ICEM CFD 5.0
Classic GUI
Classic GUI
ICEM CFD / AI*Environment 5.1
October 2004
CADFEM CZ User’s Meeting

5. New Graphic User Interface
User interface completely redesigned
Integrates Med, Tetra, Prism, Hexa and Visual3
Layout is
Clean
Modern
Intuitive
For the ICEM CFD user, this is an entirely new user interface.
Multiple interfaces are consolidated into one (Med, Tetra, Hexa, Visual3).
Layout is clean, modern and intuitive
October 2004
CADFEM CZ User’s Meeting

6. New Graphic User Interface
Tabbed menus which follow the process
Logical function groups
Model tree
For display control
Right-click to access related options and operations
For the ICEM CFD user, this is an entirely new user interface.
Multiple interfaces are consolidated into one (Med, Tetra, Hexa, Visual3).
Layout is clean, modern and intuitive
October 2004
CADFEM CZ User’s Meeting

7. New Graphic User Interface
Selection and database management
Selection manager
Selection filters
Part selection by entity
Blanking of specific entities
Improved subsets for geometry/mesh
Easier methods of limiting data displayed
October 2004
CADFEM CZ User’s Meeting

8. New Graphic User Interface
Advanced help browser
Quick-help
For the ICEM CFD user, this is an entirely new user interface.
Multiple interfaces are consolidated into one (Med, Tetra, Hexa, Visual3).
Layout is clean, modern and intuitive
October 2004
CADFEM CZ User’s Meeting

9. CADFEM CZ User’s Meeting
Geometry Import
CATIA
Unigraphics
STL
Generic CAD
IGES, ACIS, DWG, STEP …
Direct CAD
CATIA, Pro/E, I-DEAS, UG, SolidWorks …
Faceted Data
STL, NASTRAN, Patran, VRML…
SolidWorks
ANSYS ICEM CFD
October 2004
CADFEM CZ User’s Meeting

10. CADFEM CZ User’s Meeting
Geometry Import
Hexa for CATIA V5
Blocks tied to Parametric geometric space - when new CAD model is generated, blocks get automatically refitted to design change.
ICEM CFD Hexa CAA V5 is a plug-in application for CATIA V5. This application allows the user to create Hexahedral volume meshes within CATIA V5. The ICEM CFD Hexa mesh parameters and family definitions are defined with the ICEM CFD toolbar. These meshing specifications and the resulting ICEM CFD Hexa mesh are stored within the CATIA V5 Analysis document. Users can make changes to the original design and the mesh can be regenerated based on the stored mesh set-up data.
Dassault is becoming a bigger player in aerospace and automotive.
Dassault has selected ANSYS-ICEM Hexa to be the CFD Hex meshing technology for CATIA.
Hexa plug-in released ‘02
Hexa native in CATIA V5 available Q2 ’04
October 2004
CADFEM CZ User’s Meeting

11. Model Diagnosis and Repair
Build topology function
Tolerance is set automatically on import
Curves are extracted from surface model
Enables model diagnosis
Curves form basis for geometry repair
In many analysis environments, geometry may come from a wide variety of sources. As such, we need to be able to deal with the geometry that comes in, regardless of whether it is clean or appropriate for the kind of modeling to be done. ICEM CFD provides tools to create an manipulate both CAD and faceted (STL style) geometry. Automated model diagnosis makes it clear where your model has holes or other problems. The models on this page illustrate some typical repair operations, such as model simplification and closing holes.
Imported CAD
October 2004
CADFEM CZ User’s Meeting

12. CADFEM CZ User’s Meeting
Geometry tools
Body definitions
From connectivity
Point inside solid
Repair
Topology checker to find problems in surface data
Surface closing
Surface retrimming/matching
Surface extension
Unsplit surface
Split by curvature
Modify surface normals
Bolt hole detection
Button detection
Fillet detection
Point creation
From screen
Explicit
Offset
Centerpoint
Between points
Endpoint
Intersection
Along curve
Projection to curve/surface
Surface editing
Surface from curves
Driven surface
Swept surface
Revolved surface
Offset surface
Midsurface
Adjust thickness
Lofted surface
Curtain surface
Surface trimming
Merged surface
Shrink wrapped surface
Standard surfaces
Faceted Surface tools
Curve creation
From points
Arc/Circle
Surface param
Intersection
Surface projection
Concatenation
Segmentation
Surface Boundary
Faceted curve tools
October 2004
CADFEM CZ User’s Meeting

13. STL to B-Spline conversion
Circle fom points
Use geometry repair
Convert multiblock mesh to b-spline
Reverse engineering and legacy data support
STL to B-Spline conversion example
Import STL scan data, or import mesh as geometry
Clean STL geometry.
Use “circle from points” on scan cloud to create curves at desired feature lines.
Create a structured surface mesh in HEXA.
Hexa Blocking approach is flexible and a robust method to walk over details, even large holes in model
Convert mesh to B-spline surfaces
Perfect utility for reverse engineering and legacy data support
October 2004
CADFEM CZ User’s Meeting

14. CADFEM CZ User’s Meeting
Quad surface meshing
patch dependent
CAD geometry
All-quadrilateral mesh
Edge model
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CADFEM CZ User’s Meeting

15. Auto-structured meshing CADFEM CZ User’s Meeting
Quad Surface Meshing
Auto-structured meshing
Mapping = 0
Mapping = 0.2
October 2004
CADFEM CZ User’s Meeting

16. CADFEM CZ User’s Meeting
Extensions
Arbitrary boundary
Holes, prescribed curves and prescribed points
One/some transition triangles
Auto-structured
Mesh refinement
October 2004
CADFEM CZ User’s Meeting

17. Surface meshing enhancements
Unstructured quad meshing algorithm integrated into Hexa mesher.
Allows the easy creation of hybrid structured/unstructured meshes.
Ideal mesh for crash and NVH.
Initial auto blocking from CAD surfaces.
Automatic merging of regions to create a better blocking.
Geometry feature recognition in meshing.
October 2004
CADFEM CZ User’s Meeting

18. Tetra meshing (Octree)
Initial mesh of “Maximum size” elements fills a bounding box
ICEM TETRA is driven from a tetrahedral octree mesh. Some Tet meshers start with a cube and subdivide that into smaller cubes and then subdivide the cubes into tets. With ICEM TETRA we start from a Tet that has the property that it can be subdivided into 8 smaller tetrahedra that are congruent to each other and to the original tet. The edge lengths of this tet are not all equal. If two opposite edges have length 1.0 then the other four edges will have length sqrt(3)/2 = If you subdivide a cube into 5 (6!) tets then the ratio of the longest edge to the smallest edge is sqrt(2) = compared to for the tets produced by ICEM TETRA. You can't fill up space with equilateral tets. But you can fill up space with ICEM TETRA tets and, in fact, by any reasonable measure these are the highest quality tets that you can fill space with. So, actually, in a uniform-size mesh in the interior of the mesh (away from the surfaces) the mesh produced by ICEM TETRA is the mathematically optimal mesh. Near surfaces and in regions where there is a transition in size this is not true and an advancing front mesher may or may not have a better mesh quality. So an ICEM CFD user can generate a surface mesh with ICEM TETRA and smooth that to look as good as any mesh produced by any software and then fill the volume with the Delaunay mesher from Inria, see above.
October 2004
CADFEM CZ User’s Meeting

19. Tetra meshing (Octree)
Mesh is subdivided to meet specified sizes
Nodes adjusted to conform to geometry
October 2004
CADFEM CZ User’s Meeting

20. Tetra meshing (Octree)
“Flood fill” process finds volume boundaries
Improved in V5.0: Automated fixing of leaks encountered during this flood fill process.
“Flood fill” process finds volume boundaries - Elements outside the domain are discarded.
Technology roadmap: Tetra
Incremental cutting of mesh saves ½ memory.
Pointer compression saves ½ memory
Combined allows meshing 4x as many elements with same amount or memory
Improved shared memory parallelism.
Delaunay mesher complements existing octree.
Re-meshing of bad areas with Delaunay mesher.
Tet to hex converter available.
Distributed memory version under development.
October 2004
CADFEM CZ User’s Meeting

21. Tetra meshing (Octree) Patch Independence
Mesh “walks over” gaps, overlaps, holes, details
A perfect CAD model is not required
Greatly reduces the amount of required CAD cleanup
Surfaces near stamped label
Mesh “walks over” the detail
October 2004
CADFEM CZ User’s Meeting

22. Tetra meshing (Octree)
Improved: ‘bounding box’ by ‘geometry blow-up’
Improved in V5.0: Automated fixing of leaks encountered during this flood fill process.
“Flood fill” process finds volume boundaries - Elements outside the domain are discarded.
Technology roadmap: Tetra
Incremental cutting of mesh saves ½ memory.
Pointer compression saves ½ memory
Combined allows meshing 4x as many elements with same amount or memory
Improved shared memory parallelism.
Delaunay mesher complements existing octree.
Re-meshing of bad areas with Delaunay mesher.
Tet to hex converter available.
Distributed memory version under development.
October 2004
CADFEM CZ User’s Meeting

23. Tetra meshing: Improved Auto-Volume Detection
Material points are automatically created
Multiple volumes are supported
October 2004
CADFEM CZ User’s Meeting

24. Tetra meshing: Part-by-Part
Enables assemblies to be meshed in batch as component parts
Mesh is non-conformal
October 2004
CADFEM CZ User’s Meeting

25. Tetra meshing: Improved Volume Mesh Controls
Point, line, and volume density
Width parameter extends effect
Volume defined by points or by bounding box around selected entities
Point density
October 2004
CADFEM CZ User’s Meeting

26. CADFEM CZ User’s Meeting
Hybrid meshes
Tet to hex converter
We actually have two methods to convert from Tet to Hex mesh. The first method converts each tet into four hexes and will convert all the elements.  This will obviously increase the cell count dramatically. The second method converts 12 tets into one hex where possible.  In that case, the cell count can go down quite dramatically.  Using this method, regions away from the problem boundaries tend to get converted to hex elements, while regions near boundaries tend to remain as tets.  Most people who use this method tend to create tet meshes which are initially a bit fine, so that the hex mesh regions after conversion aren't too coarse.  In general, this method works better in meshes with a large volume and not too much surface area, as "long-thin" regions tend to stay as all tets.
Best used with octree tetra mesher.
October 2004
CADFEM CZ User’s Meeting

27. CADFEM CZ User’s Meeting
Prism meshing
Prism layers in pure tetra meshes
Often better then tri to hex conversion because the hexas are in the important boundary regions.
October 2004
CADFEM CZ User’s Meeting

28. CADFEM CZ User’s Meeting
Prism meshing
Prism layers in hybrid meshes
Extrusion of mixed quad/triangle meshes.
Extrusion of quad as well as triangle meshes.
October 2004
CADFEM CZ User’s Meeting

29. CADFEM CZ User’s Meeting
Hybrid meshing
New hexahedral-dominant meshing
Written by: Barry Joe, Zhou Computing Services Inc., 79 MacEwan Glen Rd NW, Calgary, AB, Canada T3K 2J3, Phone: (403) , This software is only to be used by ANSYS, Inc. under the conditions of the June 2003 software licensing agreement between ZCS Inc. and ANSYS, Inc.
Hexahedral­dominant mesh generation is much slower than the other operations. Sweep submeshes are used where possible, otherwise an advancing front approach is used. The advancing front approach takes slightly greater than linear time, but the constant of proportionality is rather large due to the amount of intersection and other basic operations. The Geompack++ hexahedral­dominant mesher has a lot of enhancements over the Geompack90 one. There are some faster procedures used and fewer elements generated in general (particularly fewer pyramids and tetrahedra in the interior of the mesh). Also, with the given surface mesh, the number of hexahedra in the layer next to boundary and constrained faces is close to optimal, using the criterion that hexahedra incident on adjacent surface quadrilaterals (sharing an edge whose dihedral angle is approximately in the range 120 ffi to 240 ffi ) should share a common face. Without changing the given surface mesh, the current approach cannot likely be improved much further. Research is continuing on determining sufficient and necessary conditions on the quadrilateral surface mesh in order to generate a complete layer of hexahedra next to the subregion boundaries and ultimately an all­hexahedral mesh.
October 2004
CADFEM CZ User’s Meeting

30. Hybrid meshing (outside of Hexa)
Easily in any order/any element type
Volume tetra -> add prisms -> convert interior to hexas.
Surface tri/quad mesh -> add prisms -> fill with tets.
Surface quad mesh -> add boundary hexas -> fill with hexas/tetras/pyramids/prisms
October 2004
CADFEM CZ User’s Meeting

31. CADFEM CZ User’s Meeting
Hexa meshing
Multi-block structured
Volume is decomposed into hex sub-volumes.
These ‘blocks’ are like a very coarse all hex mesh which roughly represents the volume to be meshed.
Structured mesh is interpolated in blocks and projected to geometry.
Geometry
Blocking
Mesh
October 2004
CADFEM CZ User’s Meeting

32. Auto 2-D blocking approach
Each Rectangular surface gets a mapped block.
Arbitrary surfaces get free blocks.
Blocks are independent of surface topology (morphable).
Geometry recognition is used to group potentially problem surfaces.
Mapped block
Free block
October 2004
CADFEM CZ User’s Meeting

33. Hex or hex dominant meshes
Multiple automatic pathways to high quality all hex or hex dominant meshes.
Auto surface blocking followed by auto volume blocking.
Direct block decomposition method.
Auto surface blocking into surface meshing into hex dominant volume meshing.
Hexa tools can be used to assist either the surface blocking or the volume blocking.
Better parametric association
Hybrid meshing framework
October 2004
CADFEM CZ User’s Meeting

34. Auto blocking from surface blocking
Start from CAD surfaces.
Generate Auto 2-D blocks based on surfaces
Generate Auto 3-D blocks by filling in 2-D blocks with Hex dominant mesher
Automatically create mesh spacing and mesh with existing Hexa techniques.
Results in a high-quality all hex mesh.
All Hex
October 2004
CADFEM CZ User’s Meeting

35. True automatic blocking
Produces great blocking but sensitive to geometric details
October 2004
CADFEM CZ User’s Meeting

36. Hex meshing: Hex dominant mesher
Start from CAD geomety
Generate all quad surface mesh as before
Automatically fills surface mesh with uniform hex dominant mesh
User can then edit the hex dominant mesh by automatic smoothing and mesh editing
HEXA_8 : 4849
PENTA_6 : 287
PYRA_5 : 640
TETRA_4 : 313
October 2004
CADFEM CZ User’s Meeting

37. CADFEM CZ User’s Meeting
HEXA: System level
Hexa will do the bookkeeping to keep track of the connectivity of hybrid meshes.
Structured blocks.
Extruded blocks
All Quad
Quad + 1 Tri
Quad dominant
All Tri
Unstructured blocks:
Filled by Tetra
Filled by Hex-Dominant mesher
Filled by Delaunay mesher.
October 2004
CADFEM CZ User’s Meeting

38. CADFEM CZ User’s Meeting
Final remarks
Thank you!
Questions?
Unique Capabilities:
All Hex mesh.
Quad dominant meshing for crash.
Hybrid meshing for size-speed-accuracy constraints.
Handling of dirty or over-detailed geometry
Very large meshes
Support for multiple CFD/FEA solvers from one pre-processor.
Handling of faceted data or existing mesh.
•It can mesh poor CAD with holes and overlapping surfaces. This is a
very unique feature and it is the strongest selling point.
•It can make high quality hex meshes for models that would not be
possible to hex mesh in other systems.
•It has direct connections to all major CAD systems in either a reader or
an embedded system native in the CAD system. This allows for
parametric mesh generation.
•It has powerful mesh editing tools. You can move nodes and elements
interactively. There are enough mesh editing tools to build a mesh by
hand if you so desired! This functionality allows a user to always get the
high quality mesh they are looking for.
•There is always more than one way to do the same job in ICEMCFD.
This allows you to always get the job done. Never will you get the
situation were you cannot continue. This makes it perfect for high end
users who have to get the job done and have strict deadlines.
•It can write out to almost any solver. How many other companies
support 100+ output interfaces. Our framework is designed to work
independently of any particular solver, so if the customer can’t find a way
to make it work, we can do this for them. This is a big selling point with
Government and other in-house codes.
•Surface based CAD editing that supports CAD or Faceted data. The
repair tools could be sold as a completely stand-alone product, but given
for free to help reduce meshing time when needed. The geometry repair
and simplification tools are on par with CADFIX. With the patchindependent
meshing tools used in ICEM CFD, however, geometry repair
is rarely needed!
Thank you for your attention!
October 2004
CADFEM CZ User’s Meeting