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 Christian.Nienhueser@ansys.com

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

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

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

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

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

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

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

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

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

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

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

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

CADFEM CZ User’s Meeting Quad surface meshing patch dependent CAD geometry All-quadrilateral mesh Edge model October 2004 CADFEM CZ User’s Meeting

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

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

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

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 = .866... . If you subdivide a cube into 5 (6!) tets then the ratio of the longest edge to the smallest edge is sqrt(2) = 1.414... compared to 1.1547 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

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

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

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

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

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

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

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

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

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

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

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) 730-1531, E-mail: bjoe@netcom.ca. 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

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

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

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

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

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

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

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

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

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