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Mesh Generation and Automated Simulation Part II: Issues in Applied Mesh Generation

Mesh Generation and Automated Simulation Part II: Issues in Applied Mesh Generation. John R. Chawner Pointwise, Inc. Revelations. Meshing is the dirty little secret of CAE practitioners. Pre-processing consumes the vast majority of man-hours for any analysis (75-90%).

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Mesh Generation and Automated Simulation Part II: Issues in Applied Mesh Generation

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  1. Mesh Generation and Automated SimulationPart II: Issues in Applied Mesh Generation John R. Chawner Pointwise, Inc.

  2. Revelations • Meshing is the dirty little secret of CAE practitioners. • Pre-processing consumes the vast majority of man-hours for any analysis (75-90%). • There is no silver bullet.

  3. Applied Meshing • Interfacing to CAD • Mesh Types & Algorithms • Mesh Quality • Automatic vs. Manual • Software Aspects

  4. CAD Issues • Interoperability • Geometry Representations • Poor Geometric Quality 1999 report: CAD interoperability problems cost US automotive industry $1 billion per year

  5. CAD mesher CAD Interoperability:Plug-in • Pros: • no translation or conversion errors, • CAD GUI familiar to users • Cons: • analyst must have access to and be trained for CAD system

  6. CAD mesher CAD Interoperability:common kernel geometry kernel • Parasolid • ACIS • Open CASCADE • Nlib • Etc. • Pros: • Portable • Cons • Limits CAD choice or… • Requires multi-kernel approach

  7. direct access CAD mesher CAD Interoperability:direct access • CAD Services (www.omg.org) • CADScript (ITI?) • CGM (Sandia) • CAPRI (MIT) • CAD Native API (e.g. Pro/TOOLKIT) • Pros: • Direct access to CAD data • No need to understand CAD GUI • Cons: • Neutral interfaces limited to least common denominator • May require CAD license

  8. CAD mesher file transfer CAD Interoperability:indirect access • Neutral files: IGES, STEP • Native files: CATIA, Pro/E, etc. • Pros: • No CAD license, no CAD familiarity • Portable • Cons: • Possible translation errors

  9. CAD Interoperability:Other Issues • Level of interoperability • Geometry (aka “dumb” geometry) • Features • Bi-directional?

  10. CAD: Representations • Solids • Manifold vs. Non-manifold • Solid vs. Partial Solid vs. No Solids • B-Rep vs. CSG • NURBs and/or analytics • Discrete • Faceted data • Sub-division surfaces

  11. Missing components Too much detail Gaps & overlaps Slivers Improper trimming (tolerances) Translation errors IGES is the whipping boy - undeservedly 70% of CAD files don’t conform to the company’s internal drafting standards CAD: Quality Issues unavoidable

  12. Repair/heal the geometry CADfix, DEXCenter (ITI) TransMagic ACIS Healing CADIQ PrescientQA Services, services, services Fault tolerance Is repair an inherently unsolvable problem? Most successful near-term solution: Change process: man in the loop to bridge CAD and CAE CAD: Dealing with Poor Quality

  13. CAD: Other Issues • Defeaturing • Adding geometry • outer boundaries • missing pieces (e.g. windscreens)

  14. Applied Meshing • Interfacing to CAD • Mesh Types & Algorithms • Mesh Quality • Automatic vs. Manual • Software Aspects

  15. Mesh Quality Almost any solver can compute a good solution on a good mesh. A poor mesh will yield a poor solution from all but the most robust solver. Mesh Density Given enough mesh points, any solver can compute a good solution. Use too few points in critical locations and no solver will compute a good solution. Importance of the Mesh You have the most influence over the solution via the mesh.

  16. structured Mesh Type Unstr. hex • Largely out of your control • Limited by your solver’s capabilities • Primary issues • Analysis goals • Accuracy • Turnaround time • Secondary issues • Cell type(s) • Linear vs. polynomial elements • Boundary conforming or not unstructured cartesian hybrid Images courtesy of Iowa State, JPL, Centaursoft, NASA, NUMECA

  17. structured Structured Grid(hex) • Pro: Structured solvers are very efficient. • Pro: Good control over hex cell quality including stretching. • Pro: Methods are mature. • Con: Structured grids take a long time to generate because of topology. • Application: CFD Grid courtesy of Raytheon.

  18. Unstructured Mesh(tet) • Pro: High degree of automation • Pro: Tet meshers are “commodity” items • Con: hard to make stretched tets • Application: linear CSM, CEM, inviscid CFD unstructured

  19. Hybrid Mesh(hex/pyramid/prism/tet) • Pro: Balances automation (tet) & structure (hex, prism). • Con: (Semi)structured mesh is near complex geometry. • Application: CFD hybrid

  20. Hexahedral Mesh(hex) Unstr. hex • Pro: Automation (semi-automation?) • Con: Difficult • Application: CSM Hinge mesh courtesy of Simulation Works

  21. Cartesian(hex/?) • Pro: Vast majority of mesh trivial. • Con: Boundaries may be “stair stepped” –or- • Con: Complex cells near boundary • Application: CFD cartesian F-16 solution courtesy of Lockheed-Martin Aeronautics

  22. Structured vs. Unstructured Unstructured provides more opportunities for automation Structured solvers are more efficient Hex vs. Tet Hex: Fewer hex cells for same volumetric coverage Hex: cells easier to stretch Hex/Tet: Which is better? Tet: Automation! Mesh Type Relative Merits F-15 solution and mesh courtesy of Cobalt Solutions and USAF.

  23. Linear FEA Unstructured tets Non-linear FEA Hex (str or unstr) Viscous CFD Structured Grid Hybrid Mesh Inviscid CFD Unstructured Tets If algorithm is sensitive to number of cells (e.g. CEM which is n3), consider higher order elements. Mesh Type by Application Femur implant mesh courtesy of Cornell Univ. and TrueGrid.

  24. Mesh Algorithm • It just doesn’t matter. • All that matters is the resulting mesh, not how it was generated. • Unlike solvers, there aren’t any underlying meshing principles.

  25. Applied Meshing • Interfacing to CAD • Mesh Types & Algorithms • Mesh Quality • Automatic vs. Manual • Software Aspects

  26. Effects of Poor Mesh Quality • Solution accuracy decreases • Discretization error increases with element distortion • Solution convergence rate decreases • Iterations increase as minimum included angle increases

  27. Signs of Poor Mesh Quality • Large jumps in mesh size • Twisted cells • Cells with one or more very short edges • Problem: no agreed upon standard for mesh quality • How to measure and compare?

  28. Know your solver’s mesh quality criteria Exploit your mesher’s solver controls Case study: 20 min. of mesh smoothing reduces run time by 4 hours. Adaptive meshing Adjust mesh as dictated by solution Point insertion Point movement Improving Mesh Quality Grids courtesty of Nabla Ltd., EPFL,

  29. Applications • Interfacing to CAD • Mesh Types & Algorithms • Mesh Quality • Automatic vs. Manual • Software Aspects

  30. Automatic • Be aware of what automatic means. • au-to-ma-tic (adj.): without human intervention • automatic: better than we’re currently doing • Example: an automatic mesher that didn’t work half the time and never let you view the mesh • Be aware of conditions required for automation

  31. Applied Meshing • Interfacing to CAD • Mesh Types & Algorithms • Mesh Quality • Automatic vs. Manual • Software Aspects

  32. Software Aspects • Good news! Most (all?) CAE software comes with its own mesher. • Bad news! You’ll have to make a business case to buy something else. • Why would you want to use something else? • Meshing offers the best opportunities for CAE process improvement.

  33. Things to Consider • Is your meshing software contributing toward your primary goals? • Accurate and timely CAE • Are you using a different mesher for each CAE application?

  34. Resources • International Meshing Roundtable • www.IMR.sandia.gov • Meshing Research Corner (Steve Owen) • www.andrew.cmu.edu/user/sowen/mesh.html • Mesh generation on the web (Robert Schneiders) • www-users.informatik.rwth-aachen.de/~roberts/meshgeneration.html • Geometry in Action (David Eppstein) • http://www.ics.uci.edu/~eppstein/gina/meshgen.html

  35. Resources cont’ • International Society of Grid Generation • www.ISGG.org • CFD-Online • www.CFD-Online.com • CFD Review • www.CFDReview.com

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