mesh generation and automated simulation part ii: issues in applied mesh generation john r. chawner...
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Mesh Generation and Automated Simulation
Part II: Issues in Applied Mesh GenerationJohn R. Chawner
Pointwise, Inc.
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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.
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Applied Meshing• Interfacing to CAD• Mesh Types & Algorithms• Mesh Quality• Automatic vs. Manual• Software Aspects
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CAD Issues• Interoperability • Geometry
Representations • Poor Geometric Quality
1999 report: CAD interoperabilityproblems cost US automotive industry$1 billion per year
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CAD Interoperability:Plug-in
CADmesher
•Pros:•no translation or conversion errors, •CAD GUI familiar to users
•Cons: •analyst must have access to and be trained for CAD system
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CAD Interoperability:common kernel
mesher CAD
geometrykernel
•Parasolid•ACIS•Open CASCADE•Nlib•Etc.•Pros:
•Portable•Cons
•Limits CAD choice or…•Requires multi-kernel approach
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CAD Interoperability:direct access
CADmesher 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
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CAD Interoperability:indirect access
CADmesherfile
transfer
•Neutral files: IGES, STEP•Native files: CATIA, Pro/E, etc.
•Pros:•No CAD license, no CAD familiarity•Portable
•Cons:•Possible translation errors
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CAD Interoperability:Other Issues
• Level of interoperability– Geometry
(aka “dumb” geometry)– Features
• Bi-directional?
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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
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CAD: Quality Issues
• 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
unavoidable
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CAD: Dealing with Poor Quality
• 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
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CAD: Other Issues• Defeaturing• Adding geometry
– outer boundaries– missing pieces (e.g.
windscreens)
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Applied Meshing• Interfacing to CAD• Mesh Types & Algorithms• Mesh Quality• Automatic vs. Manual• Software Aspects
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Importance of the Mesh
• 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.
You have the most influence over the solution via the mesh.
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Mesh Type• 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
structured
unstructured
hybrid
cartesian
Unstr. hex
Images courtesy of Iowa State, JPL, Centaursoft, NASA, NUMECA
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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
structured
Grid courtesy of Raytheon.
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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
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Hybrid Mesh(hex/pyramid/prism/tet)
• Pro: Balances automation (tet) & structure (hex, prism).
• Con: (Semi)structured mesh is near complex geometry.
• Application: CFDhybrid
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Hexahedral Mesh(hex)
• Pro: Automation (semi-automation?)
• Con: Difficult• Application: CSM
Unstr. hex
Hinge mesh courtesy of Simulation Works
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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
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Mesh Type Relative Merits
• 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!
F-15 solution and mesh courtesy of Cobalt Solutions and USAF.
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Mesh Type by Application
• 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.
Femur implant mesh courtesy of Cornell Univ. and TrueGrid.
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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.
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Applied Meshing• Interfacing to CAD• Mesh Types & Algorithms• Mesh Quality• Automatic vs. Manual• Software Aspects
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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
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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?
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Improving Mesh Quality
• 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
Grids courtesty of Nabla Ltd., EPFL,
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Applications• Interfacing to CAD• Mesh Types & Algorithms• Mesh Quality• Automatic vs. Manual• Software Aspects
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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
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Applied Meshing• Interfacing to CAD • Mesh Types & Algorithms• Mesh Quality• Automatic vs. Manual• Software Aspects
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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.
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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?
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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
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Resources cont’
• International Society of Grid Generation– www.ISGG.org
• CFD-Online– www.CFD-Online.com
• CFD Review– www.CFDReview.com