An Abstraction and Meshing Technique for Industry Problems

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An Abstraction and Meshing Technique for Industry
Problems

• Michael Hancock, Debashis Basu, Ashish Das,
• Nilanjan Mukherjee
• ( Michael.Hancock_at_sdrc.com )

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Need for Abstraction

• Interoperability through STEP, IGES and Vendor to
  Vendor translator compatibility -- a must !!
• CAD Data quality becomes a major issue.
• Sliver surfaces
• Unstitched geometry
• Industry Approach for FE Analysis
• FIX the CAD and mesh
• Fix the CAD ( geometry ) with another CAD tool
• Make additional changes considering Analysis
  Intent
• Create and mesh an Abstraction of the geometry
  using virtual topology that only refers to the
  geometry
• Geometry is never modified
• Analysis Intent built-in to some degree
• Good mesh quality achievable
• Define Boundary Conditions on the abstraction

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Abstraction Process
Legacy FE data
STL data
Create abstraction
Mesh to meet
Group to criteria
requirement
De-feature
Re-feature
Analysis
cycle(s)
Analysis
cycle(s)
Geometry abstraction
Re-mesh to
Group to criteria
requirement
De-feature
Re-feature
Synthesized geometry
Native geometry
Imported Geometry
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Analysis Intent

• What is Analysis Intent ?
• Feature Removal, Simplification
• Suppression
• Features (thru hole, blind hole, etc)
• Edges
• Vertex
• Small feature removal (auto-merge)
• Bead Abstraction
• Boundary Smoothing
• Isthmus removal
• Design Alternatives (need New features)
• Bead Creation
• Replace existing curve to a new boundary

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Virtual Topology from Surfaces

• Virtual Topology
• The area entity of the virtual topology is called
  a Section
• Section can reference one or more surfaces
• Section -gtLoops -gtCurves-gtconnectors
• Section-surface relation is tracked
• If curve is on edge(s), curve-edge relation is
  tracked
• Characteristics
• Loops are always closed
• There could be multiple loops, but at least one
• A loop cannot self-intersect
• Section must be meshable
• Adjacent sections share a common boundary

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Shell of Sections forms a Water-Tight Virtual
Volume

• Sections can be made water-tight even if there
  are unstitched surfaces with gaps and overlaps

Sections define a water-tight volume
Free edges of unstitched surfaces
Volume is not water-tight
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Section Creation Options

• Basic option
• Hole suppression
• Curve Merging
• Surface Grouping
• Tolerance
• Advanced option
• Total Curvature
• Pre-process fillets
• Combine cylinders
• Surface merge based

on target element size
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Section Creation

• Abstraction on forward creation of sections

Auto create -- std.
Auto create -- adv.
Mesh on counter-bore
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Manual Modification Tools

• Manipulating the virtual topology entities
• changes section definition
• meshing reacts to the change
• boundary conditions react to the change
• Any removal operation can be undone by an add
  operation
• Some capabilities
• Add, Remove, Replace connectors
• Split, Merge, Stretch curves
• Add, Remove, Replace curves
• Split, Merge, Stitch sections ( sometimes loops
  )
• Add, Remove loops
• Un-suppress, suppress features attached to loop(s)

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De-featuring Remove Loop

• De-featuring --- entity suppression ( remove
  hole )

At times it is helpful to
be able to create hard
points for mesh to
snap to.
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De-featuring Isthmus Removal

• De-featuring --- Isthmus removal ( replace
  curve )

The 2 isthmus sections contain 2 outer loops each.
3 sections created
over 72 surfaces
An isthmus
section
Still 3 sections !!
Replace curve
operation removes the
isthmus. Still one section
30 mm mesh
30 mm mesh
with two outer loops.
( free mapped, allow tri )
( free mapped, allow tri )
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De-featuring Auto-merge Section

• De-featuring --- Auto-merge (small feature
  removal)
• Higher order operator built

518 surfaces
410 sections
Using auto-merge with 13 mm
to suit analysis intent
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De-featuring Bead Abstraction

• De-featuring --- Bead Abstraction

Two sections on surfaces
Extract median line
defining beads
defining bead
Mesh may go across
Ensure mesh to
the entire section
capture stiffness
May remove the rail
Final representation
curves of the beads
for bead and fillet
Create median line
on fillets
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De-featuring Boundary Smoothing

• De-featuring --- Boundary smoothing

Laplacian smoothing for nodes that has no
projection space
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Re-featuring Bead Creation

• Re-featuring --- Bead Creation

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Re-featuring Replace Curve

• Re-featuring --- Replace curve to a new
  boundary
• Mesh will get projected to underlying surfaces
• Additional options
• Ignore surface for projection
• Add surface for projection (surface is not a part
  of the solid )

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Example A Shell Part

• Completely unstitched geometry

• Automated Abstraction lt5 minutes
• Manual Editing of Abstraction lt15 minutes
• Mesh Generation lt 5 minutes

393 surfaces -gt112 sections -gt 5481 elements
created
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Example A Solid Part

• 1481 surfaces, 1062 sections, 32985 par.tri,
  56578 par.tet

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CAD neutral ?

• The Abstraction works on a CAD NEUTRAL level
  while providing flexibility for analysis and
  design modifications.
• Future direction Abstraction on Mesh
• Industry reaction
• Ford Power-train says time to mesh large
  power-train model was reduced by 75 in last one
  year
• ZF Friedrichshafen AG says section meshing is a
  key in the process chain

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Abstract

• AN ABSTRACTION AND MESHING TECHNIQUE FOR INDUSTRY
  PROBLEMS.
• Michael Hancock, Debashis Basu, Ashish Das and
  Nilanjan Mukherjee
• The most commonly used data exchange methods
  between the CAD and CAE application are (a)
  Direct translators, which are vendor-to-vendor
  data exchange, (b) Indirect translators, like
  IGES, STEP, STL and the like and (c) Consistent
  kernel, like ACIS, Parasolid and the like.
  Although there has been many thoughts on creating
  a CAD neutral framework, in reality the design
  data containing surface information is handed
  over to the analysis land through one of the
  above data exchange channels. Any surface data
  from an industry model, coming through one of the
  above data exchange channels, brings imperfect
  geometry with gaps, overlaps and surface
  degeneracy. Meshing such surfaces is highly
  unlikely to produce quality mesh with a desired
  density. This paper proposes an abstraction
  technique that creates an auxiliary simplified
  topology referencing the underlying imperfect
  (sometimes perfect) geometry. A meshing strategy
  is presented that works directly on the
  abstraction layer. The synergy between the
  abstraction and meshing techniques provides one
  way of achieving CAD neutrality, trying to
  embed the concept in the process. The idea here
  is to accept any surface data from any data
  exchange channel, be it stitched or unstitched,
  and create a watertight topology layer (the
  abstraction) spanning multiple surfaces which is
  then used for meshing. An application based on
  this abstraction technique provides the
  flexibility to remove topological details
  irrelevant for finite element analysis while
  providing the facility to perform design
  modification (for example elimination of holes)
  without altering the geometry. The technique
  eliminates the need to repair any underlying
  imperfect geometry. The meshing technique
  smoothes out effects from geometry deficiencies
  and works on any unspecified (void) region within
  the abstraction to produce a quality mesh of the
  desired density.

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