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An Abstraction and Meshing Technique for Industry Problems

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Free edges of unstitched surfaces. Sections define a water-tight volume ... De-featuring --- Auto-merge (small feature removal) Higher order operator built ... – PowerPoint PPT presentation

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Title: An Abstraction and Meshing Technique for Industry Problems


1
An Abstraction and Meshing Technique for Industry
Problems
  • Michael Hancock, Debashis Basu, Ashish Das,
  • Nilanjan Mukherjee
  • ( Michael.Hancock_at_sdrc.com )

2
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

3
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
4
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

5
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

6
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
7
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
8
Section Creation
  • Abstraction on forward creation of sections

Auto create -- std.
Auto create -- adv.
Mesh on counter-bore
9
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)

10
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.
11
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 )
12
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
13
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
14
De-featuring Boundary Smoothing
  • De-featuring --- Boundary smoothing

Laplacian smoothing for nodes that has no
projection space
15
Re-featuring Bead Creation
  • Re-featuring --- Bead Creation

16
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 )

17
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
18
Example A Solid Part
  • 1481 surfaces, 1062 sections, 32985 par.tri,
    56578 par.tet

19
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

20
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.

21

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