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Copyright 2006 ABAQUS, Inc.
Meshing Imported and Native
Geometry
Lecture 7
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
L7.2
Overview
Introduction
Dependent and Independent Part Instances
Mesh Generation Techniques
Enabling Various Meshing Techniques
Mesh Compatibility
Controlling Mesh Density and Gradation
Parametric Modeling
Assigning Element Types
Checking Mesh Quality and Obtaining Mesh Statistics
Workshop 8: Structured Hex Meshing: Pipe Creep Model
Workshop 9: Free and Swept Meshing: Pump Model
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
L7.3
Overview
This lecture is intended to provide a brief overview of the meshing
capabilities of ABAQUS/CAE.
Meshing and partitioning is discussed further in the "ABAQUS/CAE:
Geometry Import and Meshing" lecture notes.
Specific issues relating to element selection criteria are discussed in
Appendix 2 of these notes.
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Copyright 2006 ABAQUS, Inc.
Introduction
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Introduction to ABAQUS
L7.5
Introduction
What is a mesh?
Approximation of the geometry of the physical part model.
Discretized geometry including many geometrically simple nodes and
elements.
Necessary for the finite element analysis program to perform a
simulation. Defined by attributes that are features of the assembly.
Features defined in the Mesh module will regenerate if you modify
parameters of part or assembly features.
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Introduction to ABAQUS
L7.6
part geometry
Discretized geometry
nodes elements
Introduction
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Introduction to ABAQUS
L7.7
Introduction
General capabilities of the Mesh module
Allows you to mesh an assembly using various levels of automation and
controls to suit the needs of your analysis
Assign mesh attributes and set mesh controls to specify:
Meshing technique
Element shape
Element type
Mesh density
Generate the mesh
Query and verify the mesh for: Number of nodes and elements
Element type
Element quality
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Dependent and Independent
Part Instances
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Introduction to ABAQUS
L7.9
Dependent and Independent Part Instances
Concept of a part instance
A part instance is a representation of the part in the assembly
A part instance can either remain dependent on the original part or be
designated independent of the original part
Independent instances can be partitioned at the assembly level.
Multiple independent instances of a given part can be partitionedeach according to its own requirements (mesh, loads, etc.)
Each independent part instance must be meshed separately
Dependent instances cannot be partitioned at the assembly level.
All dependent instances of a given part share the same geometry as
the original part.
Thus, only the original part needs to be meshed
Its dependent instances will inherit its mesh
Any partitions must be made to the original part.
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Introduction to ABAQUS
L7.10
Dependent and Independent Part Instances
Misc.
For either dependent or independent instances:
Different attributes (loads, boundary conditions, etc.) and
sets/surfaces can be created.
All instances of a part must be either dependent or independent.
No mixture is allowed for a given part.
All orphan mesh instances must be dependent.
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Introduction to ABAQUS
L7.11
Dependent and Independent Part Instances
Part
-Repair features
-Shape features
-Partitions
-Seeds
-Mesh techniques
-Element types
-Virtual topology
-Mesh
Part
-Repair features
-Shape features
-Partitions
-Virtual topology
Dependent
instance
No mesh-related
features allowed
(The geometry and the
mesh cannot be
modified.)
Mesh the
part
Independent
instance
-Partitions
-Seeds
-Mesh techniques
-Element types
-Virtual topology
-Mesh
Mesh the
assembly
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Introduction to ABAQUS
L7.12
Dependent and Independent Part Instances
Choose Independent or
Dependent when creatingpart instance
Independent not allowed if:
Part is meshed
Dependent instances ofpart already exist
Part is an orphan mesh
Dependent not allowed if:
Independent instances of
part already exist
Can easily convert between
dependent and
independent
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Introduction to ABAQUS
L7.13
Dependent and Independent Part Instances
Displaying parts or the
assembly in the Mesh module
Switch via context bar or
model tree.
All mesh module functions
can be applied to parts.
Native mesh display toggle
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Mesh Generation Techniques
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Introduction to ABAQUS
L7.15
Mesh Generation Techniques
Free meshing
Free meshing uses no preestablished mesh patterns, making it
impossible to predict a free mesh pattern before creating the mesh.
Element shape options available for free meshing two-dimensional
regions:
Quadrilateral (default) Can be applied to any planar orcurved surface.
Quadrilateral-dominated Allows some triangular elements
for transition.
Triangular Can be applied to any planar or
curved surface.
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Introduction to ABAQUS
L7.16
quad mesh
quad-dominated mesh triangular mesh
Mesh Generation Techniques
L 1
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Introduction to ABAQUS
L7.17
Mesh Generation Techniques
Element shape option
available for free meshingthree-dimensional regions.
Tetrahedralany
geometry can be meshed
with tetrahedral elements
unless the mesh seedsare too coarse.
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Introduction to ABAQUS
L7.18
Mesh Generation Techniques
Swept meshing
A mesh is created on one side
of the region, known as the
source side.
The nodes of that mesh are
copied, one element layer at a
time, along the connectingsides of the region until the
final side, known as the target
side, is reached.
The source and target sides
are automatically located byABAQUS.
source sidetarget side
nodes copied from the
source side to each element
layer and to the target side
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Introduction to ABAQUS
L7.19
Mesh Generation Techniques
Two-dimensional swept meshes
All-quad meshing of swept regions
Planar or curved surfaces
Quad-dominated meshing of degenerate revolved regions
(Degenerate regions include the axis of revolution)
Swept mesh Degenerate revolved mesh
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Introduction to ABAQUS
L7.20
Mesh Generation Techniques
Swept meshing (contd)
Swept solid regions can be filled
with:
Hex meshes
Hex-dominated meshes
Wedge meshes
General sweep paths allowed
Generalized sweep path
through the thicknessGeneralized sweep path
follows the draft angle
Extruded mesh
sweep path:
straight line
Revolved mesh
sweep path: arc
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Introduction to ABAQUS
L7.21
Mesh Generation Techniques
Requirements for sweep meshable
regions Topological
The source side may contain multiplefaces
Target face and each connecting
side must have only one face.
Geometric
Adjacent faces will be combined to
form the source side only if the
edge dihedral angles are not too far
from 180
Source Side
Target Side
Connecting side
Not sweep meshable
Sweep meshable
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Introduction to ABAQUS
L7.22
Mesh Generation Techniques
Structured meshing
The structured meshing
technique generates meshes
using simple predefined mesh
topologies.
ABAQUS transforms the mesh
of a regularly shaped region,such as a square or a cube,
onto the geometry of the region
you want to mesh.
Structured meshing generally
gives the most control over themesh.
Three-dimensional structuredmeshable regions
Simple mesh topology
structured tri
meshes
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Introduction to ABAQUS
L7.23
Mesh Generation Techniques
Mapped meshing
Special case of structured meshing
4-sided surface regions
Allows for improved mesh quality
Can be used with
Swept hex/hex-dominated meshusing advancing front algorithm
Free quad/quad-dominated
mesh using advancing front
algorithm
Free tetrahedral or triangularmesh
Mapped meshing applied
indirectly by meshing a regionand allowing ABAQUS/CAE to
apply mapped meshing where
appropriate
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Introduction to ABAQUS
L7.24
Mesh Generation Techniques
Mapped mesh example
Free tet mesh Fill 4-sided patches with
mapped tri meshes
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Introduction to ABAQUS
L7.25
Mesh Generation Techniques
Virtual topology
In some cases part instances in the assembly contain small details such
as faces and edges.
Virtual topology allows you to ignore unimportant details.
Detailed model
Bad elements
due to tiny faces
and edges
Virtual model
Unimportant
details abstracted
away
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
L7.26
Mesh Generation Techniques
Part instances that contain virtual topology can be meshed when using
one of the following mesh techniques
Free meshing
Triangular and tetrahedral elements
Quadrilateral or quadrilateral-dominated elements using the
advancing front algorithm
Swept meshing
Hex or wedge elements
Mapped meshing
Quadrilateral, triangular, or hex elements
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Introduction to ABAQUS
L7.27
Mesh Generation Techniques
Example: virtual topology + swept meshing
Adding partition
and hole to
virtual model
Detailed
bracket
Virtual model
(sweep
meshable)
All-hex
mesh
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Enabling Various Meshing Techniques
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Introduction to ABAQUS
L7.29
Enabling Various Meshing Techniques
Which regions are
meshable?
ABAQUS/CAE
automatically
determines
meshability for each
region based on itsgeometry and mesh
controls.
Regions are color
coded to indicate
their currently
assigned meshingtechnique:
free-meshing technique
structured-meshing
technique
swept-meshing technique
cannot be meshed using
current mesh technique
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Introduction to ABAQUS
L7.30
Enabling Various Meshing Techniques
Changing the element
shape from Hex to Tetchanges the technique
from unmeshable to
meshable.
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Introduction to ABAQUS
L7.31
Enabling Various Meshing Techniques
Partitioning to make regions meshable
Most three-dimensional part instances require partitioning to permit
hexahedral meshing.
Complex geometries often can be partitioned into simpler, meshable
regions.
Partitioning can be used to: Change and simplify the topology so that the regions can be
meshed using primarily hexahedral elements with the structured or
swept meshing techniques.
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Introduction to ABAQUS
L7.32
Enabling Various Meshing Techniques
Partitioning to
mesh a piston,wrist pin, and
connecting rod
assembly with hex
elements.
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Mesh Compatibility
L7.34
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
L7.34
Mesh Compatibility
Different regions of the same
part instance can be meshedusing different elements types,
such as tetrahedra and
hexahedra.
Tie constraints are
created automaticallyto connect the regions.
Allows hexahedra to be
used adjacent to contact
surfaces or in high gradient
regions where accuracy is
essential, with tetrahedra inother regions.
When a region is meshed, an
existing mesh on an adjacent
region is unaffected.
tie constraints insertedautomatically at partition
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Introduction to ABAQUS
L7.35
Mesh Compatibility
Using tie constraints to glue the
cylinder to the block: exploded
view of assembly (top) and mesh
tied surfaces
Currently it is not possible to obtain
meshes automatically that arecompatible between part instances.
If mesh compatibility is required betweentwo or more bodies, first try to create asingle part that contains all the bodies.
Multiple part instances can be
merged into a single part instance inthe Assembly module.
Different material regions can beseparated using partitions.
If the two objects must be modeled as
separate parts, consider using tieconstraints to glue two regionstogether.
Alternatively, merge instance meshesinto a conforming orphan mesh.
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
Mesh Compatibility
Merging instance meshes into a conforming orphan mesh
Mesh topology and node positions must conform.
Single step creates orphan mesh part and replaces instances
Works with any combination of dependent/independent/native/orphan
instances
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Introduction to ABAQUS
Mesh Compatibility
Example
Approach 1: Tie constraints (labor intensive in this case)
Approach 2: Merge meshes (relatively easy)
A part partitioned
into 112 meshable cells
A 1010 pattern of
dependent part instancesPart mesh
Side 1 Side 2
Side 3
Side 4
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Controlling Mesh Density and
Gradation
L7.39
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
Controlling Mesh Density and Gradation
Mesh seeds
Mesh seeds are markers
that you define along the
edges of a region to specify
the desired, ortarget, mesh
density.
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Introduction to ABAQUS
Controlling Mesh Density and Gradation
Triangular and tetrahedral meshes and quadrilateral meshes using the
advancing front algorithm match the seeds exactly. For hexahedral or quadrilateral meshes (using the medial-axis algorithm)
ABAQUS often must change the element distribution to permitsuccessful meshing.
You can prevent such adjustments by constraining the seeds alongan edge.
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Introduction to ABAQUS
Controlling Mesh Density and Gradation
You can set a typical global element length for part instances.
ABAQUS/CAE automatically creates mesh seeds along all relevantedges based on the typical element length.
New edges created by partitioning automatically inherit the globalmesh seeds.
You can override the global mesh seeds with local mesh seeds along
selected edges. Edge mesh seeds can be uniform or biased.
Edge mesh seeds propagate automatically from the selected edgeto the matching edges for swept meshable regions.
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
Controlling Mesh Density and Gradation
Global seeds (black) and local seeds (magenta)
new partition edges inherit
global seeds
biased local seeds
local seeds automatically
propagate to matching edges
on swept regions
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
Partitioning and local mesh seeding allows
you to refine the mesh in the area of a stress
concentration.
Controlling Mesh Density and Gradation
Partitioning into different
mesh regions
Partitioning creates
additional edges, which
allows more control
over local mesh
density.
Each mesh region can
have different mesh
controls.
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Parametric Modeling
L7.45
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
Parametric Modeling
A useful feature of the Mesh module is
the ability to regenerate partitions andmesh attributessuch as element-type
assignments, seeds, and mesh
controlsafter a part has been
modified.
You must always recreate the
mesh itself after modifying a model.
For example, the model shown at
right has been partitioned into
4 regions and then seeded to
specify an approximate element
size of 3.
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Introduction to ABAQUS
Parametric Modeling
You can return to the Part module and
modify the hole so that it is somewhatlarger. When you return to the Mesh
module, the partitions and the seeds
are regenerated, as shown at right.
In addition, settings in the Mesh Controls
and Element Type dialog boxes (such as
element shape, element type, and
meshing technique) are also regenerated.
(You can display these two dialog boxes
by selecting MeshControls and MeshElement Type from the main menu bar.)
If you modify the part drastically (e.g., if you delete features insteadof modifying the hole in the figure at right), the seeds and partitions
may fail to regenerate. In these cases you must create new seeds
and partitions after re-entering the Mesh module.
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Assigning Element Types
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
Assigning Element Types
The available element types
depend on the geometry of yourmodel.
You can assign the element type
either before or after you create
the mesh.
Different element types can beassigned to different regions of
your model.
Items such as loads and
boundary conditions depend on
the underlying geometry, not themesh, so performing parametric
studies on mesh density or
element types is very easy.
element name and brief description
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Checking Mesh Quality and Obtaining
Mesh Statistics
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
Checking Mesh Quality and Obtaining Mesh
Statistics
Mesh statistics
ABAQUS/CAE can generate
plots that highlight elements
whose aspect ratios,
maximum and minimum
angles, and shape factors
exceed specified limits.
The following information is
displayed in the message
area:
Total number of elements
Number of distortedelements
Average distortion
Worst distortion
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Introduction to ABAQUS
Other mesh statistics
Mesh statistics also:
Help you check whether the mesh has been generated as you
intended.
Provide information about part instance names, number of elements
of each shape, and number of nodes.
Provide information about the element types and mesh techniques
assigned to a region.
Checking Mesh Quality and Obtaining Mesh
Statistics
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Introduction to ABAQUS
Mesh analysis checks
Elements that will produce
errors or warning in the
analysis can be highlighted.
In most cases it will be obvious
from the element shape why
the input file processor issuedan error or a warning.
If necessary, you can submit a
datacheck analysis from the
Job module and review the
messages that ABAQUS writes
to the data file.
Current limitation:
Analysis checks are not
currently supported for
structural and gasket elements.
Checking Mesh Quality and Obtaining Mesh
Statistics
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Workshop 8: Structured Hex Meshing:
Pipe Creep Model
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Copyright 2006 ABAQUS, Inc.
Introduction to ABAQUS
Workshop 8: Structured Hex Meshing:
Pipe Creep Model
Workshop tasks:
1. Create face and cell partitions.
2. Assign global seeds.
3. Assign element type.
4. Generate a structured hex
mesh.
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Copyright 2006 ABAQUS, Inc.
Workshop 9: Free and Swept Meshing:
Pump Model
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Workshop 9: Free and Swept Meshing:
Pump Model
Workshop tasks
1. Change the element type ofthe pump housing elements
from C3D4 to C3D10M.
2. Assign global and edge seeds
to the gasket, cover, and bolts.
3. Mesh the gasket and boltswith hex elements using the
swept mesh technique.
4. Mesh the cover with tet
elements using the free mesh
technique.
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