lecture7 meshing

7/29/2019 Lecture7 Meshing

1/56

Copyright 2006 ABAQUS, Inc.

Meshing Imported and Native

Geometry

Lecture 7


2/56


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


3/56



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.


4/56


Introduction


5/56



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.


6/56



L7.6

part geometry

Discretized geometry

nodes elements

Introduction


7/56Copyright 2006 ABAQUS, Inc.


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



Dependent and Independent

Part Instances




L7.9


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.




L7.10


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.




L7.11


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




L7.12


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




L7.13


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




L7.15


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.




L7.16

quad mesh

quad-dominated mesh triangular mesh


L 1




L7.17


Element shape option

available for free meshingthree-dimensional regions.

Tetrahedralany

geometry can be meshed

with tetrahedral elements

unless the mesh seedsare too coarse.

L7 18




L7.18


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

L7 19




L7.19


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

L7 20




L7.20


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

L7 21




L7.21


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

L7 22




L7.22


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

L7 23




L7.23


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

L7 24




L7.24


Mapped mesh example

Free tet mesh Fill 4-sided patches with

mapped tri meshes

L7 25


25/56



L7.25


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

L7 26


26/56



L7.26


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

L7 27


27/56



L7.27


Example: virtual topology + swept meshing

Adding partition

and hole to

virtual model

Detailed

bracket

Virtual model

(sweep

meshable)

All-hex

mesh


28/56



L7 29


29/56



L7.29


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

L7 30


30/56



L7.30


Changing the element

shape from Hex to Tetchanges the technique

from unmeshable to

meshable.

L7 31


31/56



L7.31


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.

L7 32


32/56



L7.32


Partitioning to

mesh a piston,wrist pin, and

connecting rod

assembly with hex

elements.


33/56


Mesh Compatibility

L7.34


34/56



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

L7.35


35/56



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.

L7.36


36/56



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

L7.37


37/56



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


38/56


Controlling Mesh Density and

Gradation

L7.39


39/56




Mesh seeds

Mesh seeds are markers

that you define along the

edges of a region to specify

the desired, ortarget, mesh

density.

L7.40


40/56




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.

L7.41


41/56




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.

L7.42


42/56




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

L7.43


43/56



Partitioning and local mesh seeding allows

you to refine the mesh in the area of a stress

concentration.


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.


44/56


Parametric Modeling

L7.45


45/56



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.

L7.46


46/56



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.


47/56



L7.48


48/56




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


49/56


Checking Mesh Quality and Obtaining

Mesh Statistics

L7.50


50/56



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

L7.51


51/56



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.


Statistics

L7.52


52/56



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.


Statistics


53/56


Workshop 8: Structured Hex Meshing:

Pipe Creep Model

L7.54


54/56



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.


55/56


Workshop 9: Free and Swept Meshing:

Pump Model

L7.56


56/56

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.

lecture7 meshing

Documents