Explicit Dynamics Chapter 6 Explicit Meshing

7/28/2019 Explicit Dynamics Chapter 6 Explicit Meshing

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1-1ANSYS, Inc. Proprietary

2009 ANSYS, Inc. All rights reserved.February 27, 2009

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Chapter 5

Explicit Dynamics:Meshing

ANSYS Explicit Dynamics


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Expl ic i t Dynam ics: Meshing



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Training ManualWhat is required of Meshes for Explicit Applications?

Uniform element size (in finest zoned regions)

Smallest element size controls the time step used to advance thesolution in time

Explicit analyses compute dynamic stress waves that propagatethroughout the entire mesh

Element size controlled by the user throughout the mesh

Not automatically dependent on geometry

Implicit analyses usually have static region of stress concentration wheremesh is refined (strongly dependent on geometry)

In explicit analyses, the location of regions of high stress constantly changeas stress waves propagate through the mesh

Mesh refinement is usually used to improve efficiency

Mesh transitions should be smooth for maximum accuracy

Hex-dominant meshing preferred

More efficient

Sometimes more accurate for slower transients


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Training ManualMesh

Right-click Mesh in the Outline Tree to:

Insert

Method

Sizing

Contact Sizing

Refinement

Mapped Face Meshing

Match Control

Pinch

Inflation

Update

Generate Mesh

Preview Surface Mesh Show Sweepable Bodies

Preview Inflation

Clean

Rename

Important for Explicit


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Training ManualMeshing Methods

Solid Bodies

Automatic Tetrahedron

Hex Dominant

Sweep

Multizone

CFX-Mesh

Surface Bodies (Shells) Quadrilateral Dominant

Triangles

Uniform Quad / Tri Uniform Quad

Line Bodies (Beams) Automatic


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Training Manual

Tetrahedrons

Advantages

An arbitrary volume can always be filled

with tetrahedra

Can be generated quickly, automatically,

and for complicated geometry

Disadvantages

Element and node counts are higher than

for a hex mesh with a similar mesh density

Generally not possible to align the cells

with a flow direction

Not well suited for thin solids or annuli due

to non-isotropy of geometry and nature of

element

Meshing Methods Solid Bodies


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Training ManualMeshing Methods Solid Bodies

Tetrahedrons - Patch Conforming

Default Tetrahedron Mesher

All Faces, Edges, Vertices of the

geometry are respected during mesh

generation

Delaunay Method Not good for Explicit Dynamics

Curves in Geometry are Reflected in the Mesh


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Tetrahedrons - Patch Independent

Recommended Tet mesher for Explicit

Faces, Edges, Vertices not always respected

Octree Method

Element size Defined By

Maximum Element Size

Approx. number of Elements

Curves in Geometry NOT reflected in the Mesh

Max. Element Size = 2.5 mm Max. Element Size = 1.0 mm


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Training Manual

Hex Dominant

Useful for meshing bodies that cannot be swept

Recommended for meshing bodies with large interior

volumes

The hex-dominant meshing algorithm creates a quad-

dominant surface mesh first, then pyramid and

tetrahedral elements are filled in as needed

Always check interior of mesh for good element structure

Control Messages will appear to warn you if volume may

not be suitable for hex-dominant meshing

Solid Model with Hex dominant mesh :

Tetrahedrons 443 (9%)

Hexahedron 2801(62%)

Wedge 124 (2%)

Pyramid 1107 (24%)

Meshing Solid Bodies


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Hex Dominant

Sometimes produces a better (more uniform) mesh if a size control isplaced on one or more edges / surfaces of a body


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Training ManualMesh Methods - Sweeping

Sweep Sweeping from a single source face to a single target face

Thin Sweep

Good at handling multiple sources and targets for thin parts

Multizone Uses a free decomposition approach

Attempts to automatically slice geometry into

sweepable regions

Supports multi-source and multi-target

Sweep methods for generating pure hex meshes


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Training ManualMeshing Methods Sweeping

Multizone

Direct decomposition of complex geometries at the time of meshing tocreate a hex mesh

Select source andtarget surfaces forthe Multizone sweepmesher


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Training ManualSweep vs. Thin Sweep

Use Sweep When:

The side faces are not thin You only have 1 source and 1

target

The sweep direction changesalong the path

Use Thin Sweep When:

The side faces are thin

In general, thin means that theside faces are small in relation to

the source faces (aspect ratio ofsides/sources is ~ 1/5th)

You only have multiple sourcesand targets

Path is linear

Sweep

Thin Sweep


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Training ManualSweep vs. MultiZone

Use Sweep when you have a multibody part where some bodies should bemeshed with Sweep, and some with Patch Conforming Tet

Preview Sweepable Bodies shows all bodies that are sweepable

Use MultiZone when

you are meshing single body parts that are too complicated for Sweep

you have multiple sources and targets you need to respect

Example: Using Sweep, the single body part (left) must be manually sliced into a Multibody part

containing five bodies (center) to obtain a pure hex mesh (right)

With Multizone, it is meshed automatically!


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Training ManualThin Sweep vs. MultiZone

Use Thin Sweepwhen you have a thin solid part where the source and target faces

dont exactly match, and you dont care about the features on the target side

Multiple source Multiple target

Multiple sources captured Multiple targets ignored


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Training ManualThin Sweep vs. MultiZone

Use MultiZone when you have a thin solid part where the source and target faces dontexactly match, and you care about the features on both sides

Multiple source Multiple target

Multiple sources captured Multiple targets captured


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Training ManualMeshing Methods - Solid Bodies

This is the default Method for Explicit Ensures that preferred Hex meshes are generated whenever bodies can be

swept

If any bodies are not swept, the tetrahedron meshes generated for these bodies

should be remeshed by inserting a method that generates a Patch Independent

Tet. mesh for those bodies

Automatic (Patch Conforming/Sweeping)

Sweepable bodies are automatically swept

If a solid body cannot be swept it will be meshed using the Patch

Conforming Tetrahedron mesher


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Training ManualMeshing Methods Surface Bodies (Shells)

Quad Dominant Triangles Uniform Quad / Tri(not recommended for explicit)

Use sizing controls to obtain uniform element size


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Training ManualMeshing Line Bodies (Beams)

Method is Automatic

Cross-section is

assigned

Can be visualized as

line segments orshowing cross-section

Use sizing controls to

obtain uniform

element size


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Training ManualMesh Sizing

Sizing can be applied to

Body

Face

Edge

Vertex

Three options

Element Size (Body, Face, Edge)

Number of Divisions (Edge)

Sphere of Influence (Body, Face, Edge, Vertex)


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Training Manual

Element Size

Element Size specifies average element edge length for bodies,faces or edges

Often will also improves mesh quality

Mesh Sizing


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Number of Divisions

Specifies number of elements onedge(s) of bodies or faces

Bias Type

Edge is discretized to include a bias

towards one end, both ends, or the

center

Bias Factoris ratio of largest element

size to smallest element size

Effect of the Bias Factor is displayed

Mesh Sizing


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Training Manual

Number of Divisions

Behavior

Soft (default)

Size control and Number of Divisions used is controlled by the

meshing algorithm

Affected by proximity, curvature and local re-meshing

during the meshing process

Hard

Specified Number of Divisions is fixed and cannot be changed

by the meshing algorithm

Increases likelihood of mesh generation failure

Can have positive or negative effects on mesh quality

Soft HardNo EdgeSizing

Mesh Sizing


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Training Manual

Sphere of Influence

Elements associated with the scoped entities, that are within theSphere Radius have the specified Element Size

Vertex Sizing

Body Sizing

For any entity other than a

vertex, the definition of a

Sphere of Influence

requires that a Coordinate

System be defined to set

the center of the Sphere

Vertex is the center of

the sphere

Vertex sizing is applied

to all entities

Mesh Sizing

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Sphere of Influence

Elements associated with the scoped entities, that are within theSphere Radius have the specified Element Size

Surface meshing

identical

Interior meshing

different

Scoped to 1 Face Scoped to 2 Faces

Scoped to 3 Faces Scoped to 1 Body

Mesh Sizing

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Training ManualDefeaturing

Defeaturing reduces the influence that geometricfeatures have on the meshing process

Avoids generating small elements that may force explicit

calculations to run with a small time step

Two Meshing options are available for Defeaturing

Virtual topology

Defeaturing tolerance

Can only be used with for Patch Independent Tetrahedral

meshes

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Virtual Topology

Default MeshSmall feature controls smallest

element size and mesh type

Create a virtual face

from small feature

and larger bodyMesh is now uniform Hex

Feature is simply meshed over

Element size significantly improved

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Virtual

Cell

Virtual Topology & CAD

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Defeaturing Tolerance

Only forPatch Independent Tetrahedrons

Without Defeaturing Tolerance

With Defeaturing Tolerance

mesh ignores small feature

Much better element size



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Training ManualDM (DesignModeler) and Meshing

Bodies and Parts

Design-Modeler contains three different body types:

Solid body: Body has surface area and volume

Surface body: Body has surface area but no

volume

Line body: Body consists entirely of edges, no

area, no volume

By default, DM places each body into one part by itself

Individual parts will always be meshed separately

If bodies in separate parts share faces, the meshes

on those shared faces will not be matched

Multiple bodies in a single part will have matched

meshes on shared faces



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Training Manual

Example:

In DM: 1 part, 1 body consisting of1 solid

During Meshing: 1 solid ,1body

Entire solid meshed as one entity

No internal surfaces

DM

Mesh

DM and Meshing Bodies and Parts



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Training Manual

Example:

In DM: 3 parts, 3 bodies consisting of 3solids

During Meshing: 3 solids, 3 bodies

Each solid meshed independently

Nodes are not shared

Nodes do not line-up

DM

Mesh




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Training Manual

Example:

In DM: 1 multi-body part, 3 bodies / solids

During Meshing:1 multi-body part, 3 bodies /

solids

Each solid meshed independently but node

connectivity among solids is preserved

DM

Mesh




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Training ManualDM and Meshing - Slice

Slice geometry in Design Modeler to generate a Swept Hex Mesh

Original Geometry

Sliced Geometry

Default Mesh

Default Mesh



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Training ManualDM and Meshing Slice

Combine Sliced Bodies into single Part to get Mesh to Match across

body boundaries

Single Body

Parts

Multi-bodyPart



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Training ManualDM and Meshing Shared Topology & Match Mesh

Overlapping Meshes and

Contact

Explicit Dynamic Analyses

require that there are no initial

penetrations at contact surfaces

For arbitrarily shaped bodies,this requires that nodes on

surfaces that are initially in

contact must match

Shared Topology and Match

Mesh Where Possible options

allow you to do this



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Training ManualDM and Meshing - Shared Topology and Match Mesh

Specified for each Part (in DM) Applied at common boundaries of all Bodies in the

Part Options

Automatic (default)

Nodes are merged (bodies are bonded)

None

Bodies are meshed independently

Imprint

Meshes are across interfaces of bodies

contained in Multi-Body Parts

When used with Match Mesh Where

Possible option for Patch Independent

Tetrahdron meshing, can prevent unwantedinitial contact penetration

New feature in 12.0. Currently only

implemented for tetrahedra



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Training ManualDM and Meshing - Shared Topology and Match Mesh

Automatic None Imprint

No Match Mesh

Only Automatic has

matching meshes



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Training ManualDM and Meshing - Shared Topology and Mesh Matching

Automatic None Imprint

Mesh matching

where possible

All cases have

matching meshes



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Training ManualDM and Meshing - Spot Welds

Define points on source geometry

at location of the spot welds

Corresponding points on target

geometry are generated

Source geometry and target

geometry can be in contact or

separated

Meshing ensures that nodes are

generated at the spot weld points

Explicit Dynamics uses nodes todefine solid or breakable joints



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Training ManualDM and Meshing - Mid-Surface Creation

Mid-Surface

Reduces 3D geometry of constant thickness to a simplifiedshell representation

Automatically places surface body at mid point between 3Dface pairs

Allows shell element type meshing in Explicit Dynamics

3D Model Mid-Surface representation



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Training ManualDM and Meshing Surface Extension

Surface Extension

Creates a surface extension based on edgeselection

Often required afterMid-Surface operation

A thin solid modelThe model is converted

to a mid plane surface

model. The result is a

gap at the intersection

of the 2 parts

Extending the circular

edge closes the gap



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Training ManualDM and Meshing Named Selections

Create Named Selections in DM orMeshing

Can be used to apply constraintsand boundary conditions inExplicit Dynamics



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Training Manual

Edge and Face Merge for Model Simplification

Merge edges and faces based on angle criteria to simplifythe model

DM and Meshing - Cleanup and Repair of CAD



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Training Manual

Repair




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Training Manual

Face Delete

Selected faces for delete

Patch healing

Natural healing




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Training ManualFEModeler

Import a finite element (FE) model from a NASTRAN bulk data file

orABAQUS Input file.

Import FE information from WorkbenchMeshing or Advanced

Meshing

Import archived ANSYS data created using the CDWRITE

command. Navigate and visualize the data contained in the model.

Generate a geometry from an FE Model using the Geometry

Synthesis feature.

Create named components based on element selections. Generate an ANSYS, NASTRAN, or ABAQUS input deck for

downstream analysis.



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Training Manual

Import ANSYS model file into Workbench

Open the imported model in FEModeler

FEModeler



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Display Element Statistics in FEModeler

FEModeler



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Training Manual

Convert to Geometry (Geometry Synthesis / Skin Detection)

FEModeler



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Pass Geometry to Meshing

FEModeler

Explicit Dynamics Chapter 6 Explicit Meshing

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