ANSYS Explicit Dynamics - Highvec.netcae.highvec.net/files/4_Tutorials/ANSYS TUTORIALS...Explicit...

1-1 ANSYS, Inc. Proprietary

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

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Chapter 5 Explicit Dynamics: Meshing

ANSYS Explicit Dynamics

Explicit Dynamics: Meshing



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Training Manual What is required of Meshes for Explicit Applications?

• Uniform element size (in finest zoned regions)

– Smallest element size controls the time step used to advance the solution in time

– Explicit analyses compute dynamic stress waves that propagate throughout 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 where mesh is refined (strongly dependent on geometry)

• In explicit analyses, the location of regions of high stress constantly change as 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 Manual Mesh

• 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 Manual Meshing 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 Manual Meshing 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 is

placed on one or more edges / surfaces of a body




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Training Manual Mesh 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 Manual Meshing Methods – Sweeping

• Multizone

– Direct decomposition of complex geometries at the time of meshing to create a hex mesh

• Select source and target surfaces for the Multizone sweep mesher




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Training Manual Sweep vs. Thin Sweep

• Use Sweep When:

– The side faces are not “thin”

– You only have 1 source and 1 target

– The sweep direction changes along the path

• Use Thin Sweep When:

– The side faces are “thin”

• In general, “thin” means that the side faces are small in relation to the source faces (aspect ratio of sides/sources is ~ 1/5th)

– You only have multiple sources and targets

– Path is linear

Sweep

Thin Sweep




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Training Manual Sweep vs. MultiZone

• Use Sweep when you have a multibody part where some bodies should be meshed 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 Manual Thin Sweep vs. MultiZone

• Use Thin Sweep when you have a “thin” solid part where the source and target faces

don’t exactly match, and you don’t care about the features on the target side

Multiple source Multiple target

Multiple sources captured Multiple targets ignored




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Training Manual Thin Sweep vs. MultiZone

• Use MultiZone when you have a “thin” solid part where the source and target faces don’t exactly 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 Manual Meshing 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 Manual Meshing 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 Manual Meshing – Line Bodies (Beams)

• Method is Automatic

• Cross-section is

assigned

• Can be visualized as

line segments or

showing cross-section

• Use sizing controls to

obtain uniform

element size




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Training Manual Mesh 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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Training Manual

• Number of Divisions

– Specifies number of elements on

edge(s) of bodies or faces

– Bias Type

• Edge is discretized to include a bias

towards one end, both ends, or the

center

• Bias Factor is 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 Hard No Edge

Sizing

Mesh Sizing




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Training Manual

• Sphere of Influence

– Elements associated with the scoped entities, that are within the

Sphere 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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Training Manual

• Sphere of Influence

– Elements associated with the scoped entities, that are within the

Sphere 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 Manual Defeaturing

• Defeaturing reduces the influence that geometric

features 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 Mesh

Small feature controls smallest

element size and mesh type

Create a virtual face

from small feature

and larger body Mesh 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 for Patch Independent Tetrahedrons

Without Defeaturing Tolerance

With Defeaturing Tolerance

mesh ignores small feature

Much better element size




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Training Manual DM (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 of

1 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 3

solids

–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 Manual DM 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 Manual DM and Meshing – Slice

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

body boundaries

Single Body

Parts

Multi-body

Part




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Training Manual DM 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 Manual DM 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 unwanted

initial contact penetration

– New feature in 12.0. Currently only

implemented for tetrahedra




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Training Manual DM and Meshing - Shared Topology and Match Mesh

Automatic None Imprint

No Match Mesh

Only Automatic has

matching meshes




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Training Manual DM and Meshing - Shared Topology and Mesh Matching

Automatic None Imprint

Mesh matching

where possible

All cases have

matching meshes




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Training Manual DM 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 to

define solid or breakable joints




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Training Manual DM and Meshing - Mid-Surface Creation

• Mid-Surface

– Reduces 3D geometry of constant thickness to a simplified “shell” representation

– Automatically places surface body at mid point between 3D face pairs

– Allows shell element type meshing in Explicit Dynamics

3D Model Mid-Surface representation




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Training Manual DM and Meshing – Surface Extension

• Surface Extension

– Creates a surface extension based on edge selection

– Often required after Mid-Surface operation

A thin solid model The 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 Manual DM and Meshing – Named Selections

• Create Named Selections in DM or Meshing

– Can be used to apply constraints and boundary conditions in Explicit Dynamics




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Training Manual

• Edge and Face Merge for Model Simplification

– Merge edges and faces based on angle criteria to simplify

the 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 Manual FEModeler

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

or ABAQUS Input file.

• Import FE information from Workbench Meshing 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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Training Manual

• Display Element Statistics in FEModeler

FEModeler




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• Convert to Geometry (Geometry Synthesis / Skin Detection)

FEModeler




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Training Manual

• Pass Geometry to Meshing

FEModeler

ANSYS Explicit Dynamics - Highvec.netcae.highvec.net/files/4_Tutorials/ANSYS TUTORIALS...Explicit...

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