chapter 3: shell meshing - altair university · • the automesh panel creates shell mesh on...

Copyright © 2009 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. Copyright © 2009 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

Automeshing

Chapter 3: Shell Meshing

Copyright © 2009 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

Automeshing: What is it?

• The automesh panel creates shell mesh on existing surfaces

• Allows rapid generation of mesh

• Most parts from CAD software come in as surfaces

• Automeshing allows all surfaces of a part to be meshed at the same time

• Mesh on properly connected geometry assures proper connectivity of mesh


Automeshing: What is “topology”?

• Topology is how surfaces connect to adjacent surfaces of a part

• Surface connectivity is controlled by the associated surface edges

• If a surface edge is associated with more than 1 surface, those surfaces are

considered to be connected (“equivalenced”)

• Surface edges are categorized, named, and colored according to the number of

associated surfaces:

Free edge (red) • Associated with only 1

surface

• Surfaces with a free edge

between them are NOT

equivalenced at that edge

Shared edge (green) • Associated with 2 surfaces

• Surfaces are equivalenced

Suppressed edge (blue) • Surfaces are treated as though

combined into 1 surface

T-junction edge (yellow) • Associated with 3 or more

surfaces

• Example: surfaces forming a

T-connection

• Surfaces are equivalenced


Automeshing: How topology affects the mesh

• Surface edges control how mesh created on adjacent surfaces interact:

Free edge between

2 surfaces

Nodes are placed along

edge for meshing

Mesh is discontinuous;

nodes along the free edge

are not equivalenced

(Nodes only separated for

illustration)

5

5

5

2

2 3

3

Shared edge between

2 surfaces

Mesh has proper

connectivity; nodes along

the edge are equivalenced


edge for meshing

Free edges (red) Shared edges (green)

5

5

5

2

2 3

3

5


Automeshing: How topology affects the mesh

• Surface edges control how mesh created on adjacent surfaces interact:

Suppressed edge

between 2 surfaces

Edge is ignored; nodes

are not placed along

the edge

Area is treated just like 1

surface; there is no line

of nodes along the edge

Non-manifold edge

between 3 surfaces

Mesh has proper

connectivity; nodes along

the edge are equivalenced


edge for meshing

5

5

5 5

5

5

5

5

4

4 3

3

3 3

Suppressed edges (blue) T-junction edges (yellow)


Topology Refinement: What is it?

• “Topology refinement” is modifying topology in order to obtain a quality mesh

• Unlike defeaturing, this generally does not change the shape of the part

• CAD geometry has topology details that interfere with mesh quality

• Edges are created where ever there is a change in surface curvature

• Even smooth areas can be split into several faces

• The automesher will be forced to place nodes along the edges and fixed points

• This may cause small element lengths, angles, aspect ratios, etc. depending

on the shape of the model

• CAD geometry can also have surfaces with a complex outline

• Highly complex shapes can make it hard to get a quality mesh

• Adding in edges splits the surfaces into smaller and simpler regions which can be

meshed easier

• Fixed points can be added in to force a node to be placed at that location, giving

more control


Topology Refinement: Examples

Suppressing

edges

Adding

edges

Removing

fixed points

Adding

fixed points

Replacing

fixed points

Aft

er

Befo

re


Topology Refinement: Tools

Edge edit

• Toggle – Select a shared edge to suppress it

• (Un)Suppress – Select multiple edges to suppress all of them at once

Point edit

• Add – Add fixed points to help control mesh pattern

• Especially helpful along edges to control node seeding

• Suppress – Removes unwanted fixed points

• Replace – Combines 2 fixed points together at a single location

• Project – Projects fixed points onto a nearby edge

• Useful for aligning mesh between 2 edges

Surface edit – All functions add edges by cutting surfaces

• Adding edges can be useful for controlling mesh patterns in large areas

• Trim with nodes – Uses node locations to cut surfaces

• Trim with lines – Uses lines to cut surfaces

• Trim with surfs/plane – uses other surfaces or a defined plane to cut surfaces



Quick edit • Has a number of tools found in other panels

• Focused on tools with minimal user input for rapid editing

• Split surf-node: Adds an edge to or divides a surface by cutting in a straight line between 2 selected nodes

• Split surf-line: Adds an edge to or divides a surface by cutting a straight line between a node and perpendicular to the selected line.

• Washer split – Adds a circular edge around a hole in a surface

• Mostly used for creating all quad mesh around a hole

• Toggle – Same as edge edit panel

• Remove Point – Deletes a selected fixed point



Quick edit, continued

• Replace point – Same as point edit panel

• Add/remove point : point – Creates a new fixed point at the selected locations

• Add point: line – Creates a user specified number of fixed points along the

selected edge

• Project point – Same as edge edit panel


Topology Refinement: Tips

• Mesh the part

• Visually scan the part to look for poor mesh patterns

• Use element checks in order to find areas of poor element quality

• Check elems panel, QI Panel, etc.

• Suppress edges with edge edit :(un)suppress

• Use various settings for break angle to suppress many edges at once

• Sometimes suppressing all edges and then unsuppressing desired edges can

be a good method.

• Suppress all the fixed points before add more in

• Gets rid of as many fixed points as possible, leaving ones that are required

Experiment!

There is no set process, so experience is a key factor in refining topology


• Meshing Methods:

• mesh surfaces or re-mesh existing meshes with constant size

• the surfaces are meshed to optimize the quality index (QI) of the elements

generated

• set specific meshing parameters to limit how far the mesh elements can

deviate from the actual edges of the surfaces meshed

• to mesh within limits of element deviation from a surface

• Rigid Body meshing produces a poor-quality mesh, but one that

accurately models the surface shapes

Automeshing: The Automesh Panel

“Automesh” Panel Mesh > Create > 2D AutoMesh



• Meshing Mode:

• – Creates mesh using automesh panel settings

• – As automatic; also opens the meshing module to

interactively edit the mesh



• Mesh Parameters:

• - Target size of elements to create

• - Type of elements to be created

• - advanced option allows you to select mapped and

free type of mesh

• – Quad dominant mesh; will use trias if necessary

• – Equilateral (60-60-60) tria mesh

• – Quad mesh with use of trias to create as rectilinear

a mesh as possible

• – Isosceles (45-45-90) tria mesh

• – Only quad elements created


Quads Mixed

Trias R-trias




• Select surfaces to be meshed

• Destination component:

• Elements will be created in the current component

• Elements will be created in the component of the surface(s)

being meshed

• Identify and Select:

• Click to select surfaces that have not been meshed

• Click to select surfaces that failed to mesh in a previous operation




• Mesh Connectivity:

• Keeps the re-meshed selection connected to the adjoining mesh

with the same connectivity

• Keeps the re-meshed selection connected to the adjoining mesh

but updates the connectivity

• Separates the re-meshed selection from the adjoining mesh so

that they are no longer connected

• Activates all manual adjustments and options previously used to

mesh




• Mesh Parameters:

• will produce a more orthogonal quad dominated mesh

• To keep elements roughly the same size

• To prevent the mesh from producing skewed elements

• Forces opposite surface edges to use consistent edge densities



• Meshing Mode:

• As automatic; also opens the meshing module to interactively edit the mesh


• General Controls

• Creates and updates the mesh preview using the current settings

• User updates some mesh parameters on a surface, then clicks mesh to view the results

• Accepting and Canceling

• Cancels out of the meshing module; no mesh is created

• Creates the mesh as per the mesh preview and returns to the automesh panel

• Offers Fit and controls from the View panel

• Only applies to surfaces that are currently being meshed



• elem type Determine the type of elements to be used in the mesh

Automeshing: The Meshing Module

• Density – Control element density along edges

• Mesh Style – Set method and type that determines mesh pattern and smoothing

“Automesh” Panel Mesh > Create > 2D AutoMesh Interactive


Map as

Rectangle

Free

(Unmapped)

Quad Mixed Trias R-trias

Me

sh

Me

tho

ds

• mesh method:

• Autodecide Automatically selects one of the below methods

• map as rectangle For rectangular shaped areas; can sometimes

be used for other shapes as well

• map as triangle Triangular shaped areas

• map as pentagon Pentagonal shaped areas

• free (unmapped) For areas of any shape


• Mesh Style continue

• Element type and method combine to determine the overall “style” of your mesh

• This “style” can be viewed by mesh that transitions between different density values:

Mesh Types



• bias style:


• Biasing – Controls the distribution of nodes during node seeding

Biasing

• Checks – Evaluates element quality of the generated mesh



Automeshing: Process and Strategy

• Use geometry cleanup tools to address surface connectivity

• Automesh the surfaces

• Set the approximate element size you want

• Use type and algorithm to set the overall mesh “style”

• Alter individual densities to find better mesh patterns

• Change some biasing on problem surfaces (this can change mesh

pattern as well)

• Use HyperMesh’s mesh editing tools to fix the 10-20% elements that have

poor quality or bad mesh pattern

• Don’t delete a mesh that is 90% good. Keep it and fix the 10% that is bad

• Experience is key: EXPERIMENT!

• Goal: get the mesh to be approximately 80-90% good quality

chapter 3: shell meshing - altair university · • the automesh panel creates shell mesh on...

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