Mechanical_Intro_14.5_L04_Meshing.pdf

8/10/2019 Mechanical_Intro_14.5_L04_Meshing.pdf

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2012 ANSYS, Inc. December 19, 20121 Releas e 14.5

Introduction to ANSYSMechanical

14.5 Release

Lecture 4Meshing Techniques


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Chapter OverviewIn this chapter controlling meshing operations is described.

Topics:A. Global Meshing ControlsB. Local Meshing ControlsC. Meshing TroubleshootingD. Virtual TopologyE. Workshop 4.1 Mesh ControlF. Submodeling


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Meshing in MechanicalThe nodes and elements representing the geometry model make up the mesh: A default mesh is automatically generated during a solution. It is generally recommended that additional controls be added to the default

mesh before solving. A finer mesh produces more precise answers but also increases CPU time

and memory requirements.

Generate the mesh or preview the surface ofthe mesh before solving (previewing thesurface mesh is faster than generating theentire mesh).


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A. Global Meshing ControlsPhysics Based Meshing allows the user to specifythe metrics used in measuring element quality tobe based on the kind of analysis being done.Physics preferences are: Mechanical Electromagnetics CFD Explicit

Different analysis types define acceptable orfavorable element shapes differently. For thiscourse we limit the discussion to Mechanical.


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Global Meshing Controls Relevance is the most basic global size control and is set in the Defaults area

of the mesh details. Relevance is set between 100 and +100 (zero = default).

- Relevance = coarsemesh

+ Relevance = finemesh


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Global Meshing Controls Sizing Control:

These settings assume the Use Advanced SizeFunction is set to Off.

Relevance Center: sets the mid point of the Relevance slider control. Element Size: defines the maximum element size used for the entire model. For most static structural applications the default values for the remaining global

controls are usually adequate.

Coarse FineMedium

-100 +1000

-100 +1000

Relevance Center


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8/48 2012 ANSYS, Inc. December 19, 20128 Releas e 14.5

Global Meshing Controls The Fixed size function provides minimum and maximum element size controls.

Curvature as the name implies, is driven by the curvature encountered in thegeometry. For models dominated by lots of curved features this control provides away to refine the mesh over much of the model without using local controls.

For models composed of mostly linear features the control will have a lesserimpact.

Curvature = 20 deg. Curvature = 75 deg.



Global Meshing Controls Proximity provides a means to control the mesh density in regions of the modelwhere features are located closer together. In cases where the geometry containslots of detail this can be a quick way to refine the mesh in all areas withoutapplying numerous local controls.

As mentioned earlier proximity and curvature can be combined. The choice ofcontrol is dictated by the geometry being meshed.

Num Cells = 2 Num Cells = 5



Global Meshing Controls Again some of the advanced mesh settings are beyond the scope of theintroductory course or apply to other physics. Several controls have potentialapplication in linear static analysis.Shape Checking: Standard Mechanical linear stress, modal and thermal analyses. Aggressive Mechanical large deformations and material nonlinearities. Number of retries : if poor quality elements are detected the mesher will retry using a

finer mesh.



B. Local Meshing Controls

Local Mesh Controls can be applied to either a Geometry Selection or a Named

Selection. These are available only when the mesh branch is highlighted.Available controls include : Method Control Sizing Control Contact Sizing Control Refinement Control Mapped Face Meshing Match Control Pinch Control Inflation Control


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Local Meshing Controls Method Control : Provides the user with options as to howbodies are meshed:

Automatic (default): Body will be swept if possible. Otherwise, the Patch

Conforming mesher under Tetrahedrons is used.

Continued . . .


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Tetrahedrons Method: All Tetrahedra mesh is generated (not usually requested for

mechanical applications). Can use Patch Conforming or Patch Independent Meshing

algorithms.

Local Meshing Controls

Patch Conforming Patch Independent

Underlying Geometry


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Hex Dominant Method : Creates a free hex dominant

mesh:Recommended for meshing bodies with large interiorvolumes.

Not recommended for thin or highly complex shapes.

Useful for meshing bodies that cannot be swept.

Solid Model with Hex dominantmesh (approximate percentages):

Tetrahedrons 443 (9.8%)

Hexahedron 2801(62.5%)

Wedge 124 (2.7%)

Pyramid 1107 (24.7%)



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Sweep Method (hex and possibly wedge shapes): Source/Target Selection: Manually select the start/end faces for sweeping or

allow the mesher to choose. Can include size controls and/or biasing along sweep.


Source

Target

Note: the geometry shown here has 6different possible sweep directions.


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Local Meshing Controls MultiZone Method:

A patch independent mesher that automatically decomposes solid geometry toaccomplish sweep meshing (like a user might slice a model for meshing).

Standard Free Mesh

MultiZone Mesh


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Local Meshing Controls Surface Body Methods:

Quadrilateral Dominant (default): attempts tomesh with as many quad elements as possible, fillsin with triangles.

Triangles: all triangular shapes are used.

MultiZone Quad/Tri: Depending on settings, quador tri shapes are created using a patch independentalgorithm.

Note, each method contains a unique set of optionsin the details allowing additional configuration.


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Local Meshing Controls Sizing (3 configurations): Element Size (element edge length) OR Number of Divisions. Sphere of Influence (see next page)

Soft control may be overridden by other mesh controls.Hard may not.

Entity Element Size # of Elem. Division Sphere of InfluenceBodies x xFaces x xEdges x x x

Vertices x


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Local Meshing Controls Sphere of Influence: Center is located using a coordinate system. All scoped entities within the sphere are affected by size

settings.

Only the portion of the scoped face or body within thesphere is included in the scope of the mesh control.


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Local Meshing Controls Contact Sizing: generates similar-sized elements on contact

faces for face/face or face/edge contact regions. Element Size or Relevance can be specified. Can drag and drop a Contact Region object onto the Mesh

branch as a shortcut.


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Local Meshing Controls Element Refinement:

An initial mesh is created using the global and local size settings, then elements aredivided at the scoped locations (up to 3 times).

For example shown the scoped facehas a refinement level of two.

Note: the refinement method generally offers less control or predictability over the final meshsince an initial mesh is split. This splitting process may adversely affect other meshing controls

as well.


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Local Meshing Controls Mapped Face Meshing: generates structured meshes onsurfaces:

Mapped quad or tri mesh also available for surface bodies.

See next slide for advanced options . . . .


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For some geometry mapping will fail if an obvious pattern is not recognized.

By specifying side, corner or end vertices a mapped face can be achieved.See next page for additional examples.


Original mapping failedas indicated next to themesh control.

By setting side and end verticesthe mapped mesh succeedsresulting in a uniform sweep.


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Side, Corner and End controls for vertices, to define strategy forMapping

Vertex Type Intersecting Grid Lines Angle Between EdgesEnd 0 0 135

Side 1 136 224

Corne r 2 225 314



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Local Meshing Controls Inflation Control: useful for adding layers of elements along specific boundaries.

Boundary


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Local Meshing Controls Pinch: allows the removal of small features by pinching outsmall edges and vertices. Master: geometry that retains the original geometry profile. Slave: geometry that changes to move toward the master. Can be automatic (mesh branch details) or local (add Pinch

branch).


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C. Meshing TroubleshootingMesh Metric (requested in the statistics section): Select individual bars in the graph to view the elements graphically.

Note: each mesh metric isdescribed in detail in theMeshing Users Guide.


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. . . Meshing TroubleshootingIf the mesher is not able to generate a mesh an error message will be returned: Double click the message field in the status bar to open the messages window. Double click individual messages to show the error in a separate window.

When possible Mechanical cangraphically display theproblem regions (RMB in themessage window). Using awireframe view will makefinding these areas easier.


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D. Virtual TopologyVirtual topology is a feature that can aid you in reducing the number of elementsin the model, simplifying small features out of the model, and simplifying loadabstraction.

For meshing certain CAD models you may want to group faces and/or edges togetherallowing you to form virtual cells in order to reduce or improve the elements.

You can split a face to create two virtual faces, or split an edge to create two virtualedges for improved meshing.

Virtual Cells can be created automatically.

Several Examples Follow . . .


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. . . Virtual Topology

In this example one edge of this multibody

part has a size control assigned whichcauses irregularities in the overall mesh.

Shown in the upper right, 3 edges arevirtually split to accommodate improvedelements shapes.

Initial Mesh

Final Mesh

Virtual Split Edges

Size Control


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. . . Virtual TopologySurface Model Example:

VirtualCell


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. . . Virtual Topology

Virtual Topology branch is added below the Modelbranch: Individual virtual entities do not appear in the tree.

Instead, a statistics section in the details lists virtualentities.

An automatic virtual topology function will attempt tocreate virtual cells based on the details settings.

Automatic Virtual Topology: Low, Medium, High: Indicates how aggressively

virtual topology will be searched for. Edges Only: Searches for adjacent edges to be

combined.


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. . . Virtual TopologyVirtual Cells can be created manually:

Select the entities (faces shown here) to be included in the virtual cell. Choose Merge Cells in the context menu (or RMB > Insert > Virtual Cell).


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. . . Virtual TopologyIn some instances it may be desirable to split a face to allow a specific meshingoperation.

With the Virtual Topology branchhighlighted, select 2 vertices as the desiredsplit point.

Choose Split Face at Vertices to completethe operation.

Vertex selection may be comprised ofexisting vertices or virtual hard verticescan be created (see following slides).

Note, selected vertices must both beassociated with the face to be split.


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. . . Virtual TopologyVirtual split faces can be accomplished by creating virtual split edges (note the splitfaces utilize the vertices generated when the edges are split).

Virtual Split Edge at +: splits atthe selection point along theedge.

Virtual Split Edge: requires afractional entry indicating theposition along the edge wherethe split will be located (e.g. 0.5results in the line split in half).


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. . . Virtual TopologyA Virtual Hard Vertex feature allows the creation of hard points which can beused to split faces where no natural vertex exists.

With the Virtual Topology branch highlightedselect the face where the hard vertex is to belocated.

Note, a + sign will appear at the cursorlocation.

Choose Hard Vertex at + (or RMB > Insert> Virtual Hard Vertex at +).


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. . . Virtual TopologyVirtual entities can be reviewed, edited or deletedfrom the context toolbar (highlight Virtual Topologybranch):

Use the arrow keys to cycle through next/previousvirtual entities.

The virtual entity is highlighted graphically and the

status bar (bottom of graphics window) indicates thecurrent selection.

The Edit icon allows access to an editor window

where modifications to the virtual entity definitioncan be made.

Use Delete to remove unwanted virtual entities.


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. . . Virtual TopologyKeep in mind that the topology can change! Example: a chamfer is added to the top surface in this virtual cell. The interior lines are

not recognized anymore.

Mesh using virtualtopology

Original meshElements edge is shown as a solidline and the original chamfer and topsurface is shown as a dotted line.

The chamfer representation is nolonger present.


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Workshop 4.1 Mesh Control

Goal: Use the various mesh controls to enhance

the mesh for the solenoid model.

E. Workshop 4.1 Mesh Control
http://localhost/var/www/apps/conversion/tmp/Workshops/WB-Mech_120_WS_03.2.ppthttp://localhost/var/www/apps/conversion/tmp/Workshops/WB-Mech_120_WS_03.2.ppthttp://localhost/var/www/apps/conversion/tmp/Workshops/WB-Mech_120_WS_03.2.ppthttp://localhost/var/www/apps/conversion/tmp/Workshops/WB-Mech_120_WS_03.2.ppthttp://localhost/var/www/apps/conversion/tmp/Workshops/WB-Mech_120_WS_03.2.ppthttp://localhost/var/www/apps/conversion/tmp/Workshops/WB-Mech_120_WS_03.2.ppthttp://localhost/var/www/apps/conversion/tmp/Workshops/WB-Mech_120_WS_03.2.ppt


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F. SubmodelingSubmodeling is a technique where a coarsely meshed model can be solvedfollowed by a subsequent solution using only a portion of the coarse model with

a more refined mesh. Submodeling is available for structural and thermalanalysis types with solid geometry.

As shown in the example below, and explained shortly, one of the key conceptsin submodeling is the designation of the cut boundaries defining the submodel.

Cut Boundaries


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. . . SubmodelingSubmodeling Example:

The model shown below is initially solved using a coarse mesh. As expected we see stress concentrations in regions containing detailed

geometry along with a coarse mesh.

Based on these results we choose to create a submodel to explore the regioncircled in more detail.


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. . . SubmodelingAlthough there are numerous geometry modeling techniques that can be usedto create the submodel, we have chosen to slice a body from the full modelusing the DesignModeler application. This new body is our submodel and amore refined mesh is created in the Mechanical application.


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. . . SubmodelingThe submodel schematic is set up as shown here:

Original Full Model

Since the full model and submodel are

comprised of different geometry, we cantsimply drag and drop a new structuralsystem on to the existing one as this wouldlink the geometry. Instead we create a newstand alone system and drag the full modelsolution onto the submodels setup cell.


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. . . Submodeling

After opening the new (submodel)

system open in Mechanical we cansee a new Submodeling branch hasautomatically been inserted in thetree. If we RMB we can choose thetype of result to import(displacement in this example).

In the details of the imported loadwe choose scope to which the loadsare applied. The scope here is thecut boundaries of the submodel.

Note, there are numerous mappingoptions available when transferring loadsnot all of which will apply tosubmodeling. For a complete discussionsee the section of the documentationcovering External Data Import.


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. . . Submodeling

RMB to import the load from the full

model. When completed, the importcan be reviewed graphically.

Add any additional boundaryconditions to the submodel to matchthose on the full model and solve.


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. . . SubmodelingTo insure that the cut boundary is far enoughfrom the high stress region a check should beperformed to compare full and submodelresults near the cut boundary.

Here an array of probes is used but path plots,surface plots, etc. are options as well. Wesimply want to verify that the results near thecut boundary are not drastically differentbetween the full and sub models. If they are,

it is usually an indication that the boundaryneeds to be moved further from the highstress region.

Full Model

Submodel

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