general preprocessing chapter three. training manual general preprocessing procedure august 26, 2005...

General Preprocessing

Chapter Three

August 26, 2005Inventory

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General Preprocessing Procedure

Chapter Overview

• In this chapter, performing analyses without the use of the Wizards will be covered:– Geometry

– Contact

– Meshing

– Named Selections

– Coordinate Systems

• The capabilities described in this section are generally applicable to the ANSYS DesignSpace Entra licenses and above and are noted in the lower-left hand tables.

ANSYS License AvailabilityDesignSpace Entra xDesignSpace xProfessional xStructural xMechanical/Multiphysics x


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Introduction

• In the previous chapter, the Simulation GUI was introduced by the use of the Simulation Wizards

• In this chapter, navigating through the GUI without the Wizards will be covered.

Model shown is from a sample Mechanical Desktop assembly.


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… Introduction

• The Outline Tree is the main way of setting up the analysis– The Context Toolbar, Details View, and Graphics Window

update, depending on which Outline Tree branch is selected

– Use of the Outline Tree will be emphasized in this chapter

Use of the Outline Tree is the means by which users navigate through the Simulation GUI.


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A. Geometry Branch

• After importing a model either (a) directly from a supported CAD system or (b) from the Context Toolbar in a blank database, the Geometry branch lists available parts.

• In Simulation, there are three types of bodies which can be analyzed.– Solid bodies are general 3D or 2D

volumes/areas/parts.

– Surface bodies are only areas.

– Line bodies are only curves.



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… Types of Bodies

• Solid bodies are geometrically and spatially 3D or 2D:– 3D solids are meshed with higher-order tetrahedral or

hexahedral solid elements with quadratic shape functions

– 2D solids are meshed with higher order triangle or quadrilateral solid elements with quadratic shape functions

• Currently 2D geometry can be obtained from:

• DesignModeler, ProEngineer, Solid Edge, SolidWorks and Unigraphics

– Each node has three translational degrees of freedom (DOF) for structural or one temperature DOF for thermal

Geometry Interface AvailabilityDesignModeler xPro/ENGINEER xUnigraphics xSolidWorks xInventor xSolid Edge xMechanical Desktop xCATIA V4 xCATIA V5 xACIS (SAT) xParasolid xIGES x


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… Types of Bodies

• Considerations for 2D Geometry:– Geometry must consist of surface models lying on the XY

plane

– The “2D” switch must be set on the Project page prior to import

– Geometry type cannot be changed from 2D to 3D (or vice versa) after import

– Plane stress, plane strain and axisymmetric behaviors are supported

– Be sure to consult the Simulation documentation for all details regarding 2D analysis


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… Types of Bodies

• Surface bodies are geometrically 2D but spatially 3D:– Surface bodies are meant to represent structures which are

thin in one dimension (through-thickness), so that thickness is not explicitly modeled but supplied as an input value.

– Surface bodies are meshed with linear shell elements

– Each node has three translational and three rotational DOF for structural applications but one temperature DOF for thermal

Geometry Interface AvailabilityDesignModeler xPro/ENGINEER xUnigraphics xSolidWorks xInventorSolid Edge xMechanical DesktopCATIA V4 xCATIA V5 xACIS (SAT) xParasolid xIGES x


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… Types of Bodies

• Line bodies are geometrically 1D but spatially 3D:– Line bodies are meant to represent structures which are thin

in two dimensions compared to the length, so the cross-section is not explicitly modeled.

– Currently, only DesignModeler supports creation of line bodies since it can define cross-sections and orientations of lines.

– Line bodies are modeled with linear beam elements

– Each node has three translational and three rotational DOF for structural analysis and one temperature DOF for thermal

Geometry Interface AvailabilityDesignModeler xPro/ENGINEERUnigraphicsSolidWorksInventorSolid EdgeMechanical DesktopCATIA V4CATIA V5ACIS (SAT)ParasolidIGES


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… Multibody Parts

• For many applications, bodies and parts are the same. In DesignModeler, however, multibody parts are possible.

• Support of mixed surface and solid bodies in the same part is not supported for most CAD systems. An assembly may contain surfaces and solids, but a single part cannot.

– In DesignModeler, multiple bodies can be joined together to form a multibody part. This means that if the parts share common boundaries, the nodes are shared at that interface.

• No contact is needed in these situations if the nodes are shared.



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… Multibody Parts

• Multibody parts allows the user to define bodies with common nodes, as shown below:


Multibody parts made of surface and line bodies share nodes at common boundaries.This allows modeling of shells with stiffeners.

Multibody parts made of solid bodies share nodes at common boundaries.Material properties can be different for each body.


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… Material Properties

• To assign material properties to a body, select that body from the tree and select a “Material” from the pull-down menu– Materials can be selected from external XML

files

– New material data can be added or imported in the “Engineering Data” application. The new material will then be available from the pull-down menu.

– For surface bodies a thickness needs to be supplied as well

• Thicknesses will import directlyfrom DesignModeler, if defined.



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… Geometry Worksheet

• A summary of bodies and assigned materials is available– Select “Geometry” branch and then the “Worksheet” tab



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B. Contact

• When multiple parts are present, a means of defining the relationship between parts is needed.– Contact regions define how solid and/or shell parts interact

with each other.

– Spot welds provide a means of defining shell assemblies.

• Without contact or spot welds, parts will not interact with each other– In structural analyses, contact and spot welds prevent parts

from penetrating through each other and provide a means of load transfer between parts.

– In thermal analyses, contact and spot welds allow for heat transfer across parts.

• Contact will be introduced first, then spot welds.

ANSYS License AvailabilityDesignSpace EntraDesignSpace xProfessional xStructural xMechanical/Multiphysics x


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… Solid Body Contact

• When an assembly is imported, contact surfaces are automatically detected and created– The mating relationships are not used from the CAD software.

Proximity of surfaces is used instead to define contact.

– Tolerance for contact detection is available under the “Contact” branch as a slider bar in “Tolerance Slider”

Model shown is from a sample SolidWorks assembly.



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– Contact elements, which act as a ‘skin’ on the surface of the contacting regions, provides the relationship between parts.

– Each part maintains a separate mesh. This means that one small part will not drive mesh density of the entire assembly and/or the user can make parts of interest have a finer mesh than other parts


Note the non-matching mesh at the interface between parts.Mix of hexahedral elements contacting tetrahedral elements is possible.


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• When a contact region is highlighted in the “Contact” branch, parts are made translucent for easier viewing

– Selecting a contact pair makes the other bodies not involved in that contact region translucent

– Amount of translucency is controlled via “Tools > Options… > Simulation: Contact: Transparency”. Transparency can be turned off in the Details view of the “Contact” branch


The contacting bodies are partially transparent.Bodies not in the contact region are more fully transparent.The contacting surfaces which are oriented with surface normals pointing towards the view are opaque for easier viewing.


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• If a geometric entity is highlighted, use right-mouse button in the Graphics window to quickly select associated contact– The right-mouse pop-up menu allows the user to select the

corresponding body in the “Geometry” branch or highlight all associated contact regions under the “Contact” branch



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• Defining a contact pair involves selecting “contact” and “target” surfaces.– In ANSYS DesignSpace, the distinction between “contact” and

“target” is unimportant. Select surfaces for one body as “contact” and choose the surfaces for the other as “target”.

– Using “Contact” from the Context Toolbar allows manual definition of contact regions


Selection of contact and target surfaces is performed in the “Details” view.The contact bodies associated with selected surfaces will be listed below. Ensure that unique bodies are for each “Contact” and “Target” body.The “Contact” surfaces will be shown in red while “Target” surfaces will be displayed in blue.


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… Selection Planes

• Selection planes allow for users to easily select surfaces which are hidden from view by other surfaces.– User selects a plane; if more planes lie directly underneath the

cursor, selection planes appear. Selection planes are color-coded with the same color as its parent part and are ordered by depth from the cursor.



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… Selection Planes

• The use of selection planes streamlines the identification of contact regions– Example below shows two surfaces selected from two parts.

When moving the cursor over selection planes, those surfaces will get highlighted.Use of wireframe mode may make visualization easier.One can select a particular surface or even use Ctrl-select to select multiple surfaces.In this example, two surfaces highlighted on the screen are selected to define a contact pair manually. Without selection planes, the selection of the specific surfaces would be tedious.

ANSYS License AvailabilityDesignSpace Entra /DesignSpace xProfessional xStructural xMechanical/Multiphysics x

Selection Point


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… Renaming Contact Regions

• Select the “Contact” branch and right-click and select “Rename Based on Geometry” to rename all contact pairs based on part names



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… Verifying Contact Regions

• Although Simulation automatically detects contact, one should review each contact pair to ensure that contact is properly defined.


In this example, because of the tolerance used by automatic contact detection, some fillets shown here are included in the contact definition. The user may wish to remove the fillets from the contact region definition, especially in the case of bonded contact, in order to prevent spurious behavior.


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… Advanced Solid Body Contact

• For ANSYS Professional licenses and above, advanced contact options are available.– Auto detection dimension and slider

– Asymmetric contact

– Contact results tool

– More contact formulations available

– Pinball control (see next page)

ANSYS License AvailabilityDesignSpace EntraDesignSpaceProfessional xStructural xMechanical/Multiphysics x


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… Advanced Solid Body Contact

• The Pinball region represents a contact detection zone:– Contact calculation/detection occurs when contact gap is within the

pinball radius

– The pinball radius dimension may be entered to ensure that bonded contact is established for a large clearance or gap

– Pinball radius is displayed as a sphere in the graphics window

The pinball region enables the user to verify that contact is detected for a large gap.



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. . . 2D Solid Body Contact

• Surface contact for solids composed of 2D plane geometry is defined on edges rather than faces


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… Surface Body Contact

• For ANSYS Professional licenses and above, mixed assemblies of shells and solids are also supported



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• Shell contact includes edge-to-face or edge-to-edge contact– Shell contact is not turned on by default.

– Tolerance controls absolute search distance for detecting contact. Useful for shell assemblies with gaps.

– User can turn on detection of face-to-edge or edge-to-edge contact

– Priority can be set to prevent multiple contactregions from being formed in a given regionby setting priority.



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• Another example of the use of the pinball region is below:– Surfaces represent midplanes of thin structure. At the “T”

intersection of two shells, a gap is present

– If the pinball region is large enough, bonded contact can be established between the shells despite the gap.

– For bonded regions the radius should be set large enough to fill any ‘gap.’

– The default Pinball region is based on the size of the underlying mesh (solid body) or thickness (surface body).

If needed, use the “Label” button on the Graphical Toolbar to move the “Contact Region” label & pinball sphere to a location which may be more convenient.



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… Spot Weld

• Spot welds provide a means of connecting shell assemblies at discrete points– Spot weld definition is done in the CAD software. Currently,

only DesignModeler and Unigraphics define spot welds supported by Simulation

– Spot welds can be created in Simulation manually at discrete vertices.

ANSYS License AvailabilityDesignSpace EntraDesignSpace xProfessional xStructural xMechanical/Multiphysics xDesignModeler xPro/ENGINEERUnigraphics xSolidWorksInventorSolid EdgeMechanical DesktopCATIA V4CATIA V5ACIS (SAT)ParasolidIGES


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… Contact Worksheet

• The “Worksheet” tab of the “Contact” branch provides a summary of various contact and spot weld definitions:


Right-click on the spreadsheet to hide/show specific columns.


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C. Meshing

• The nodes and elements of the mesh participate in the finite element solution– A “default” mesh is automatically generated during initiation

of the solution

– The user can “preview” the mesh to check whether it is adequate or not for his/her needs

• Note: previewing a mesh will create the nodes and elements. A solve following a preview will not require re-meshing.

Model shown is from a sample Inventor assembly.



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… Meshing

• The user needs to balance the computational cost with the numerical accuracy of the mesh– A finer mesh produces more precise answers but also

increases CPU time and memory requirements

– Ideally, having a solution not dependent on the mesh density is what users want (i.e., answers do not change appreciably as mesh is refined)

• Convergence controls (discussed later) aid in this

– A finer mesh does not compensate for incorrect assumptions and inputs, however!


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… Global Meshing Controls

• Basic meshing controls are available under the “Mesh” branch– With “Global Controls” as “Basic” (default), user

has control with a single slider bar• “Relevance” setting between –100 and +100

Relevance = -100

Nodes: 9968

Elements: 5808

Relevance = 0

Nodes: 19040

Elements: 10909

Relevance = +100

Nodes: 40764

Elements: 24687



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• User can change to “Advanced” global controls

• Five options are available to user:– “Element Size” defines average element edge size

– “Curv/Proximity” places more elements near curvature or proximity of edges to each other

• Set slider bar from –100 to +100. If “Element Size” left to “Default”, “Curv/Proximity” behaves the same as “Relevance”

– “Shape Checking” defines element shape quality tests used• Standard: For linear analyses

• Aggressive: For nonlinear or field type analyses



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• Five options are available to user (cont’d):– “Solid Element Order” allows users to toggle

between lower- or higher-order solid elements.• Higher-order solid elements are default

– “Initial Size Seed” controls what geometry the mesh seeding is based on


Part-Based Mesh SeedingNodes: 52,484Elements: 19,816(Mesh seeding is based on parts, so less uniform between parts)

Assembly-Based Mesh SeedingNodes: 13,001Elements: 5,666(Mesh seeding is more uniform between parts)


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… Solid Element Shape

• The “Method” control provides the user with options as to how solid bodies are meshed:– “Auto Sweep if Possible”: sweeps volumes

using brick or wedge shaped elements

– “All Tetrahedrons”: meshes all volumes with tetrahedral elements

– “Hex Dominant”: attempts to mesh using brick shapes but will contain some pyramid and tet shapes. (only available with ANSYS Structural licenses and above)



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… Solid Element Shape

• Sweep Meshing:– Sweep-meshable volumes will have hex (and

possible wedge) elements. Other volumes willbe meshed with tet elements.

– Sweep-meshing is done in cases where a volume has the same topology in one direction.

– Right-click on “Mesh” branch gives user ability to see what volumes may be ‘swept’ with “Preview Sweep”. Sweepable solid bodies will be selected.


For model shown on right, the solid body in middle is swept-meshed with hexahedral (and pentahedral) elements, whereas other volumes are meshed with tetrahedral elements.


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… Local Mesh Controls

• Sizing:– “Element Size” specifies average element edge length

– “Number of Divisions” specifies number of elements on edge(s)

– “Sphere of Influence” elements within the sphere have a given average element size

– Available options above depend on which entities are scoped:


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


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“Sphere of Influence” (shown in red) has been defined. Elements lying in that sphere for that scoped entity will have a given average element size.

Scoped to single surface

Scoped to 3 surfaces

• Sphere center located using local coordinate system

• All scoped entities within sphere are affected by size settings


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• Contact Sizing provides a way of generatingsimilar-sized elements on contact faces– An “Element Size” or “Relevance” can be specified

for a given contact region

In this example, the contact region between the two parts has a Contact Sizing specified (by Element Size). Note that the mesh is now consistent at the contact region.



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• Element refinement divides existing mesh– An ‘initial’ mesh is created with global and local

size controls first, then element refinement is performed at the specified location(s).

– Refinement level of “1” is recommended. This breaks up the edges of the elements in the ‘initial’ mesh in half.

For example shown, the left side has refinement level of 1 whereas the right side is left untouched with default mesh settings.



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… Mapped Face Meshing

• Mapped Face Meshing allows for the generationof structured meshes on surfaces:– In example below, mapped face meshing on the

internal cylindrical face provides a more uniformmesh pattern.

– If surface cannot be mapped mesh for any reason, meshing will continue and this will be shown in Outline Tree with icon:



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… Mapped Face Meshing

• Mapped quad or tri mesh also available for surface bodies



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. . . Match Face Meshing

• Matches mesh pattern on symmetry faces to facilitate cyclic symmetry analyses typical of rotating machinery

• For cyclic symmetry the nodal locations on each cut face must be identical except for the offset (see below)

Cut Boundaries

Full Model Cyclic Symmetry Model

Matched Faces


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. . . Match Face Meshing

• Procedure:– Insert “Match Face Meshing” control under Mesh branch

– Identify faces of symmetry boundary

– Identify the coordinate system (Z axis is rotation axis)

Rotation CS


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• Part Relevance allows controlling mesh by parts– Control is given with a slider (-100 to +100)


Part Relevance=+100

Part Relevance=-100


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… Part Proximity

• Part Proximity specifies that the proximity ofsmall lines to each other will affect mesh density

– Degree is controlled by global “Relevance” or “Curv/Proximity”

Default Mesh Default Mesh w/ Part Proximity



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… Hex-Dominant Meshing

• Advanced Structural Meshing introduction:– The hex-dominant meshing algorithm creates a quad-

dominant surface mesh first, then pyramid and tetrahedral elements are filled in as needed.

• “Hex Dominant” option for the “Element Shape” branch is only available with ANSYS Structural licenses and above

• “Control Messages” will appear to warn user if volume may not be suitable for hex-dominant meshing

ANSYS License AvailabilityDesignSpace EntraDesignSpaceProfessionalStructural xMechanical/Multiphysics x


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… Hex-Dominant Meshing

• Example of hex-dominant mesh shown below:– 10,918 brick (39%)

– 6,289 tetra (23%)

– 907 wedges (3%)

– 9,631 pyramids (35%)

ANSYS License AvailabilityDesignSpace EntraDesignSpaceProfessionalStructural xMechanical/Multiphysics x


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… Meshing Failures

• If the mesher is not able to generate well-shaped elements, an error message will be returned:

– The problematic geometry will be highlighted on the screen, and a named selection group “Problematic Geometry” will be created, so the user may review the model.



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… Meshing Failures

• Meshing failures can be caused by a number of things:– Inconsistent sizing controls specified on surfaces, which

would result in the creation of poorly-shaped elements

– Difficult CAD geometry, such as small slivers or twisted surfaces

– Stricter shape checking (“Aggressive” setting in Mesh branch)

• Some ways to avoid meshing failures:– Specify more reasonable sizing controls on geometry

– Specify smaller sizing controls to allow the mesher to create better-shaped elements

– In the CAD system, use hidden line removal plots to see sliver or unwanted geometry and remove them

– Use virtual cells to combine sliver or very small surfaces– This option will be discussed next



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… Virtual Topology

• Virtual Topology allows users to combine surfaces for the purposes of meshing– “Virtual Topology” branch can be added under the “Model”

branch

– A “Virtual Cell” is a surface defined by a group of adjacent surfaces. Select surfaces first, then add “Virtual Cell”

– Interior lines of original surfaces will no longer be honored by meshing process

– For other operations such as applying Loads and Supports, a virtual cell can be referenced as a single entity



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• When creating virtual cells, select surfaces first, then add a virtual cell:

• Only after Simulation determines that surfaces can be joined will a new Virtual Cell branch be created.

– When a virtual cell is added, the entities cannot be changed• Details View will show the “Geometry” field as grey (unmodifiable)

• Since surfaces need to be evaluated before the virtual cell is defined, the surfaces cannot be changed afterwards

• A “Model” branch containing a “Virtual Topology” branch cannot be duplicated or copied



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… Virtual Topology Example

• Consider the example below:


The small chamfer causes a finer element mesh near cylindrical area.

Virtual cell (red) of two surfaces (top surface and chamfer surface) created

Original model contains a small cylindrical surface, which forms the chamfer

Resulting mesh is not driven by small features anymore.


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… Virtual Topology Example

• Keep in mind that topology changes slightly, however!– Because chamfer is added to top surface in virtual cell, the

interior lines are not recognized anymore.

– Because of this, the position of the mesh is slightly lower than originally expected, and the topology changes slightly.

On the right, the higher-order element’s edge is shown as a solid yellow line. The contour of the original chamfer and top surface is shown as a dotted blue line.

The midside node of the element is projected onto the top surface, but the chamfer representation is no longer present because no nodes lie on the boundary between the chamfer and top surface.



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• Typical error messages which may be encountered:– Surface normals differ

significantly, preventingcreation of virtual cell:

– More than one surface needs to be selected:

– Adjacent surfaces need to be selected:

– Virtual cells containing other virtual cellscannot be created:



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D. Named Selections

• The Named Selection Toolbar provides functionality for grouping together geometric entities:

– Named Selections allow users to group together vertices, edges, surfaces, or bodies together

– Named Selections can be used for defining mesh controls, applying loads and supports, etc.

– Provides an easy method to reselect groups that will be referenced often

• Defining contact regions

• Scoping results

• Etc.


Create Defined Names Manipulate Show/Hide Supress/Unsuppress


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… Defining Named Selections

• To create Named Selections:– Select the vertices, edges, surfaces, or bodies of

interest, then click on the “Create Selection Group” icon

– Enter a name in the dialog box

– The new group will appear in the Named Selection Toolbar as well as in the Outline Tree

• Note:– Only one type of entity can be in a particular

Named Selection. For example, vertices and edges cannot exist in the same Named Selection.

– Named Selection groups can be imported fromsome CAD systems (see Chapter 10)



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… Using Named Selections (1)

• For mesh controls and loads/supports, Named Selections can be referenced directly:– In the Details view, change “Method” from “Geometry

Selection” to “Named Selection”

– Select the “Named Selection” from the pull-down menu• Simulation will filter non-applicable types of Named Selections.



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… Using Named Selections (2)

• Named Selections can be used in other situations where geometry must be picked:– Select “Geometry” from the Details view to enter picking mode

– Toggle the Named Selection to select from the Toolbar

– Select the applicable choice:• “Select Items in Group”, “Add to Current Selection”, “Remove from

Current Selection”

– Then, click on “Apply” in the Details view


1

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… Named Selections and Bodies

• Named Selection groups can be hidden or suppressed:– If surface Named Selections are hidden, the associated bodies

will be hidden graphically. Show/hide and Suppress/Unsuppress functionality work on bodies, not individual lines or surfaces.



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E. Coordinate Systems

• The Coordinate Systems branch is usually not displayed by default, but it can be added from the Model tree

– Coordinate systems can be used for mesh controls, point masses, directional loads, and results

• Initially the “Global Coordinate System” is added, based on the origin of the CAD model.

• Local Coordinate Systems can be imported from some CAD systems (see Chapter 10)



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… Coordinate Systems

• Coordinate Systems can be defined by selecting “Coordinate System” icon from the Context toolbar– “Type” (Details view), can be cartesian or

cylindrical.

– Local coordinate systems are defined by selecting geometry to specify the origin

– The orientation can then be changed by selecting “X/Y/Z Direction” and selecting an appropriate surface to define the direction

• Reversing the direction can be done in the Graphics window after selecting a surface.



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… Coordinate Systems

• Coordinate systems can be used from pull-down menus in the Details view for appropriate operations

Sizing w/ Sphere of Influence Option

Force Load (Directional Load)

Point Mass

Directional Results



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• Workshop 3 – Mesh Control

• Goal:– Use the various DS mesh controls to

enhance the mesh for the crankshaft model.

F. Workshop 3 – Mesh Control

general preprocessing chapter three. training manual general preprocessing procedure august 26, 2005...

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