In this chapter we will cover the basics of creating meshes for CAD models. This will be sufficient for completing the exercises in the first few chapters. The topic will be covered in more detail in Chapter 5, "Meshing."
Autodesk® Simulation is compatible with most FEA software products and most major CAD products. It works from native CAD files as well as STL, IGES, ACIS, and STEP files. For native CAD formats not directly supported, the models can typically be exported using one of the supported universal formats.
Meshing operations are performed on solid models and 3-D plate/shell models using quadrilateral or triangular elements representing the surface of the model.
The interface also offers enhanced user control over the geometric properties of the generated solid brick mesh. Users can control internal angles of quadrilaterals and quadrilateral warpage, adapting to requirements of some FEA systems.
At the lowest interface level, the simulation program retrieves an existing solid model for FEA processing from another source such as a CAD solid modeler or from another finite element program. The engineer can automatically improve the mesh for more accurate and faster FEA results. At the highest level, the engineer can intervene to enhance the model, including adding local mesh refinement, adding manually constructed elements, or merging in additional parts or assemblies.
CAD Solid Models Supported directly:
• ACIS files (*.sat)
• AutoCAD (*.dwg, *.dxf)
• Inventor files (*.ipt, *.iam)
• Inventor Fusion (*.dwg)
• Mechanical Desktop (*.dwg)
• IGES files (*.igs, *.iges)
• STEP files (*.stp, *.step)
• Stereolithography files (*.stl)
Note: More files can be brought into Autodesk Simulation via Autodesk Fusion
To open these models, access the "Application" pull-down menu, select the "Open"
command, and select the file type you need in the CAD Files section of the "Files of type:"
drop-down box. The model will be opened in the FEA Editor environment. You can also use the "Merge" command to create assembly files from multiple part or subassembly files. The models will be combined using the same position and orientation as the CAD solid models.
When you first open a CAD solid model in the user interface, you may be asked if you want to use a process called "surface-knitting." Whether or not you see a surface-knitting prompt depends upon the settings under "Application Menu: Options: CAD Import." The options for the "Knit surface on import" settings are "Yes" "No" and "Ask each time." This process is required if an internal or external fluid part is to be automatically derived within Autodesk Simulation from the imported CAD geometry. It also enhances mesh matching between parts by splitting the surfaces where two parts meet so that the two intersecting surfaces and their feature lines are identical for each part. If surface-knitting is performed, it also lets the user apply a load — such as pressure or convection — to a surface that partially intersects an adjacent part without having it act on the portion of the surface where the two parts meet. In
other words, the load will act everywhere on the surface except where it is coincident with another part, since this portion will actually be identified by a new surface number after knitting has been completed. After the model has been imported, you will need to re-import the original model if you want to change whether to knit the surfaces or not.
It is not necessary to perform surface-knitting on assemblies if the only purpose is to enhance mesh matching. Meshing features called "virtual imprinting" and "smart bonding" are sufficient to enhance connectivity between adjacent parts. Smart bonding was discussed in the
"Introduction" chapter. Virtual imprinting identifies coincident surfaces between adjacent parts and meshes these intersecting regions one time, producing an identical mesh on both parts where they meet. It does not actually divide the larger surface into two subsurfaces as the surface-knitting operation does. So, if this behavior is desired or if fluid part derivation is to be performed, you must still do surface-knitting.
NOTE: By default, the surface-knitting operation is disabled for a new or clean installation of the current software version. For the purpose of this manual's exercises and examples, it will be assumed that the surface-knitting option is set to "No." If this is not the case for a given PC, either change the setting under "Application Menu:
Options: CAD Import" or simply answer "No" whenever prompted unless instructed otherwise. The setting may also be changed by clicking on the "Options" button within the File: Open dialog when a CAD file type is selected.
Creating a Mesh
When a CAD solid model is opened in the FEA Editor environment and the "Mesh: 3D Mesh Settings" command is chosen; the "Model Mesh Settings" dialog pictured in Figure 2.2 will appear.
Figure 2.2: Model Mesh Settings Dialog
The "Solid" radio button in the "Mesh type" section will be selected. By default, the program will automatically create a surface mesh on all parts and verify that they each enclose a watertight volume. The solid mesh will be generated during the analysis phase. The options specified in the "Model Mesh Settings" dialog will be applied, by default, to all of the parts in the model. If you want to apply certain mesh settings to a specific part(s), right-click on the part(s) in the display area or on the heading(s) for the part(s) in the tree views. Select the
"CAD Mesh Options…" pull-out menu and then choose the "Part…" command. A dialog identical to the "Model Mesh Settings" dialog will appear. The mesh settings specified in this dialog will only be applied to the selected part(s). This functionality will allow you to mesh certain parts as brick elements and other parts as plate elements, for example.
Click on the slider bar in the "Mesh size" section and drag it to the desired mesh coarseness or fineness. Press the "Mesh model" button to create the mesh. When the mesh is complete, you will have the option to view the mesh results. Pressing the "View Mesh Results" button, within the "Mesh panel options; will access the "Meshing Results" dialog shown in Figure 2.3.
Figure 2.3: Meshing Results Dialog
If the "Model" button is depressed, the mesh results for the entire model will be displayed. If the
"Part" button is depressed, you will be able to toggle through the results for each part in the assembly. After you are finished reviewing the mesh results, press the "Close" button.
Model Mesh Settings – Options
By pressing the "Options…" button within the "Model Mesh Settings" dialog, a different
"Model Mesh Settings" dialog will appear. There are three icons on the left side of this dialog that will each access different options.
• The "Surface" icon will access options that are used to control the surface mesh.
• The "Solid" icon will access options that are used to control the solid mesh.
• The "Model" icon will access options that will affect all parts of the model.
The dialog accessed by the "Surface" icon is shown in Figure 2.4.
Figure 2.4: Model Mesh Settings Dialog with the Surface Icon Active
Mesh Settings – "Surface" Section: The options within the "Surface" section of the mesh settings dialog control the size of the mesh, how to proceed when automatic mesh size reduction is necessary, and whether second order elements are to be generated.
Mesh size section:
Size: The value in this field controls the size of the mesh that will be applied. The type of value shown depends on the selected option in the "Type" drop-down box. If the
"Percent of automatic" option is selected, this value will be a ratio of the default mesh size that was determined when the model was opened in the FEA Editor environment. If the "Absolute mesh size" option is selected, this value will be the length of one side of an element in the current units system. The actual element length will typically vary slightly from the requested size because the number of elements along an edge or across a surface must be a whole number.
Type: In this drop-down box, you can choose to have the size defined as a percent of the default value calculated when the model was opened ("Percent of automatic") or an absolute size ("Absolute mesh size").
Retries section:
If a successful mesh cannot be formed with the currently specified mesh size, the mesh engine will try again after reducing the size by the value in the "Retry reduction factor"
field. It will repeat this process, if necessary, until the number of retries specified in the
"Number of retries" field is reached. If all of the retries fail, the original mesh size will be used and any problem surfaces will not be meshed.
Generate 2nd order elements:
There is an option in the element definition screen to include midside nodes in the finite element solution for brick, shell, tetrahedron and certain other element types. When midside nodes are included, they are – by default – placed at the midpoint of straight
lines connecting the vertices of each element. This is true even when a surface mesh lies on a curved surface of a CAD-based model. Activating the "Generate 2nd order elements"
option causes midside node placement to be based on the surface of the CAD model so that they follow the curvature of the part(s). In other words, they don't have to lie along a straight line between two element corner vertices.
Mesh Settings – "Model" Section: Please refer to Chapter 5 for information regarding options within this section of the mesh settings dialog.
Mesh Settings – "Solid" Section: The options within the "Solid" section of the mesh settings dialog are beyond the scope of this introductory level course. Please consult the program Help files for further information. The appropriate help file section may be accessed by pressing the "Help" button within the model mesh setting dialog or via the "Contents,"
"Index" or "Search" command in the HELP pull-down menu.