How to Sketch & Mesh 2D Models

Autodesk Simulation sketching and meshing capabilities can be used to build 2D models directly within the FEA Editor environment. These integrated modeling tools complement Autodesk Simulation’s full suite of modeling capabilities, including universal and proprietary CAD format support, and make it easy to perform first-pass or proof-of-concept analyses. For example, you can quickly create and analyze a 2D static stress model to gain an understanding of its loading and behavior within the linear range as preparation and justification for a subsequent 3-D nonlinear analysis or Mechanical Event Simulation (MES).

The general process for building 2D models in the FEA Editor environment consists of four steps:

1. Create a sketch of the wireframe of each part.
2. Optionally specify edge divisions, and refinement points.
3. Generate the 2D mesh.
4. Optionally, specify surface contact between adjacent parts of an assembly (default is bonded).

Sketching Parts

Before beginning a sketch, plan ahead to determine the parts of your model and how they fit together. Keep the following requirements in mind:

• Each part must consist of a single enclosed area and must be a uniform material.
• Line elements (beam/truss etc.) can be added to models containing 2D Elements but must be drawn in the YZ plane at X=0.
• The 2D mesh generator can be used to mesh wireframes that are not on the YZ plane, but the resulting elements cannot be defined as 2D. They can be 3-D planar elements of the planar mesh can be extruded into solid elements.
• 2D element parts cannot be added to models with containing solid element parts.
• The lines where two parts meet must be identical and coincident.

The sketch mode is initiated from the context menu that appears when you right-click on a plane in the Browser (tree view). For 2D modeling, we use the Sketch command from the Plane 2 < YZ(+X) > context menu, as shown in Figure 1.

In the display area, a grid drawn in the YZ plane will appear. Use the commands in the Draw tab of the ribbon within the Draw panel to create sketch entities. You can add lines, circles, construction vertices, rectangles, arcs, fillets, tangent lines, and splines (see Figure 2).

There are also commands for defining a new sketch plane and for setting the “From Point” (that is, the reference point for a subsequent relative coordinates entry). Additionally, entities can be defined as construction objects or actual FEA lines; in which case arcs, circles, and splines are divided into a discrete number of straight line segments approximating the curved geometry. You must specify construction objects when drawing a wireframe for 2D meshes. Once sketch entities are drawn, a new part number is added to the tree view, with the sketch number nested beneath it.

You can toggle the visibility of the sketch plane and grid by right-clicking on the plane heading and clicking the Visibility command. You can also modify grid settings (spacing and color) via the Sketching tab of the Options dialog. This can be accessed by clicking the Application Menu button in the upper-left corner and then clicking the Options button at the bottom of the menu.

The Modify and Pattern panels of the ribbon’s Draw tab contain the various tools for performing operations on sketch entities. You can edit attributes, move points, and divide, intersect, trim, extend, move, copy, rotate, scale, and mirror sketch entities (see Figure 3).

Whether you are drawing sketch entities, copying them, or modifying them, the status bar at the bottom of the screen will provide prompts to step you through the process. Specify the part number for each entity within the draw command dialog boxes when creating them. You can also select any of the construction objects that comprise your 2D sketch and change the part number by using the Attributes command located in the Modify panel. Each part must consist of a wireframe outline that encloses one contiguous area. Multiple internal boundaries, such as slots and holes, are allowed.

Once the entire sketch is complete for all parts, specify options for the 2D meshing operation. Select the numbered sketch entity groups beneath each part heading in the tree view (add to the selection set by holding down the Ctrl key).

HINT: If you have numerous parts in your simulation an easier way to select all sketches is to select the sketch plane with a left click, right click and pick “select all sketches”. This command will automatically select all the sketches drawn within that sketch plane.

All parts must be meshed simultaneously so that the meshes are properly matched. Next, right-click one of the selected headings and choose the Create 2D Mesh... command (see Figure 4) to access the 2-D Mesh Generation dialog box.

Specifying Edge Divisions and Refinement Points

Before performing the meshing operation, you can perform the following meshing-related operations:

• Divisions... – To control how many elements are created along a particular edge; select the construction object(s) representing the desired edges, right-click, and then choose the 2D Mesh Divisions command. The Sketch Entity Mesh Divisions dialog box will appear. Activate the checkbox, specify the number of divisions to be made along the selected edge, and click OK (see Figure 5).

Note that for an all quadrilateral 2D mesh, it is mathematically impossible to generate a mesh if there is an odd number of mesh divisions on the boundary taken as a whole. To avoid a problem, by design for all quadrilateral meshes any user-specified odd-numbered mesh divisions are rounded up to the next even number.

• Add Refinement Point... – Add a refinement point to the sketch by right-clicking at the location where you want to define the refinement point and choosing the Add Refinement Point... command. The Create Refinement Point dialog box appears (see Figure 6). Specify parameters for the refinement point including choosing between two types of refinement—Mesh size or Divide factor. Also enter the “Effective radius,” which defines a circular area about the specified point, within which the refined element size is imposed. You can also manually enter the coordinates of the refinement point.

Generating the 2D Mesh

After using the Create 2D Mesh... command to access the 2D Mesh Generation dialog box, you can specify the element shape, mesh density, or mesh size (see Figure 7). The default mesh density should be a good first approximation for most models. Afterward, you can right-click on the 2-D Mesh heading near the bottom of the Browser (tree view) and choose the Edit command. This will return you to the 2-D Mesh Generation dialog box, where you can change the settings and remesh the model.

Once the mesh is generated, you can also add refinement points using the Mesh tab, Refinement Points panel or add/change mesh division specifications, as previously described, to selectively refine regions. The model must be remeshed after doing so.

The "Advanced" section provides additional meshing parameters (including Angle, Geometric Ratio, Close Factor, Refinement Factor and Tolerance), but, for most cases, the default values will suffice.

With the mesh defined, you can specify data needed for the analysis; including the element type, element definition, materials, loads, constraints, and analysis parameters. Then analyze the model.

Specifying Contact - Please refer to the separate article, How to Define Contact in 2D Analysis, for instructions on performing this step.

In conclusion, the ability to sketch and mesh multiple parts in the FEA Editor environment provides flexibility and convenience for 2D modeling. For a step-by-step demonstration of these sketching and meshing capabilities, see the Modeling with 2D Elements Tutorial. For more information about sketching and meshing tools in the FEA Editor environment, see the Autodesk Simulation In-Product Help or Online Wiki Help.