Harmonic Response - Problem Description
Tutorial 6: Automeshing a Simple Solid Model - -Surface Automeshing
The Surface Automeshing tool is used to automatically generate a mesh of plate elements for specified geometry faces.
This generates plate elements on the surface of the model. The tool can be configured to mesh at a specified density and to mesh all or a selected number of faces. Meshes can be created using Tri3, Tri6, Quad4 or Quad8 elements. Note that if a quadrilateral element type is specified, a small number of triangular elements may still be generated. Hence, these meshes are often referred to as 'quadrilateral dominant' or 'quad dominant' meshes.
Note
In order to use the Solid Automeshing tool, a Surface Automesh must first be created. The resulting surface mesh must fully enclose and define the solid.
The decision as to which element type to generate can be made by determining whether a plate or a brick mesh is
quality element than the three-node triangular element. However, triangular elements should be used for generating tetrahedral meshes. The decision to create either linear or quadratic elements is dependent on the required analysis. A suitable mesh density is also dependent on the analysis type and the density necessary to provide an acceptable solution with the available computer resources.
The Surface Automeshing tool, as shown in Figure 6.10 below, can function in two modes. In Auto mode (default option) the Maximum Edge Length specifies the mesh density, either as an absolute length or a percentage of the maximum model size. Clicking the Custom tab allows detailed control of the surface mesh creation parameters. For this tutorial, the default automatic settings are used.
Figure 6.10 - Surface Meshing dialog box
Maximum Edge Length sets the largest edge length of any element created by automeshing. This Maximum Edge Length can be previewed graphically by clicking the Preview button, which temporarily displays nodes along the edges. Previewing the mesh can be achieved by,
1. Choose Tools/Automeshing/Surface Mesh (alternatively, press CTRL+M).
2. The Maximum Edge Length can be specified as either an absolute value or a percentage.
3. Enter a Maximum Edge Length % value of 3.
4. Click Preview.
This function gives an indication of the mesh density by displaying nodes at the specified edge length. You should use this function if you are uncertain about the mesh density required to correctly represent the geometry. The node density displayed is only indicative. The actual density may be somewhat different as the preview does not take into account transitioning or the local edge curvature. Figure 6.11 below shows the resulting display using a Maximum Edge
Length of 3%.
Figure 6.11 - Mesh Density Preview with Maximum Edge Length of 3%
The Maximum Edge Length can be adjusted until a satisfactory mesh density is found. For this tutorial, a Maximum Edge Length of 5% will be used to create a linear triangular mesh. This can be achieved by:
1. If the Surface Meshing dialog box is not already open, choose Tools/Automeshing/Surface Mesh.
2. Enter a Maximum Edge Length % value of 5.
3. Click the Target tab.
4. Click the Tri3 button.
5. Under Plate Property select Face Number(the resulting plates are assigned a plate property number equal to their parent face number).
6. Click Mesh.
The resulting mesh should look like Figure 6.12 below.
Figure 6.12 - Automeshed Model with Mesh Density Irregularities
Observe that the meshing operation has preserved the restraint and pressure attributes. To see the plate outlines more clearly,
1. Choose View/Entity Display (alternatively, press CTRL+ALT+E).
2. Click the Plates Display Options tab.
3. Under Colours, click the box next to Outline.
4. Choose Black as a colour.
5. Click OK.
6. Click OK to apply the changes and close the Entity Display dialog box.
The mesh however, contains some anomalies. Although the mesh density is uniform on most faces, some faces have a mesh density that sharply varies across the surface. This can be seen as a clustering of nodes in Figure 6.12 above.
Unless the geometry has intentionally small features in relation to surrounding features (such as a hole in the sheet), this indicates a problem with the geometry.
By zooming into these regions, small 'sliver' faces can be seen in between two larger faces. These sliver faces are the cause of the clustering in our model. Figure 6.13 below shows one of these sliver faces.
Figure 6.13 - Meshed Sliver Face causing Mesh Density Irregularities
This sliver face was not an intended feature of the CAD model. Its purpose was simply to maintain visual coherence of this area of the model. For an FEA model however, this sliver face disrupts the uniformity of the mesh and should be removed. Cleaning the geometry with a larger Minimum Edge Length can do this. This will ensure that the Clean Geometry tool collapses the sliver face, hence joining the two adjoining faces. A Minimum Edge Length of 0.05 should be large enough to remove any small faces in this model.
This operation could be performed only on those faces by choosing the selected option. However, this zipping tolerance is still a safe global zipping tolerance for this model as there are no intentionally small features that need to be maintained. Removal of the sliver faces can be achieved by:
1. Choose Edit/Undo, to undo the previous automesh operation.
2. Display the geometry by clicking the Geometry Toggle button.
3. Choose Tools/Clean/Geometry.
4. Enter 0.05 in the Minimum Edge Length box.
6. Click Apply.
7. Click OK to close the Information dialog box.
8. Click Close to close the Clean Geometry dialog box.
The model is ready to be automeshed again. Follow the same steps as before to automesh the model. Figure 6.14 below shows the remeshed model with a 5% mesh density. The mesh is more uniform without noticeable clustering.
Figure 6.14 - Remeshed Model with Sliver Faces Removed