Defining the connecting sides for a bottom-up swept mesh is a complex task. Connecting sides are required only when creating a swept mesh that uses orphan element faces as the source side. However, even when the connecting sides are not required, using them can save you a significant amount of time after you create the mesh. Connecting sides help control the shape of the swept mesh, and they enforce associativity between the mesh and geometry or force the mesh to match the existing mesh along the connecting sides, depending on whether you are working with geometry or a mesh. In other words, connecting sides force a bottom-up swept mesh to be more similar to a top-down swept mesh. Without connecting sides, you may need to edit the mesh shape, merge nodes, or otherwise modify the mesh to comply with adjacent geometry or meshes, and you will need to manually associate the mesh with the surrounding sides if there are any loads or boundary conditions applied to those sides.
Connecting sides for a bottom-up mesh may be a combination of geometric faces, element faces, and two-dimensional elements. Elements or element faces must be quadrilaterals, and a single connecting side cannot contain both mesh entities and geometric faces. The criteria below and the examples that follow should help you select valid connecting sides or create valid connecting sides from sides that are initially invalid:
Each geometric face must have four logical sides.
The corners of each geometric face must be consistent with the mesh sweeping operation.
Connecting sides must share a common edge with the source side.
Geometric faces or mesh entities must form a regular grid pattern.
Each geometric face must have four logical sides
In Figure 17–99 the top face contains a hole and the front face includes a triangular face. Neither face can be used as a connecting side because of these additional features.
If you select sides such as these, Abaqus/CAE will display an error message indicating that the faces are topologically not four-sided.In addition to topological tests, Abaqus applies geometric tests to determine whether faces that have four or more edges should be considered four-sided. The geometric tests evaluate the angles at the vertices of the face and determine whether a good quality structured mesh can be generated. You can override the geometric tests by using the mesh controls to assign the structured meshing technique to these faces.
For example, in Figure 17–100 the front face is topologically four-sided; however, because the sides meet the top edge along a curve, Abaqus/CAE does not recognize all four of the corners.
When you finish selecting connecting sides, Abaqus/CAE highlights faces that are not geometrically four-sided and displays an error message. You can use the mesh controls to redefine the region's corners as highlighted in the figure so that the face can be used as a connecting side. For more information on using mesh controls on the faces of bottom-up mesh regions, see “Improving the quality of boundary meshes for a bottom-up region,” Section 17.11.6.The corners of each geometric face must be consistent with the mesh sweeping operation
The four corners shown on the selected face in Figure 17–101 do not match the corners of the adjacent source side (the bottom side of the part). In this case you can finish selecting the connecting sides; but when you attempt to apply the bottom-up swept mesh, Abaqus/CAE indicates that it cannot map the mesh on the connecting sides into a regular grid. If you use mesh controls to remove the corner at the semicircular cut and add a corner at the bottom left corner of the side, you can use the face as a connecting side.
Connecting sides must share a common edge with the source side
The common edge shared between the source side and connecting sides cannot extend beyond the edges of the source side. In addition, if you use two-dimensional elements or element faces as the source side and select geometric faces as a connecting side, or vice versa, the element edges must be associated with the geometric edge or Abaqus will not recognize them as sharing a common edge. As an alternative to associating the elements with the geometric edge, you can create a bottom-up boundary mesh on the geometric face and manually merge the nodes of the source side and connecting side meshes (for more information, see “Creating the boundary mesh for a bottom-up region,” Section 17.11.5). If a connecting side does not share a common edge with the source side, Abaqus/CAE indicates that it cannot map the mesh on the connecting sides into a regular grid.
Geometric faces or mesh entities must form a regular grid pattern
The geometric faces or element faces and two-dimensional elements that you select must form a regular grid pattern. Figure 17–102 provides two examples of acceptable connecting face patterns.
The combination of the three four-sided faces in Figure 17–103 makes it unacceptable as a connecting side. In this case when you indicate that you are done selecting connecting sides, Abaqus/CAE displays an error message stating that the selected geometric faces do not form a grid pattern.
