The Edge Substitution tool in CFD-GEOM is designed to make it easy to modify surface boundaries. It allows you to replace the lines/curves of a surface with edges, and vice versa.This is very useful if you have already created surfaces from lines/curves and realize that you want to use edges to add control to the mesh size. Indeed, with this tool you do not need to delete the surface(s), create the edges, and then reconstruct the surface from those edge(s). Instead, you simply need to create the edges and directly substitute them into the surface.

Figure 1.  Edge Substitution tool under Trimmed Surface Modification Options

To substitute in/out edges, use the following steps:

1. Select the Trimmed Surface Modification Tools --> Edge Substitution tool
2. Select the set of Edges to substitute into the surface (Ctrl-A will select all displayed edges). Note that any edge that is already used in the surface will be silently ignored if selected.
3. Click "Substitute In" to substitute the selected edges into the surface, as shown in Figure 2. Notice that the edge grid points are colored cyan, indicating they are now part of the surface.

Figure 2.  Edge substitution options

Now that the edges have been substituted into the surface, they will be used when re-meshing the surface. Figures 3 and 4 show the mesh without and with edge substitution. Note that upon substituting in/out edges, the mesh will be destroyed.

Figure 3.  Mesh without using edges during surface mesh creation	Figure 4.  Mesh using edges during surface mesh creation

To remove the edges from the surface, go back to the Edge Substitution tool, select the edges to be removed from the surface, and click on "Substitute Out."

In some cases, edge substitution may not be possible. An example is shown in Figure 5. In this example, two surfaces have been created. The selected edge (shown in red) was created using the top line of each surface, and therefore cannot be substituted into either surface.

Figure 5.  Case where edge substitution is not possible

Edges can be a nice way to control the mesh size in the entire model or in certain regions where mesh quality/size may be of particular importance. This tool now enables you to make these modifications quickly and easily.

Regards,
ESI CFD Support Team