What are the limitations of Define Surface?

In version 6.0 of our 3D programs we introduced NURBS for easier definition of more general surfaces. However, we also still permit users to define surfaces from a Coons Patch routine based on the bounding segments of the surface. A basic description of each is given in Advantages of NURBS in CAE Modeling – this particularly demonstrates the difference in describing curved surfaces. The only case of creating geometry where you need to be concerned about the limitations of the Coons Patch is when you are using the “Define Surface” tool rather than sweeping/extruding, using the 2D mode, or using the primitives to create surfaces.

To the left is an example of a surface that works well as a Coons Patch. If you can readily draw a 3 or 4 sided figure and put a grid across corresponding sides to make the surface, it will work. To the right is a surface that will be tricky, if possible at all to make work as a good single surface. To see this, look at the meshes obtained if one chooses various groupings of the sides.

This skinny “L” shape was deliberately chosen for the fact that while it seems a simple enough shape, the surface mesh clearly doesn’t stay within the presumed limits. The math of the Coons patch simply isn’t that sophisticated. It is recommended that you avoid interior angles greater than 180 degrees. Our old method of fixing these problems is shown to the left. Our new method is shown to the right. In this case the surface was drawn on a 2D plane, or displayed on a 2D plane after drawing so that all closed regions would be recognized. These are then interpreted as 3D surfaces when you switch to 3D mode. Now it isn’t possible to draw quadrilateral elements, but the triangular elements mesh most surfaces better and show no problem with the surfaces that are properly recognized rather than defined from the Coon’s Patch.

Another tricky aspect of the Coons Patch is surfaces containing holes. For more on this see the following question: How can I draw a model without overlapping surfaces?

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