Dr. Mesh on Modern Art and Surface Wrapping
Images courtesy of James Clement & Alex Read

If there’s one tool that over the years has saved me more time than any other, it has to be the surface wrapper. Gone is the time when I had to spend days if not weeks, closing gaps, fixing non-manifold surfaces, removing double surfaces, extracting the fluid volume and all the other tedious operations that needed to be done before volume meshing. A bit of clever surface wrapping and I can go from any old CAD to volume mesh in no time at all. So this edition of Dr Mesh is dedicated to a couple of hints on getting the most out of the surface wrapper. It also provides the perfect opportunity for me to share my recently discovered flair for a little known modern art form: selfportraits through the medium of CAD!

Click on the thumbnails below to view figures.

A common desire when meshing is the ability to treat two areas of the model in different ways: coarsen and remove geometric features in one, while retraining all detail and sharp corners in the other.

My starting point is some non-ideal CAD (obviously ignoring its aesthetic value). My portrait is made of lots of different closed bodies (shown in different colors in figure 1) that overlap and intersect. My objective is to automatically produce a high quality surface mesh, demonstrating how we can use contract prevention, volume sources and lines to improve feature capture and to focus mesh density in the specific areas.

Contact Prevention
This feature simply requires the user to specify which types should not intersect. The wrapper will then do its level best to prevent these two from touching. Figure 2 shows two wraps: on the right I specified contact prevention for all the parts, and on the left I have removed head, nose and mustache. With the contact prevention on all parts, two differences are seen. First, the nose and mustache no longer join, and second the capture of the sharp corner between the mustache and the head is much improved. Using contact prevention, even in places where we know the surfaces touch, is a neat way of getting the wrapper to focus its attention on intersections and thereby retain the sharp corners.

Generally it is not necessary to put lines on the model, as the wrapper automatically identifies sharp angles and will attempt to resolve them. Where lines are useful is if you need to resolve a feature on a flat surface, or that is defined by a shallow angle. For example, in figure 3 we can see two wraps. In the first wrap (upper) a line and new region type have been defined on the central section of the lamp (where the bulb would be). These have been removed in the second (lower) wrap. As you can see, the interface between the two regions has been well resolved in the wrapped mesh, despite it being on a flat surface.

Volume sources are a method of specifying areas of local refinement in the mesh. In figure 4 you can see two pink boxes (or sources) around the left eye and eyebrow. These sources can be a number of different shapes – bricks, spheres, cylinders and cones – and I can either specify a relative mesh size or an absolute value. Here, it’s set to 5 % of the base
size. By using volume sources I can get highly detailed capture of specific features, without significantly increasing my total mesh count.

Having used the volume sources in the surface wrapping, they can also be applied in the volume meshing (see figure 5) – another of STARCCM+’ s time saving devices!

Whether the CAE world will ever catch onto to the exciting possibilities of CAD based art, I can only hope (I’m currently working on a portrait of Her Majesty the Queen), but it’s certainly been sold on the benefits and timesavings of surface wrapping.

Yours,

Dr. Mesh
Dr Mesh (Ph.D. CFD)