Using Faceted Geometries for Analysis and Design

Many analysis and design tasks start from a previous configuration that is (unfortunately) defined by tessellated surfaces, such as in a stereo-lithography file. Sometimes these definitions come from legacy CAD tools that are no longer supported, and sometimes they come from laser scans of a real configuration (such as a wing with ice growing on the leading edge). While these files define a watertight configuration that look good in the eye-ball norm, the fact that the surfaces are faceted yields less-than-desirable results when used in CFD and FEM tools. This paper describes two techniques for transforming faceted representations into smooth boundary representations. The first, called Slugs, generates a static representation by breaking the facets into contiguous groups and then fitting each group with a B-spline; the configuration is called “static” since it cannot easily be modified, except possibly via free-form deformation tools that operate on the associated B-splines. The second, called Plugs, automatically modifies the design parameters of a user-defined parametric model to best-fit the points in the tessellated surfaces. This paper describes these two techniques and the various enhancements in them since their introduction is previous publications.