An Algorithm for Projecting Points onto a Patched CAD Model

Proceedings, 10th International Meshing Roundtable, Sandia National Laboratories, pp.353-362, October 7-10 2001

MESHING
RESEARCH
CORNER

10th International Meshing Roundtable
Newport Beach, California, U.S.A.
October 7-10, 2001

Centre for Applied Scientific Computing
Lawrence Livermore National Laboratory
Livermore, CA, 94551
Email: henshaw@llnl.gov

Abstract
We are interested in building structured overlapping grids for geometries defined by computer-aided-design (CAD) packages. Geometric information defining the boundary surfaces of a computation domain is often provided in the form of a collection of possibly hundreds of trimmed patches. The first step in building an overlapping volume grid on such a geometry is to build overlapping surface grids. A surface grid is typically built using hyperbolic grid generation; starting from a curve on the surface, a grid is grown by marching over the surface. A given hyperbolic grid will typically cover many of the underlying CAD surface patches. The fundamental operation needed for building surface grids is that of projecting a point in space onto the closest point on the CAD surface. We describe a fast algorithm for performing this projection, it will make use of a fairly coarse global triangulation of the CAD geometry. We describe how to build this global triangulation by first determining the connectivity of the CAD surface patches. This step is necessary since it often the case that the CAD description will contain no information specifying how a given patch connects to other neighbouring patches. Determining the connectivity is difficult since the surface patches may contain mistakes such as gaps or overlaps between neighbouring patches.

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