Control of Boundary Representation Topology in Multidisciplinary Analysis and Design

Abstract

Multidisciplinary Analysis and Design requires the use of a single geometric represen- tation for a configuration that is shared amongst the various Computer-Aided Engineering (CAE) disciplines. A natural choice for this geometric hub is a Computer-Aided Design (CAD) system. Unfortunately, each vendor's CAD system is unique; therefore an engi- neering organization must decide on a single system that interfaces with all the various CAE analyses to be utilized. This has been a major impediment to the deployment of multidisciplinary analysis and design systems throughout industry. CAPRI provides a vendor-neutral system for accessing a variety of CAD systems through a unified and simple programming interface that also includes the ability to regenerate components. One of the outputs of CAPRI is the underlying CAD system's Boundary Representation (BRep) that is comprised of a set of topological entities such as Nodes, Edges, and Faces. These entities contain both connectivity information as well as internal pointers into the CAD system's geometry kernel that are used to provide uniform geometric data. While CAPRI has been very successful at providing a vendor-neutral CAD interface, it does not manipulate the structure of the BRep (except for the splitting of periodic curves and surfaces). Therefore, CAPRI's BRep contains both essential configuration information, or design intent, as well as artifacts of the model-building operations. As a result, small changes in design variables can cause discontinuous changes in the model's BRep even though the geometry may have smoothly morphed. These abrupt changes in topology will then cause discontinuous changes in the resulting mesh (or grid) and may propagate to the predicted physics. This is a serious impediment to automated multidisciplinary design, and in some cases may even force termination of the entire design procedure. The work presented in this paper extends CAPRI's methodologies and software such that a BRep can be manipulated. A merge operation allows one to remove topological entities such that the new BRep mirrors the original design intent. A split operation allows one to divide parts of the BRep such that automatic grid generation can be directly supported. These operations are made possible by the use of a lightweight Ferguson spline modeller that generates a skin that is directly coupled to the CAD system's geometric data. This paper describes a simple topological algebra with 2 functions that can be applied to BReps. With this algebra, it is now possible to generate a BRep whose topology remains fixed for continuously-varying geometric changes. This allows for the generation of a continuously varying engineering representation of any configuration from a (possibly discontinuous) BRep that is produced by CAD system regeneration.