Thomas Hughes, Institute for Computational Engineering and Sciences, University of Texas at Austin

Friday April 13th, 4pm, Phillips 332 (refreshments served in Phillips 330 starting at 3:30)

Computational Geometry and Computational Mechanics

Abstract: Geometry is the foundation of analysis yet modern methods of computational geometry have until recently had very little impact on computational mechanics. The reason may be that the Finite Element Method (FEM), as we know it today, was developed in the 1950’s and 1960’s, before the advent and widespread use of Computer Aided Design (CAD) programs, which occurred in the 1970’s and 1980’s. Many difficulties encountered with FEM emanate from its approximate, polynomial based geometry, such as, for example, mesh generation, mesh refinement, sliding contact, flows about aerodynamic shapes, buckling of thin shells, etc. It would seem that it is time to look at more powerful descriptions of geometry to provide a new basis for computational mechanics.

At least two themes have recently emerged with this spirit. One is based on the Isogeometric concept utilizing NURBS and the other is based on Subdivision Surfaces. Both approaches have been demonstrated to have considerable potential and several advantages over typical FEMs.

The purpose of this talk is to explore the new generation of computational mechanics procedures based on modern developments in computational geometry. The emphasis will be on the Isogeometric approach in which basis functions generated from NURBS (Non-Uniform Rational B-Splines) are employed to construct an exact geometric model. For purposes of analysis, the basis is refined and/or its order elevated without changing the geometry or its parameterization. Analogues of finite element h- and p-refinement schemes are presented and a new, more efficient, higher-order concept, k-refinement, is described. Refinements are easily implemented and exact geometry is maintained at all levels without the necessity of subsequent communication with a CAD (Computer Aided Design) description. In the context of structural mechanics, it is established that the basis functions are complete with respect to affine transformations, meaning that all rigid body motions and constant strain states are exactly represented. Standard patch tests are likewise satisfied. Numerical examples exhibit optimal rates of convergence for linear elasticity problems and convergence to thin elastic shell solutions. Extraordinary accuracy is noted for structural vibrations calculations. A k-refinement strategy is shown to converge toward monotone solutions for advection-diffusion processes with sharp internal and boundary layers, a very surprising result. The current state of mathematical convergence results will be summarized and initial efforts directed toward development of unstructured NURBS refinement will be presented. It is argued that Isogeometric Analysis is a viable alternative to standard, polynomial-based, finite element analysis and possesses many advantages.