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Regge Finite Elements with Applications in Solid Mechanics and Relativity

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Regge Finite Elements with Applications in Solid Mechanics and Relativity Regge Finite Elements with Applications in Solid Mechanics and Relativity A DISSERTATION SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Lizao Li IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Doctor of Philosophy Advisor: Prof. Douglas N. Arnold May 2018 © Lizao Li 2018 ALL RIGHTS RESERVED Acknowledgements I would like to express my sincere gratitude to my advisor Prof. Douglas Arnold, who taught me how to be a mathematician: diligence in thought and clarity in communication (I am still struggling with both). I am also grateful for his continuous guidance, help, support, and encouragement throughout my graduate study and the writing of this thesis. i Abstract This thesis proposes a new family of finite elements, called generalized Regge finite el- ements, for discretizing symmetric matrix-valued functions and symmetric 2-tensor fields. We demonstrate its effectiveness for applications in computational geometry, mathemati- cal physics, and solid mechanics. Generalized Regge finite elements are inspired by Tullio Regge’s pioneering work on discretizing Einstein’s theory of general relativity. We analyze why current discretization schemes based on Regge’s original ideas fail and point out new directions which combine Regge’s geometric insight with the successful framework of finite element analysis. In particular, we derive well-posed linear model problems from general relativity and propose discretizations based on generalized Regge finite elements. While the first part of the thesis generalizes Regge’s initial proposal and enlarges its scope to many other applications outside relativity, the second part of this thesis represents the initial steps towards a stable structure-preserving discretization of the Einstein’s field equation. ii Contents Acknowledgementsi Abstract ii Contents iii List of Tables vi List of Tables vi List of Figures vii List of Figures ix 1 Introduction 1 1.1 From Regge Calculus to Regge finite elements: a brief introduction.......1 1.2 Outline............................................4 2 Generalized Regge finite element6 2.1 Definition of generalized Regge family.........................9 2.2 Basic properties...................................... 14 2.2.1 Bernstein decomposition for Lagrange elements............... 17 2.2.2 Geometric decomposition for Regge elements................. 19 2.3 Affine and approximation properties.......................... 27 2.3.1 Affine property................................... 27 2.3.2 Approximation properties of the canonical interpolant........... 29 2.4 Coordinate representations and implementable degrees of freedom....... 34 2.4.1 Coordinate representations........................... 34 2.4.2 Implementable degrees of freedom....................... 37 iii 3 Geodesics on Generalized Regge metrics 41 3.1 Introduction......................................... 41 3.2 Review of the smooth geodesic theory......................... 47 3.3 Global geodesics on Regge metrics........................... 49 3.4 Local geodesics on Regge metrics: variational approach............... 51 3.5 Local geodesics on Regge metrics: geometric approach............... 57 3.6 Hamiltonian structures of local geodesics....................... 60 3.7 A robust algorithm for generalized local geodesics.................. 63 3.8 Error analysis........................................ 66 3.9 Numerical examples: Kepler and Schwarzchild systems.............. 76 3.9.1 Kepler system................................... 76 3.9.2 Jacobi’s formulation................................ 78 3.9.3 Numerical examples for Kelperian orbits................... 79 3.9.4 Schwarzschild system............................... 87 4 Rotated generalized Regge finite elements with applications in solid mechan- ics 89 4.1 Rotated generalized Regge finite element....................... 90 4.2 Solving the biharmonic equation via the Hellan-Herrmann-Johnson mixed for- mulation........................................... 92 4.2.1 Hellan-Herrmann-Johnson continuous mixed formulation......... 93 4.2.2 Hellan-Herrmann-Johnson discretization................... 94 4.2.3 Discretization of biharmonic equation in higher dimensions using ro- tated Regge elements............................... 97 4.3 Solving the elasticity equation via the Pechstein-Schöberl mixed formulation. 103 4.3.1 Continuous mixed formulation......................... 103 4.3.2 Rotated Regge element discretization..................... 107 4.3.3 Numerical experiments.............................. 109 4.4 Connection with numerical relativity.......................... 117 5 Model problems in relativity for discretization 119 5.1 The fully nonlinear space-time Einstein equation.................. 121 5.2 Linearized space-time Einstein equation........................ 122 5.3 Matrix calculus notation................................. 124 5.4 Model linear Cauchy problem.............................. 125 5.5 Model linear source problem............................... 129 iv 5.6 Fourier analysis: well-posedness and weak hyperbolicity.............. 130 5.7 Regularized well-posed model problems on bounded smooth domains...... 135 5.8 Regge elements discretization.............................. 143 6 Two failure modes of Regge Calculus 146 6.1 Failure due to the infinite dimensional kernel.................... 147 6.1.1 Well-posedness at continuous level....................... 148 6.1.2 Discretization.................................... 150 6.1.3 Numerical examples and discussion...................... 151 6.1.4 Implications for Regge Calculus......................... 158 6.2 Failure due to the space-time scheme for the second-order time derivative... 158 6.2.1 Regge-calculus style derivation of the model problem............ 159 6.2.2 Initial-value problem and well-posedness................... 160 6.2.3 Regge calculus-like space-time discretization and finite element view.. 161 6.3 Numerical example.................................... 162 6.3.1 von Neumann stability analysis......................... 165 6.3.2 Implication for Regge calculus.......................... 172 Bibliography 173 v List of Tables 2.1 Bernstein-style Basis in 2D................................ 17 2.2 Bernstein-style Basis in 3D................................ 17 3.1 Convergence rates comparison for geodesic solvers.................. 47 3.2 Convergence rate for a fixed maximum time. hm is the mesh size. The rates in the parenthesis are for the cases without turning p at interior facets....... 80 3.3 The error growth rate in time t. The rates in the parenthesis are for the cases without turning p at interior facets........................... 84 4.1 2D biharmonic degree 0.................................. 100 4.2 2D biharmonic degree 1.................................. 101 4.3 2D biharmonic degree 2.................................. 101 4.4 3D biharmonic degree 0.................................. 102 4.5 3D biharmonic degree 1.................................. 103 4.6 3D biharmonic degree 2.................................. 103 4.7 2D elasticity degree 1................................... 110 4.8 2D elasticity degree 2................................... 110 4.9 2D elasticity degree 3................................... 111 4.10 3D elasticity degree 1................................... 111 vi List of Figures 1.1 A triangulated surface...................................1 1.2 Flat: sum of angles equals 2¼...............................2 1.3 Nonflat: sum of angles does not equal 2¼........................2 2.1 Degrees of freedom in 1D for r 0,1,2,...........................6 Æ 2.2 Degrees of freedom in 2D for r 0,1,2,...........................7 Æ 2.3 Degrees of freedom in 3D for r 0,1,2,...........................8 Æ 2.4 Interior degrees of freedom in 3D for r 2,3,4,......................8 Æ 2.5 Mesh and non-meshes in 2D............................... 10 2.6 REG1 assembly on a 3-triangle mesh.......................... 12 2.7 Geometric decomposition of P 3 on a triangle...................... 19 2.8 Edge-based Bernstein decomposition for P 3 on a triangle. The chosen edge is in red. Basis associated with edges are in black while those associated with cells are in blue....................................... 24 2.9 Edge-based Bernstein decomposition for P 3 on a tetrahedron. The chosen edge is thickened. Basis associated with edges are in red, those associated with triangles are in blue, and those associated with cells are in black......... 24 fˆ 2.10 Pictures for Xr ........................................ 38 2.11 Subdivision-based degrees of freedom in 2D...................... 38 2.12 Subdivision-based degrees of freedom in 3D...................... 39 3.1 Illustration of the first geodesic idea........................... 43 3.2 Illustration of the second geodesic idea. In the middle two figures, the red line indicates the tangential direction while the blue indicates the normal direction. 44 3.3 Keplerian orbits. Left to right, top to bottom r 0,1,2,3............... 45 Æ 3.4 Schwarzschild orbits. Left to right, top to bottom, r 0,1,2,3............ 46 Æ 3.5 Failure of having a well-defined tangent space at a point.............. 52 vii 3.6 Pathology of generalized geodesic............................ 53 3.7 Definitions of quantities when a curve crosses an interior facet.......... 54 3.8 Possible bad initial conditions. The green cell is chosen............... 64 3.9 Left: a good crossing. Right: a bad crossing that needs a restart.......... 65 3.10 A geodesic is stuck in a cell...............................
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