An overlapping additive Schwarz preconditioner for boundary element approximations to the Laplace screen and Lamé crack problems

We study a two-level overlapping additive Schwarz preconditioner for the h-version of the Galerkin boundary element method when used to solve hypersingular integral equations of the first kind on an open surface in ℝ3. These integral equations result from Neumann problems for the Laplace and Lamé equations in the exterior of the surface. We prove that the condition number of the preconditioned system is bounded by O(1 + log2(H/δ)), where H denotes the diameter of the subdomains and δ the size of the overlap

Schnelle Löser für Partielle Differentialgleichungen

The workshop Schnelle Löser für partielle Differentialgleichungen, organised by Randolph E. Bank (La Jolla), Wolfgang Hackbusch (Leipzig), and Gabriel Wittum (Frankfurt am Main), was held May 22nd–May 28th, 2011. This meeting was well attended by 54 participants with broad geographic representation from 7 countries and 3 continents. This workshop was a nice blend of researchers with various backgrounds

Multilevel methods for the h-, p-, and hp-versions of the boundary element method

AbstractIn this paper we give an overview on the definition of finite element spaces for the h-, p-, and hp-version of the BEM along with preconditioners of additive Schwarz type. We consider screen problems (with a hypersingular or a weakly singular integral equation of first kind on an open surface Γ) as model problems. For the hypersingular integral equation and the h-version with piecewise bilinear functions on a coarse and a fine grid we analyze a preconditioner by iterative substructuring based on a non-overlapping decomposition of Γ. We prove that the condition number of the preconditioned linear system behaves polylogarithmically in H/h. Here H is the size of the subdomains and h is the size of the elements. For the hp-version and the hypersingular integral equation we comment in detail on an additive Schwarz preconditioner which uses piecewise polynomials of high degree on the fine grid and yields also a polylogarithmically growing condition number. For the weakly singular integral equation, where no continuity of test and trial functions across the element boundaries has to been enforced, the method works for nonuniform degree distributions as well. Numerical results supporting our theory are reported

Parallel solution of elasticity problems using overlapping aggregations

summary:The finite element (FE) solution of geotechnical elasticity problems leads to the solution of a large system of linear equations. For solving the system, we use the preconditioned conjugate gradient (PCG) method with two-level additive Schwarz preconditioner. The preconditioning is realised in parallel. A coarse space is usually constructed using an aggregation technique. If the finite element spaces for coarse and fine problems on structural grids are fully compatible, relations between elements of matrices of the coarse and fine problems can be derived. By generalization of these formulae, we obtain an overlapping aggregation technique for the construction of a coarse space with smoothed basis functions. The numerical tests are presented at the end of the paper

Steklov Spectral Geometry for Extrinsic Shape Analysis

We propose using the Dirichlet-to-Neumann operator as an extrinsic alternative to the Laplacian for spectral geometry processing and shape analysis. Intrinsic approaches, usually based on the Laplace-Beltrami operator, cannot capture the spatial embedding of a shape up to rigid motion, and many previous extrinsic methods lack theoretical justification. Instead, we consider the Steklov eigenvalue problem, computing the spectrum of the Dirichlet-to-Neumann operator of a surface bounding a volume. A remarkable property of this operator is that it completely encodes volumetric geometry. We use the boundary element method (BEM) to discretize the operator, accelerated by hierarchical numerical schemes and preconditioning; this pipeline allows us to solve eigenvalue and linear problems on large-scale meshes despite the density of the Dirichlet-to-Neumann discretization. We further demonstrate that our operators naturally fit into existing frameworks for geometry processing, making a shift from intrinsic to extrinsic geometry as simple as substituting the Laplace-Beltrami operator with the Dirichlet-to-Neumann operator.Comment: Additional experiments adde

Efficient domain decomposition algorithms for the solution of the helmholtz equation

The purpose of this thesis is to formulate and investigate new iterative methods for the solution of scattering problems based on the domain decomposition approach. This work is divided into three parts. In the first part, a new domain decomposition method for the perfectly matched layer system of equations is presented. Analysis of a simple model problem shows that the convergence of the new algorithm is guaranteed provided that a non-local, square-root transmission operator is used. For efficiency, in practical simulations such operators need to be localized. Current, state of the art domain decomposition algorithms use the localization technique based on rational approximation of the symbol of the transmission operator. However, the original formulation of the procedure assumed decompositions that contain no cross-points and consequently could not be used in the cross-point algorithm. In the context of the perfectly matched layer problem, we adapt the cross-point technique and combined with the rational approximation of the square root transmission operator to yield an effective algorithm. Furthermore, to reduce Krylov subspace iterations, we present a new, adequate and efficient preconditioner for the perfectly matched layer problem. The new, zero frequency limit preconditioner shows great reduction in the required number of iterations while being extremely easy to construct. In the second part of the thesis, a new domain decomposition algorithm is considered. From theoretical point of view, its formulation guarantees well-posedness of local problems. Its practicality on the other hand is evident from its efficiency and ease of implementations as compared with other, state of the art domain decomposition approaches. Moreover, the method exhibits robustness with respect to the problem frequency and is suitable for large scale simulations on a parallel computer. Finally, the third part of the thesis presents an extensible, object oriented architecture that supports development of parallel domain decomposition algorithms where local problems are solved by the finite element method. The design hides mesh implementation details and is capable of supporting various families of finite elements together with quadrature formulas of suitable degree of precision