Error estimates for the Scaled Boundary Finite Element Method

• Published in: Computer methods in applied mechanics and engineering 2021-06, Vol.379, p.113765, Article 113765
• Main Authors: Coelho, Karolinne O., Devloo, Philippe R.B., Gomes, Sônia M.
• Format: Article
• Language: English
• Subjects: A priori error estimates; Approximation; Convergence; Dirichlet problem; Duffy’s approximations; Eigenvalues; Eigenvectors; Errors; Estimates; Exact solutions; Finite element analysis; Finite element method; Harmonic functions; Operators (mathematics); Orthogonality; Polynomials; Scaled boundary finite element method; Shape functions; Singularity (mathematics)
• ISSN: 0045-7825; 1879-2138
• DOI: 10.1016/j.cma.2021.113765

The Scaled Boundary Finite Element Method (SBFEM) is a technique in which approximation spaces are constructed using a semi-analytical approach. They are based on partitions of the computational domain by polygonal/polyhedral subregions, where the shape functions approximate local Dirichlet problems with piecewise polynomial trace data. Using this operator adaptation approach, and by imposing a starlike scaling requirement on the subregions, the representation of local SBFEM shape functions in radial and surface directions is obtained from eigenvalues and eigenfunctions of an ODE system, whose coefficients are determined by the element geometry and the trace polynomial spaces. The aim of this paper is to derive a priori error estimates for SBFEM’s solutions of harmonic test problems. For that, the SBFEM spaces are characterized in the context of Duffy’s approximations for which a gradient-orthogonality constraint is imposed. As a consequence, the scaled boundary functions are gradient-orthogonal to any function in Duffy’s spaces vanishing at the mesh skeleton, a mimetic version of a well-known property valid for harmonic functions. This orthogonality property is applied to provide a priori SBFEM error estimates in terms of known finite element interpolant errors of the exact solution. Similarities with virtual harmonic approximations are also explored for the understanding of SBFEM convergence properties. Numerical experiments with 2D and 3D polytopal meshes confirm optimal SBFEM convergence rates for two test problems with smooth solutions. Attention is also paid to the approximation of a point singular solution by using SBFEM close to the singularity and finite element approximations elsewhere, revealing optimal accuracy rates of standard regular contexts. •A priori error estimates for Galerkin SBFEM approximations of harmonic solutions is provided.•The SBFEM approximation spaces is a type of Duffy’s approximation space.•The SBFEM shape functions are orthogonal to Duffy’s approximations that vanish at the element’s skeleton.•The rate of convergence is dominated by the approximation at the skeleton’s element.•Optimal convergence rates are obtained even with point singularity inside the element.