Spectral method for matching exterior and interior elliptic problems

TL;DR

This paper introduces a spectral method combining interior and exterior elliptic problem solutions using Chebyshev approximations, enabling accurate and efficient computation of coupled boundary value problems in two dimensions.

Contribution

It develops a novel spectral approach with influence matrices and Chebyshev expansions for coupled interior-exterior elliptic problems, including a preprocessing step for basis calculation.

Findings

Solutions converge exponentially with increased resolution.

The influence matrix is well-conditioned.

Method successfully applied to electrostatic potential calculations.

Abstract

A spectral method is described for solving coupled elliptic problems on an interior and an exterior domain. The method is formulated and tested on the two-dimensional interior Poisson and exterior Laplace problems, whose solutions and their normal derivatives are required to be continuous across the interface. A complete basis of homogeneous solutions for the interior and exterior regions, corresponding to all possible Dirichlet boundary values at the interface, are calculated in a preprocessing step. This basis is used to construct the influence matrix which serves to transform the coupled boundary conditions into conditions on the interior problem. Chebyshev approximations are used to represent both the interior solutions and the boundary values. A standard Chebyshev spectral method is used to calculate the interior solutions. The exterior harmonic solutions are calculated as the convolution of the free-space Green's function with a surface density; this surface density is itself the solution to an integral equation which has an analytic solution when the boundary values are given as a Chebyshev expansion. Properties of Chebyshev approximations insure that the basis of exterior harmonic functions represents the external near-boundary solutions uniformly. The method is tested by calculating the electrostatic potential resulting from charge distributions in a rectangle. The resulting influence matrix is well-conditioned and solutions converge exponentially as the resolution is increased. The generalization of this approach to three-dimensional problems is discussed, in particular the magnetohydrodynamic equations in a finite cylindrical domain surrounded by a vacuum.