Does the Helmholtz boundary element method suffer from the pollution effect?

TL;DR

This paper proves that the $h$-version Galerkin boundary element method for the Helmholtz exterior Dirichlet problem does not suffer from the pollution effect in nontrapping obstacle scenarios, clarifying its efficiency at high frequencies.

Contribution

It provides a rigorous proof that the $h$-BEM avoids pollution effects for nontrapping obstacles, unlike the well-understood $h$-FEM.

Findings

The $h$-BEM does not suffer from pollution in nontrapping cases.

Theoretical bounds on degrees of freedom growth with frequency.

Clarification of BEM efficiency at high frequencies.

Abstract

In $d$ dimensions, accurately approximating an arbitrary function oscillating with frequency $\lesssim k$ requires $\sim k^d$ degrees of freedom. A numerical method for solving the Helmholtz equation (with wavenumber $k$ and in $d$ dimensions) suffers from the pollution effect if, as $k\to\infty$, the total number of degrees of freedom needed to maintain accuracy grows faster than this natural threshold (i.e., faster than $k^d$ for domain-based formulations, such as finite element methods, and $k^{d-1}$ for boundary-based formulations, such as boundary element methods). It is well known that the $h$-version of the finite element method (FEM) (where accuracy is increased by decreasing the meshwidth $h$ and keeping the polynomial degree $p$ fixed) suffers from the pollution effect, and research over the last $\sim$ 30 years has resulted in a near-complete rigorous understanding of how quickly the number of degrees of freedom must grow with $k$ to maintain accuracy. In contrast to the $h$-FEM, at least empirically, the $h$-version of the boundary element method (BEM) does $\textit{not}$ suffer from the pollution effect (recall that in the boundary element method the scattering problem is reformulated as an integral equation on the boundary of the scatterer, with this integral equation then solved numerically using a finite-element-type approximation space). However, the current best results in the literature on how quickly the number of degrees of freedom for the $h$-BEM must grow with $k$ to maintain accuracy fall short of proving this. In this paper, we prove that the $h$-version of the Galerkin method applied to the standard second-kind boundary integral equations for solving the Helmholtz exterior Dirichlet problem does not suffer from the pollution effect when the obstacle is nontrapping (i.e., does not trap geometric-optic rays).