An extension of the approximate component mode synthesis method to the heterogeneous Helmholtz equation

An extension of the approximate component mode synthesis (ACMS) method to the heterogeneous Helmholtz equation is proposed. The ACMS method has originally been introduced by Hetmaniuk and Lehoucq as a multiscale method to solve elliptic partial differential equations. The ACMS method uses a domain decomposition to separate the numerical approximation by splitting the variational problem into two independent parts: local Helmholtz problems and a global interface problem. While the former are naturally local and decoupled such that they can be easily solved in parallel, the latter requires the construction of suitable local basis functions relying on local eigenmodes and suitable extensions. We carry out a full error analysis of this approach focusing on the case where the domain decomposition is kept fixed, but the number of eigenfunctions is increased. This complements related results for elliptic problems where the focus is on the refinement of the domain decomposition instead. The theoretical results in this work are supported by numerical experiments verifying algebraic convergence for the interface problems. In certain, practically relevant cases, even exponential convergence for the local Helmholtz problems can be achieved without oversampling.