Exact one-way methods for acoustic waveguides

Exact one-way re-formulations of the Helmholtz equation are useful for waveguide problems, since the resulting equations can be efficiently solved as ‘initial’ value problems by range marching methods. Some numerical methods for these re-formulations are reviewed in this paper. This includes a switched method that avoids the singularities of the operators and the large range step methods that give exact solutions for range independent regions and allow large range steps for weakly range dependent regions. For waveguides with curved bottoms, a method based on a local orthogonal transformation is described. As an interesting application, the scattering problem of periodic waveguides is considered.