Exact semi-separation of variables in waveguides with nonplanar boundaries
| Autor: | Athanassoulis, Gerassimos A., Papoutsellis, Christos E. |
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| Rok vydání: | 2017 |
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| Druh dokumentu: | Working Paper |
| DOI: | 10.1098/rspa.2017.0017 |
| Popis: | Series expansions of unknown fields $\Phi=\sum\varphi_n Z_n$ in elongated waveguides are commonly used in acoustics, optics, geophysics, water waves and other applications, in the context of coupled-mode theories (CMTs). The transverse functions $Z_n$ are determined by solving local Sturm-Liouville problems (reference waveguides). In most cases, the boundary conditions assigned to $Z_n$ cannot be compatible with the physical boundary conditions of $\Phi$, leading to slowly convergent series, and rendering CMTs mild-slope approximations. In the present paper, the heuristic approach introduced in (Athanassoulis & Belibassakis 1999, J. Fluid Mech. 389, 275-301) is generalized and justified. It is proved that an appropriately enhanced series expansion becomes an exact, rapidly-convergent representation of the field $\Phi$, valid for any smooth, nonplanar boundaries and any smooth enough $\Phi$. This series expansion can be differentiated termwise everywhere in the domain, including the boundaries, implementing an exact semi-separation of variables for non-separable domains. The efficiency of the method is illustrated by solving a boundary value problem for the Laplace equation, and computing the corresponding Dirichlet-to-Neumann operator, involved in Hamiltonian equations for nonlinear water waves. The present method provides accurate results with only a few modes for quite general domains. Extensions to general waveguides are also discussed. Comment: 24 pages plus five appendices (23 pages); 5 figures plus 2 figures in appendix E |
| Databáze: | arXiv |
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