Flexible subgrids in FDTD calculations

A very prosperous method for numerical simulations of electromagnetic field problems is the finite integration technique (FIT) which in time domain and with cartesian grids is computationally equivalent to the finite difference time domain (FDTD) method. To resolve geometrical details very accurately in an orthogonal grid without resolving the mesh in the whole domain subgrids are very useful. A subgridding scheme which allows the refinement of the mesh in arbitrary sub-domains is presented. The so-called flexible subgrids are proposed for 3D time domain calculations. The iteration scheme for FIT time domain calculations is consistently extended to the calculation domain including subgrids. The stability conditions are proven for the extended system and it is shown that the subgrid produces very few numerical reflections.