Known Issues When Modeling Using PMLs

When modeling with PMLs be aware of the following:

Use of One Single Perfectly Matched Layer Node

A separate Perfectly Matched Layer node must be used for each simply connected PML region. That is, to use one and the same Perfectly Matched Layer node, all PML domains must be in contact with each other. Otherwise the PMLs do not work properly.

Element Quality

The coordinate scaling resulting from PMLs also yields an equivalent scaling of the mesh that can effectively result in a poor element quality. (The element quality displayed by the mesh statistics does not account for this effect.) This typically happens when the geometrical thickness of the PML deviates much from one wavelength (local wavelength rather than free space wavelength). The poor element quality causes poor convergence for iterative solvers and makes the problem ill-conditioned in general.

For this reason, it is strongly recommended that you use swept meshing in the PML domains. The vector element formulations (the ones using two or more components of a vector field variable) in the RF Module are particularly sensitive to low element quality. The sweep direction should be selected the same as the direction of scaling. For Cartesian PMLs and regions with more than one direction of scaling it is recommended to first sweep the mesh in the domains with only one direction of scaling, then sweep the domains with scaling in two directions, and finish by sweeping the mesh in the domains with PML scaling in all three directions.

Complicated Expressions

The expressions resulting from the stretching get quite complicated for spherical PMLs. This increases the time for the assembly stage in the solution process. After the assembly, the computation time and memory consumption is comparable to a problem without PMLs. The number of iterations for iterative solvers might increase if the PML regions have a coarse mesh.

Erroneous Results

PML regions deviating significantly from the typical configurations shown in the beginning of this section can cause the automatic calculation of the PML parameter to give erroneous results.

Use the Same Material Parameters or Boundary Conditions

The PML region is designed to model uniform regions extended toward infinity. Avoid using objects with different material parameters or boundary conditions that influence the solution inside an PML region.