PMLs in Multiphysics

The coordinate stretching used in the PMLs is by default controlled by one of the physics interfaces in the model, which provides a typical wavelength. If each PML region contains a single active physics, and the PML regions are disjoint, you can set up separate PML nodes and choose different physics interfaces as wavelength source. If, instead, there are multiple physics active in the same PML domains or in adjacent domains — such as when an air-water interface extends into the PML — you must choose a single typical wavelength. Either choose a wavelength provided by one of the interfaces, or set a user-defined wavelength.

The way the stretching functions are defined, it usually makes the most sense to select the longest wavelength of propagating waves actually excited and propagating into the PML. Any shorter wavelengths must be accounted for by increasing the mesh resolution and curvature factor in the PML.

In the Acoustics Module, MEMS Module, and Structural Mechanics Module, you can control the typical wavelength passed from the physics interface to the PML, by changing the Typical Wave Speed property in the physics interface’s Settings window. The default wave speed generally corresponds to a compressional or pressure wave, which is the fastest wave type and therefore of longest wavelength. In the RF Module, the default for the typical wavelength is 2π/k, where k is the local wavenumber.