A stable absorbing boundary layer for anisotropic waves
  For some anisotropic wave models, the PML (perfectly matched layer) method of open boundaries can become polynomially or exponentially unstable in time. In this work we present a new method of open boundaries, the phase space filter, which is stable for all wave equations.   Outgoing waves can be characterized as waves located near the boundary of the computational domain with group velocities pointing outward. The phase space filtering algorithm consists of applying a filter to the solution that removes outgoing waves only.   The method presented here is a simplified version of the original phase space filter, originally described in [22] for the Schrodinger equation. We apply this method to anisotropic wave models for which the PML is unstable, namely the Euler equations (linearized about a constant jet flow) and Maxwell's equations in an anisotropic medium. Stability of the phase space filter is proved. 
Avy Soffer and Chris Stucchio