In the context of wave-like equations, an open-boundary condition is one that "lets the wave go through without reflection".
Here are some resources that cover such boundary conditions:
- https://www.asc.tuwien.ac.at/~arnold/pdf/graz/graz.pdf lots of slides
- http://hplgit.github.io/wavebc/doc/pub/._wavebc_cyborg002.html mentions them and gives a 1D formula. It mentions that things get complicated in 2D and 3D TODO why.The other page: http://hplgit.github.io/wavebc/doc/pub/._wavebc_cyborg003.html shows solution demos.