Upstream MHD boundary conditions

[ekeever1]

The very naive implementation of upstream boundary conditions in Imogen simply does not work for MHD, especially radiating MHD, flows. The problems are easily verified by trying to run a radiating MHD flow - upstream errors grow until the shock moves upstream and collides with the boundary, usually resulting in a negative-pressure simulation crash.

The upstream pulse emitted by a shock front's transient relaxation causes permanent errors in the magnetic field density at the upstream limit; The magnetic BC needs, at the absolute very least, to be rewritten to set a divergence-free update.

Small-signal transparent MHD boundary conditions are apparently known as well.