Hydrodynamic (HD) shocks are formed when supersonic fluid meets subsonic fluid. When the flow is entirely subsonic,
waves can propagate upstream. This allows the signal of any barriers to be sent back through the fluid,
which can then more easily flow around the object. When the flow is supersonic, sound waves cannot propagate
back upstream to “warn” the fluid of the upcoming barrier. A shock wave arises when the fluid runs into the
Magnetodhydrodynamic (MHD) shocks arise when the fluid is plasma, where there is separation between the positive and negative charges.
This can give rise to magnetic fields, which allow for more kinds of waves to propagate through the fluid.
There are two kinds of magnetosonic pressure waves, fast and slow waves.
The slow waves travel slower than sound speed, and the fast waves travel faster than the sound speed.
The fast MHD waves make modeling of plasma shocks difficult, since they can carry information back
upstream even when the flow is supersonic. There are also Alfven waves, which are oscillations
along the magnetic field lines. They oscillate tangentially, like waves along a plucked string.
Future interest: Include radiative cooling in the modeling.