Editing Compressible flow (section)

===Non-isentropic 1D channel flow of a gas - normal shock waves===
Normal shock waves are shock waves that are perpendicular to the local flow direction. These shock waves occur when pressure waves build up and coalesce into an extremely thin shockwave that converts kinetic energy into [[thermal energy]]. The waves thus overtake and reinforce one another, forming a finite shock wave from an infinite series of infinitesimal sound waves. Because the change of state across the shock is highly irreversible, [[Entropy in thermodynamics and information theory|entropy]] increases across the shock. When analysing a normal shock wave, one-dimensional, steady, and adiabatic flow of a perfect gas is assumed. Stagnation temperature and stagnation enthalpy are the same upstream and downstream of the shock.

[[File:Rankine-Hugoniot Relationships.PNG|thumb|center|The Rankine-Hugoniot equations relate conditions before and after a normal shock wave.]]

Normal shock waves can be easily analysed in either of two reference frames: the standing normal shock and the moving shock. The flow before a normal shock wave must be supersonic, and the flow after a normal shock must be subsonic. The Rankine-Hugoniot equations are used to solve for the flow conditions.