It seems to me that not much attention is paid to what actually happens in a plane shock wave. We can see the result of a compressible fluid flowing through a shock wave in that the velocity falls, the pressure rises, the temperature rises and the density rises but which of these changes is the one that “drives” the process? I think that it is necessary to think how the steady flow comes into existence. As the back-pressure rises to some pressure above that at P a shock wave moves towards the throat until it comes to rest relative to the nozzle when the speed of the wave relative to the flow and towards the throat equals the speed of the flow. On one side of the wave the speed is supersonic and the density low, on the other the speed is subsonic and the density high. The high-speed flow of untold numbers of molecules suddenly changes its density by a factor of 2 or more. The molecules must be packed closer together to increase the number of collisions made by each molecule every second and with this change an increase of the mean kinetic energy of the molecules. This can only come from the mechanical energy possessed by the gas in the speed of its mass centre. Mechanical energy is lost back into the random energy of the molecules. As I have already pointed out this wave may seem thin to us but on a molecular scale it is quite thick.
Such a process could not operate in the reverse direction and take mechanical energy “instantly” from the random energy of the motion of the molecules. We had to use a carefully designed nozzle to make this change.