system
Given the following post, respond as if you're commenting on Reddit:
human
Why does choked flow happen and why it is related to the speed of sound ?

I'm diving into the venturi effect (example: rocket nozzles and chocked flow happening in the admission valve of a ICE) and it seems that when the gas approaches the speed of sound, the flow starts to choke. Why does this happen ?
gpt
To preface this answer: information about a medium travels at the speed of sound in the medium. So if you change the conditions of a flow at one point, the effect on the medium travels at the speed of sound upstream in the medium.

If you imagine a venturi nozzle and a flow condition where the speed of sound is achieved at the throat, and your downstream flow is travelling at greater than the speed of sound, any disturbances downstream of the throat won't propagate upstream. Picture yourself splashing around in a river where the flow travels faster than the wave speed; no one upstream of the river will be able to see your splashing around. Now there are two ways to achieve a choked flow scenario from here: 

* ~~Increase the pressure upstream of the throat~~ **EDIT:** Forcibly increase flowrate upstream of the nozzle  
* Decrease the pressure downstream of the throat.

If you try ~~increasing the pressure~~ forcing the flowrate to increase upstream of the nozzle (perhaps by increasing power on a compressor), you end up forcing air to get to Mach 1 before the throat. If you picture it does that instantaneously before the throat, because of how supersonic flow behaves it will actually decelerate to subsonic flow just downstream of the Mach transition. And speed up again and slow down again, etc. etc.. This just creates a bunch of shocks in your convergent nozzle where heat is generated but the flow doesn't speed up. And once it gets to the throat, it hits mach 1 anyways and accelerates in the divergent section.

If you decrease pressure downstream, on the other hand, you get a situation of underexpansion at the outlet of your pipe system. Unlike subsonic flow where the information about the new reduced downstream pressure propagates through the system quickly and stabilizes the flow pressures and velocities, the underexpansion remains at the outlet in the form of an expansion fan. Remember: information can't travel in a medium faster than the speed of sound, and if the flow travels faster than the speed of sound, the information downstream never propagates to the rest of the system.

EDIT: Changed "increase pressure upstream" to "increase flowrate upstream". Your behavior from the reservoir leading up to the throat will not depend on absolute pressure of the reservoir.