De Laval nozzle (thing)

It is used in rocketry, where its use can achieve speeds of 4 km/s or more; jet engines, and in wind tunnels where it is capable of producing smoother airflow than other techniques.

It consists of 3 parts- a convergent nozzle, a throat, followed by a divergent section.

The way it operates is as follows:

From the point of view of the gas in the convergent section, the nozzle is just a hole that leads to a lower pressure area, so it starts to accelerate towards it. As the gas travels through the narrowing at the convergent section the gas is forced to accelerate, finally reaching the speed of sound (*) at the throat.

Having reached sonic speed, it flows on into the divergent section, where the gas expands and cools, pushing sidewise at an oblique angle to the wall, accelerating it to hypersonic speeds.

The shape of the divergent part of the nozzle is often a bell shape which is the classic rocket shape, but a simple cone shape gives 99% of the efficiency and is sometimes used (notably on Russian rockets who deliberately chose it due to more cost effective construction).

The divergent section of the nozzle can multiply the speed of the gas by 2.7 times the speed of sound or more, the exact ratio depends on the area ratio at the throat and the exit from the nozzle.

The energy to raise the speed of the gas comes from lowering the temperature of the gas- in some high altitude/very high expansion nozzles the exit temperature can approach the freezing point of water (i.e. ice/snow!). Care must be taken to avoid this, since it results in sudden and uneven contraction of the jet.

Under normal atmospheric conditions a phenomena called mach diamonds form in the exhaust when the nozzle is working correctly.

*- note that 'the speed of sound' here is the speed of sound in the hot gas not the speed of sound in air at ground level- the speed of sound in hot gas is typically 2 or 3 times faster, or more, depending on temperature.