Inside a converging-diverging nozzle the flow changes personality three times: subsonic in the converging part, exactly sonic at the throat, and supersonic after it โ with a shock possible when the nozzle runs off-design.
A flow regime describes how fast a gas is moving compared with the speed of sound in that gas. It is written as a Mach number: below 1 is subsonic, exactly 1 is sonic, and above 1 is supersonic. Inside a converging-diverging nozzle the flow passes through all three in turn.
The change from subsonic to supersonic happens in a very short distance and cannot be seen from outside. A cutaway with the flow drawn in makes the sequence โ and the way it can go wrong off-design โ easy to follow.
Gas collects at the inlet slow and compressed. The flow here is well below the speed of sound; the stored pressure is the energy the nozzle will convert into speed.
In the converging part the flow is below the speed of sound, so it behaves like an ordinary fluid. As the passage narrows, the gas speeds up as it approaches the throat.
At the narrowest point the gas reaches exactly the speed of sound. Passing this sonic condition is what flips the behaviour of the flow in the section that follows.
Past the throat the duct widens, and because the flow is already sonic, widening now speeds it up instead of slowing it. The gas becomes supersonic and keeps accelerating as it expands.
If the nozzle is over-expanded, the flow may not stay supersonic all the way out. A normal shock โ a thin, abrupt disturbance โ can form and the flow drops suddenly back to subsonic, wasting energy. This is shown here only as a conceptual condition.
The cross-sectional area and the pressure difference across the nozzle. Starting from below the speed of sound, the flow reaches Mach 1 exactly at the throat and then becomes supersonic after it.
Once the flow reaches the speed of sound at the throat, a widening passage accelerates it further. Below the throat a widening passage would slow it down, so the narrow throat is the turning point.
It is a very thin disturbance where supersonic flow drops abruptly to subsonic flow. It appears when the nozzle is over-expanded, and it wastes energy, so engines try to avoid it.
Yes. If a shock forms in the diverging section, the flow behind it is subsonic even though the duct is still widening.
Only when the pressure difference is large enough. With a small difference the throat may never reach the speed of sound, and the whole nozzle stays subsonic.