In a converging-diverging nozzle with a choked throat and supersonic exit flow:
- The flow velocity stays unchanged throughout both the nozzle sections
- The gas accelerates through both the converging and diverging sections
- The gas decelerates through the diverging section as its pressure rises
Explanation from PART66Online
In a convergent-divergent (de Laval) nozzle the gas arrives subsonic, so it accelerates throughout: it speeds up in the converging section to reach Mach 1 at the throat, then continues accelerating in the diverging section as the now supersonic flow expands. The area effect reverses with Mach number, so a convergent duct would instead decelerate flow that was already supersonic on entry, which is why the throat must reach Mach 1 first. Velocity does not stay constant, and the gas accelerates rather than decelerates in the divergent part.
ewanroles13 asked · 1h ago
The wording of the question suggests the airflow is supersonic before reaching the convergent-divergent nozzle
Community Comments (1)
The marked answer stands. What the question was describing is a nozzle at its design condition, with the throat choked and the exit flow supersonic. The gas arrives subsonic, accelerates through the converging section to reach Mach 1 at the throat, and then keeps accelerating through the diverging section as the now supersonic flow expands.
Your reading was the reasonable one to take, and it is exactly why the stem needed changing. The area effect reverses either side of Mach 1: a converging duct accelerates subsonic flow, but it decelerates flow that is already supersonic. So on your reading of the old stem none of the three options would have been correct, which makes the wording a real defect rather than a matter of taste. Elsewhere in the bank that distinction is made explicit, with separate items keying air that enters a converging duct at sonic speed as decelerating.
The stem now reads:
In a converging-diverging nozzle with a choked throat and supersonic exit flow:
The explanation shown after answering has been tightened in the same way, and it now states that the area effect depends on the local Mach number. Please keep the reports coming.
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