So I was clarifying a few things in a comments section of an answer, and I had quite a few questions so I thought I'd ask my main one in a separate post.

In the second comment on this answer, it says that air will continue to accelerate in supersonic flow, where it would decelerate in subsonic flow. I understand the part about disturbances reaching further out, but what would make air continue to accelerate in supersonic flow, when it wouldn't in subsonic flow? Also, this affects the pressure distribution of the wing, but why doesn't it affect the whole wings lift distribution evenly? (The answer was super helpful, but I didn't understand this part)



1 Answer 1


For supersonic flow to accelerate, it must have a negative pressure gradient in flow direction. The acceleration is "powered" by a loss of pressure and will end if pressure downstream is higher. In that case, air will abruptly decelerate in a shock.

In transsonic flow you will have accelerating air in a small supersonic pocket, followed by a shock. Ahead of the shock pressure is low so there is a pressure gradient which drives the acceleration. Typical examples are supersonic regions on the upper side of wings or convergent-divergent nozzles where the speed of sound is reached at the end of the convergent part ("throat") and the divergent part accelerates the now supersonic flow further. Once the flow crosses into the freestream at the end of the nozzle, it is recompressed in a sequence of shocks.

Now you might ask why the flow will have a negative pressure gradient when outside pressure is higher. In supersonic flow there is no mechanism which allows the higher outside pressure to travel upwards into the divergent nozzle - only when the flow exits the nozzle will that pressure travel into the core of the supersonic stream with the speed of sound, leading to the diamond pattern of shocks. Ahead of those shocks it is only the nozzle geometry which controls whether and how much the flow expands and accelerates.

  • $\begingroup$ ah okay. So basically the shockwave on top of the wing acts kind of like a barrier of sorts, shifting the lift distribution forward? $\endgroup$
    – Wyatt
    Commented Dec 14, 2023 at 0:31
  • $\begingroup$ I’m not 100% familiar with aerodynamics at Mach speeds, so that’s just what I interpreted it as. $\endgroup$
    – Wyatt
    Commented Dec 14, 2023 at 16:53
  • 1
    $\begingroup$ @Wyatt I have to admit that I do not understand your comments. Keep in mind that small pressure changes cannot travel upstream in supersonic flow. $\endgroup$ Commented Dec 14, 2023 at 19:22
  • $\begingroup$ ah okay I'll try to explain it better. My thinking is that the shockwave where the pressure recovers over the wing will be like a wall, to where the air gets rapidly decelerated, changing the pressure distribution. $\endgroup$
    – Wyatt
    Commented Dec 14, 2023 at 19:50
  • $\begingroup$ @Wyatt Yes, of course. The shock causes an increase of pressure, but if it is heavy enough, also a flow separation downstream. The supersonic area ahead of the shock causes a drop in pressure compared to the fully subsonic pressure distribution. See here or here for more. $\endgroup$ Commented Dec 15, 2023 at 0:50

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