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How does a shockwave form over say an aerofoil from a subsonic upstream?

Wouldn’t the flow have to be supersonic before it ‘shocks’?

Also, does the flow in a shockwave jump from let’s say it is at M=1.2 (before shockwave), to M<1 after please? Or gradually decrease?

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  • $\begingroup$ A subsonic velocity doesn't mean air is subsonic everywhere along the airfoil. Lift is created by accelerating air. Related: Do commercial airliners use supercritical airfoils? $\endgroup$
    – mins
    Commented Oct 31, 2023 at 16:31
  • $\begingroup$ Hi, yeah I see what you’re saying, thanks. $\endgroup$
    – James
    Commented Oct 31, 2023 at 19:28

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The airfoil accelerates the flow such that there is a region of supersonic flow which then can shock down.

The flow through a shock does experience an abrupt jump in properties. Velocity, Mach, density, pressure, and temperature all change very quickly.

Some other properties do not jump. For example, mass flow per area normal to the shock does not jump.

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  • $\begingroup$ Hello! But how does the shockwave actually form in flow that is originally subsonic (but high speed like M=0.9) please? I couldn’t find a nice explanation of shockwaves online $\endgroup$
    – James
    Commented Oct 31, 2023 at 17:26
  • $\begingroup$ The flow first gradually accelerates to supersonic. Then it encounters an adverse pressure gradient that wants to cause it to slow down. However, information can not propagate forward in a supersonic flow, so a shock is triggered. There is a ton of information on shockwaves available online and in books. Keep looking. $\endgroup$
    – Rob McDonald
    Commented Oct 31, 2023 at 18:00
  • $\begingroup$ Hello, thank you for the reply. So the adverse pressure gradient is like the catalyst that makes it jump from say M=1.3 (before the shockwave) to subsonic please? But how does it make the shockwave happen so quick please? $\endgroup$
    – James
    Commented Oct 31, 2023 at 19:23
  • $\begingroup$ @James shockwaves always happen quickly - otherwise they wouldn't be a shock. Disturbances in a flow travel at the characteristic wave speed. For this kind of problem, that is the speed of sound. When the flow is faster than the speed of sound, that means that disturbances (information) can not flow upstream. So, when the flow encounters something (downstream) that requires a change (a blockage, a turn, a pressure gradient), the flow must react abruptly. Conversely, in subsonic flow, the disturbance can propagate forward such that it can be 'felt' early and a gradual adjustment can be made. $\endgroup$
    – Rob McDonald
    Commented Oct 31, 2023 at 20:04
  • $\begingroup$ You’re super clever, thank you rob!! $\endgroup$
    – James
    Commented Oct 31, 2023 at 20:25

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