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I know that transonic aerodynamic buffet is caused by the separated turbulent boundary layer striking the airframe (horizontal stabilizer, wings, fuselage) with considerable force causing a high amplitude vibration, which physically shakes the whole aircraft. And during a buffet, a shockwave will be apparently moving back and forth just like in the video link I provided below:

Since the shockwave is usually stationary on the airfoil, then why it starts suddenly oscillating once we enter a buffet boundary? Any ideas?

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    $\begingroup$ I just wanted to ask the same question. In my textbook it says "we assume that a pressure disturbance upstream of the shock on the upper surface causes the shock to move more forward". So perhaps any kind of natural pressure disturbance present in the atmosphere will cause this kind of oscillation? I don't know. $\endgroup$
    – Daniel
    Commented Sep 30, 2018 at 13:05
  • $\begingroup$ I don't know about the subject matter, but this 2013 paper says: Although the buffet problem has been [studied] for sixty years, the physical mechanism for buffet onset is still not fully understood. Perhaps you'll be able to glean something from that paper. If you do, we encourage answering your own question too. $\endgroup$
    – user14897
    Commented Oct 1, 2018 at 20:50
  • $\begingroup$ @ymb1 Thanks I will look it up :) $\endgroup$
    – Darjan
    Commented Oct 1, 2018 at 20:58
  • $\begingroup$ Very little about air or water is "usually stationary", it will move until forces are in balance. The video, based on observations of the trailing edge, could also be the result of oscillations in pitch caused by the buffet. $\endgroup$
    – Robert DiGiovanni
    Commented Oct 6, 2018 at 19:03

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Viewing the airfoil as a rotating cylinder, the highest velocity on it's surface occurs at the top of the cylinder (90° using polar coordinates) therefore that's where the shockwave begins and hence where flow separation will start

I know that as the free stream velocity gets higher the point at which the velocity on the surface on the airfoil (cylinder) goes supersonic occurs closer and closer to the leading edge therefore the first tendency of the shockwave is to move "forwards" (towards 0° on the cylinder representation)

I also know that flow separation occurs under an adverse pressure gradient however between 90° and 0° the pressure gradient on a cylinder is favourable (due to geometry), therefore perhaps that is why the flow reattaches when the shockwaves moves <90°

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An approach to understanding this would be to try to create it. Also helpful would be research on how to avoid it. Aircraft routinely break the sonic barrier these days, using power to move through the "barrier" with little risk of damage. Earlier aircraft, lacking the knowledge of "area rule" and effect of shock waves on control surfaces, in addition to less power, had more difficulty. The keys: proper design and sufficient power. Avoid staying in transsonic speeds. Either above or below.

Now to understand the phenomena. Air, like water, can absorb energy in waves. Waves can harmonicly reinforce one another and combine into a larger wave. What may be happening in the video is that oscillations in the shock wave front, under certain conditions, will combine into a larger wave and move back and forth over the wing.

Possible cures for this behavior would be to change to configuration of the wing. Here a wealth of information exists. My first pick would be the delta, as it's strong vortex flow might tend to break up an oscillation. Other wing types or modifications such as dog tooth and turbulators could be tried.

But the best solution may be not to hang around transsonic too long.

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