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I was wondering the other day, during the trans sonic regime, when the velocity over the airfoil becomes supersonic,why does the shock wave standing on the airfoil moves downstream when the free stream velocity is further increased?

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With a higher flight velocity (still subsonic), the supersonic pocket on the upper side of the wing starts earlier and takes longer until it collapses into the shock. That is quite normal, as the speed over the whole upper wing is increased with increasing flight speed.

Think of what happens at the extreme case of accelerating flight speed towards Mach 1: Now the shock will move towards the trailing edge with increasing Mach number and will stay there once the flight speed is above Mach 1 (NOTE: This is valid only for straight wings; correct the Mach number upwards for sweep effects).

Things become nasty when the shock interacts with lift creation and starts to fluctuate back and forth - this creates a buzzing sound and might even cause flutter. The mechanism is that the stronger shock causes flow separation and lift loss, which in turn shrinks the shock, which allows lift to recover so the shock grows again. With the growth, the shock moves back and jumps forward when it becomes weaker. This back-and-forth jumping of the shock creates not only a fluctuation of lift, but also of pitching moment.

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