Can someone post a link to a video that shows an example of this either in a wind tunnel or using some other examples?

When an aircraft is flying below the speed of sound, the pressure disturbances will be moving faster than the airplane (think ripples on a pond again), and those disturbances that travel ahead of the aircraft influence or “warn” the approaching airflow. This “pressure warning” can be observed in a smoke wind tunnel as it causes the upwash well ahead of the wing.

Quoted from Flight Theory and Aerodynamics

  • $\begingroup$ You may try to search for "upwash", to find things like this. $\endgroup$
    – mins
    Jul 8 '17 at 11:12

An approaching body needs to push air out of the way. Directly ahead of it, it will compress the air such that the local pressure increase will push the air out of the way. When the body is a lift-producing wing, the lower pressure on the upper side will suck the air towards it. So what can be observed are two effects:

  1. Air gets compressed ahead of any body approaching at subsonic speed, the more the faster the body moves, and
  2. When lift is created, this pattern will be distorted such that the flow will turn in the direction of the lift-producing suction.

The propagation speed of small pressure changes can be observed with your ears. Sound is just that: Small fluctuations in pressure. An approaching wing is just like an approaching siren, only that the siren emits an oscillating pressure variation and the wing an aperiodic one.

For the video my recommendation is

Note how the flow lines diverge ahead of the airfoil: This is where the flow is slowed down. At higher angles of attack you will also see how the flow lines bend up ahead of the approaching airfoil.

  • $\begingroup$ I’d just add that a punch coming at your face also sends airflow with it, even though it does not hit the skin. That’s a practical demo of stagnation flow, carried by the punch :) $\endgroup$ Jan 2 '18 at 17:23

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