What's that visible airflow over the airplane wing and what causes it?
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$\begingroup$ I just saw the same thing on an Airbus a321neo. It seemed to only happen when the front slats were down. $\endgroup$– hazzey ♦Jun 17 at 20:15
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2 Answers
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Specifically, what you are seeing here is a vortex which is basically a sideways tornado. Vortices occur when moving air forms a twisted stream when flowing over an object with a complicated shape. The air pressure in the core of the vortex is lower than ambient and if the air is near the dew point temperature, the moisture condenses out at the core and "decorates" the vortex so it becomes visible.
Vortices commonly form at the sharp corner of an extended flap or aileron and at the trailing edge corner of a wingtip. They slightly reduce the lift generated by the wing in the vicinity of the tip. Tip winglets are designed to minimize this effect by blocking air under the wing from spiraling up and around the tip of the wing and cancelling the low pressure zone on the top of the wingtip.
answered Jun 17 at 16:44
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The airflow will be there visible or not.
What makes it visible are the atmospheric conditions and moisture or humidity in the air so the water particles show the airflow.
In wind tunnels the airflow is inspected using smoke wands to show the airflow.
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1$\begingroup$ Specifically, the lower pressure above the wing causes the moisture in the air to condense like a cloud making it visible. $\endgroup$– Eric SJun 17 at 13:24
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$\begingroup$ The dark streaks are leaks, likely oil + dirt. It is common to see condensation spirals at the wing tips , under certain atmospheric conditions. $\endgroup$ Jun 17 at 14:41