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enter image description here

When at high angle of attack the boundary layer tends to:

A. stagnate and stop
B. loop around the upper camber line
C. decrease air pressure along the chord

The answer is A, but as I see it, B seems like a better answer. What's your opinions?

For your information, the material explains that B is the wrong answer because "As the boundary layer becomes stagnate and stops, the airflow will separate and loop around the upper surface and a stall occurs," which I don't find satisfying.

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Weird language used here to describe aerodynamics. Anyways, when AoA is increased gradually beyond stall limit, the boundary layer on the upper side starts facing too stiff adverse pressure gradient. It cannot overcome it and separates. This is what is "stagnate and stop" means. This separation point will typically move from trailing edge towards leading edge gradually till a point where we see fully stalled aerofoil right from the Leading edge. Once separated, the airflow will create recirculation zone. I think that's what is being conveyed by "loop around upper camber line". The flow would look like this: Image Source

enter image description here

You can easily find a number of experimental and CFD flow visualizations for this phenomenon.

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  • $\begingroup$ Where is this image taken from? $\endgroup$ – Ruslan Sep 15 '17 at 11:33
  • $\begingroup$ Wiki. Added the link now. Wouldn't take it directly so had to download and attach image. $\endgroup$ – jayS Sep 15 '17 at 12:00
  • $\begingroup$ @jayS, there is a “use this file on the web" link from the image page that gives you URL for hot-linking. You should still also make the image a link to the image info page. $\endgroup$ – Jan Hudec Sep 16 '17 at 7:57
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The question is about the boundary layer, which indeed stagnates and stops at high AoA, as shown for the laminar boundary layer in the picture. Then the airflow will detach from the upper surface, after the point where the boundary layer has stopped.

By Ariadacapo - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=42807305

B. makes no sense to me, I'm only aware of one camber line.

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    $\begingroup$ Don't worry, a turbulent boundary layer will decelerate, stop and reverse (detach) just as a laminar will, only at higher pressure gradients. It would be better to say "if it was attached". On highly swept wings and those with a sharp leading edge the flow will detach right at the leading edge. $\endgroup$ – Peter Kämpf Sep 15 '17 at 13:24

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