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I was wondering if the void seen in this picture (on top of airfoil, not the wake from the airfoil) is from the boundary layer thickening, or from the fact that as you go further back on an airfoil, the pressure rises, "bending" less air towards the wing. Which causes it and why?

(It might be neither of those ideas, I'm no professional)

Thanks! Image

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  • $\begingroup$ Do you have any idea about the Reynolds Number at which that picture was taken? For that low an angle of attack there should be no separation at the usual speeds. $\endgroup$ Nov 22, 2023 at 21:49
  • $\begingroup$ Unfortunately no, but here’s the link to the video it was from : m.youtube.com/watch?v=rCpZpKZLz14 $\endgroup$
    – Wyatt
    Nov 22, 2023 at 21:51
  • $\begingroup$ @PeterKämpf (it’s at timestamp 2:00 in the video) $\endgroup$
    – Wyatt
    Nov 22, 2023 at 21:57
  • $\begingroup$ This answer should also help. $\endgroup$
    – sophit
    Nov 23, 2023 at 19:27

2 Answers 2

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It is both due to the boundary layer and the not perfectly spaced smoke trails injected in the airflow: if you watch carefully, there's an additional trail underneath the airfoil which is missing above it.

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  • $\begingroup$ Ah I see, thanks. Happy thanksgiving! $\endgroup$
    – Wyatt
    Nov 24, 2023 at 4:09
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The nomenclature of the art is laminar --> boundary layer --> turbulence and flow separation

As we can see in the video, the first smoke line above does bend less towards the trailing edge, but maintains its form until past the trailing edge.

So, yes, the boundary layer will thicken, and there is drag$^2$, but this is a consequence of creating lift. Heavier turbulence eventually breaks the pressure gradient. Ideally, you want this to happen after the wing has passed.

This early NACA video$^1$ seemed to focus on ways to reduce drag for higher speed flight, and is worth watching in its entirety.

$^1$ see 1st comment by Wyatt below question
$^2$ higher aspect ratio wings eliminate some of this less productive area, resulting in a higher Lift to Drag ratio at the expense of some strength.

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  • $\begingroup$ Ah yeah I did notice that the flow line above didn’t go turbulent until the wing had passed. Thanks, and happy thanksgiving! $\endgroup$
    – Wyatt
    Nov 24, 2023 at 17:42

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