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I've been researching this for the past ~45 minutes, and I still can't find a solid answer. Why do flat wings create a vortex/vortices? In this answer, it says that the detached flow will create a vortex (at high AoA) instead of re-attaching, but I still don't really understand why and how the vortex is created.

I get why high sweep wings, like the Concorde's will make a vortex, but I don't see why a flat wing would make one. Also, do flat wings make multiple vortices across span for the same reason, or just one? (It's just confusing because why doesn't an airfoil-shaped wing make the same vortex?

That also makes me wonder, what direction does the vortex face? Does it face the same direction as a tip vortex, or perpendicular to that direction?

Thanks a lot!

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  • $\begingroup$ That vortex is called Von Karman vortex and is typically formed behind stalled body. $\endgroup$
    – sophit
    Commented Jan 6 at 10:29
  • $\begingroup$ @sophit I see. What might make a stalled body create a vortex? $\endgroup$
    – Wyatt
    Commented Jan 6 at 16:23
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    $\begingroup$ Detached flow is turbulent. You might as well ask, how could turbulent flow NOT have eddies and vortices. $\endgroup$ Commented Jan 6 at 17:17
  • $\begingroup$ @CamilleGoudeseune well yeah true, but I was curious why the main vortex is created on flat wings, like what causes it. $\endgroup$
    – Wyatt
    Commented Jan 6 at 17:37
  • $\begingroup$ By "flat wing" do you mean "flat plate" like the answer you cite, or do you mean "unswept" like how you say that it's the opposite of the Concorde? $\endgroup$ Commented Jan 7 at 21:55

2 Answers 2

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At a sufficiently high angle of attack, a rectangular thin-plate "airfoil" suffers flow separation at its upper surface, just like any conventional airfoil would.

(A thin plate is not at all special in this regard. It just has poor L/D and poor structural stiffness compared to conventional airfoils. Its post-stall behavior hardly differs from that of other airfoils, cambered or not.)

This separated aka detached flow is turbulent. Turbulence has vortices. That's all there is to it. Some physicists even define turbulence as "when an ordered fluid flow breaks into small vortices." Questions about how or why turbulence causes vortices, or whether turbulence just is vortices, would be better for the physics SE, or maybe even the philosophy SE.

Based on observation, the largest vortices will be spanwise, but small chordwise vortices also appear, and everything in between. This is turbulence, after all. It's messy.

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how a vortex is created

One key ingredient is sustained wind shear. We can see this on wingtips as high pressure is drawn up and over the end of the wing. This flow is in a different direction than the low pressure area and the free stream, creating a rolling motion driven by the energy of the motion of the aircraft.

Notice that not only the degree of sweep affects vortex formation, but also the 2 dimension shape of the airfoil.

The delta wing (a paper airplane favorite) is essentially two wingtips put together, making a strong lifting vortex.

A straight wing works differently, using lift created by attached airflow for much more efficient lift. Here we don't want vorticies, which is why straight airfoils are thicker and rounded.

"Vortex generators" can increase the critical stalling AoA of a straight wing (anything is better than separated airflow), but at a cost of higher drag.

While deltas are versatile at many speeds and can be made thin like flat plates, straight airfoils are far better lifters.

A straight wing can produce a vortex perpendicular to the wing tip vortex at its trailing edge, but this is only when at a very high drag, impractical angle of attack. The ones vortex generators make are parallel to the wing tip vorticies.

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  • $\begingroup$ I think you might be confused about what I'm asking. I was asking why a wing without the curvature from an airfoil has a vortex in the front of it. Imagine a piece of ply-board being used as a wing. I think this answer would definitely help convey what I mean. (read middle - end of the first paragraph) $\endgroup$
    – Wyatt
    Commented Jan 7 at 22:16
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    $\begingroup$ @Wyatt The picture in your referred answer is showing (in blue) an area of lower pressure, not (necessarily) a vortex. But notice how tiny it is compared to an airfoil. But remember, on a paper airplane scale and speed (Reynolds number), most of the lift comes from higher pressure on the bottom "surfing" the air. This is why toy gliders can have thin wings (which actually lower drag). One of the great discoveries of around 105 years ago was that once you go faster, attached flow provides fantasticly higher lift/drag ratios. This lead to thicker wings on monoplanes. $\endgroup$ Commented Jan 7 at 22:33
  • $\begingroup$ @Wyatt please read the end of the second paragraph of your referenced answer. It seems to link the vortex behavior of the tip of the straight wing and delta wings. $\endgroup$ Commented Jan 7 at 22:39
  • $\begingroup$ hmm, I could see where a vortex might be created, but the one I'm thinking of would turn in a direction perpendicular to they way tip vortices turn. $\endgroup$
    – Wyatt
    Commented Jan 7 at 23:05
  • $\begingroup$ @Wyatt yes, that would be at the trailing edge. The most desirable, lowest drag way to make lift is to have clean (laminar) flow until the wing is past. This is not unlike the end of a power boat separating from its wake. The increase in speed from less drag is noticable. $\endgroup$ Commented Jan 7 at 23:22

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