There are several induced drag explanations out there saying that tip/wake vortices “come from” downwash and downwash “comes from” lift. I’m using quotes because the concept is that there are no cause and effect events but simultaneous events that occur in a flow.
Apologies if I describe the above incorrectly, feel free to correct me.
What I want to ask is if someone can “adjust/adapt” these explanations to describe what happens on an airfoil placed wall-to-wall in a wind tunnel that has no (or extremely small) induced drag. I’m not talking about theoretical infinite wings and mathematical explanations, I’m looking for a physical description similar to the ones given above for finite wings.
The only difference between wind tunnels and free flight (in THIS case) is that in the wind tunnel the upper and lower surface flows can interact only at the TE while in free flight they can interact (touch, blend, etc.) at the TE and at the wing tip.
In this NASA page, it is said that “The wing tip vortices produce a downwash of air behind the wing which is very strong near the wing tips and decreases toward the wing root.” Although this is not technically correct (see wake vortex), the importance of exposing the wing tip to the flow seems to be underestimated in the explanations above so I’m looking for an explanation of the wall-to-wall airfoil behaviour through the above explanations.
About Peter Kampf’s reply:
Now I think we’re getting somewhere…
Wake vortex is certainly more important than tip vortices, this is why I used the word wake with tip when referring to vortices. Perhaps I wasn’t very consistent/clear with that (like the NASA page) so apologies for any confusion.
You correctly say that the effect of the wake vortex is reduced by the tunnel walls and there are no tip vortices, which means induced drag is greatly reduced for a wall-to-wall airfoil (essentially a side effect taken care of by grating). If it was possible to add infinite size end plates with zero drag on real aircraft wings (acting as wind tunnel walls), the induced drag would be greatly reduced.
This is all I’ve been trying to say, that the only explanation for the induced drag increasing drastically when we move from a wall-to-wall model (2D) to a finite wing (3D), is that the wing tip is exposed to the flow.
- Yes, a vortex sheet leaves the wing.
- Yes, there's no spanwise spillage of lower flow towards the upper flow.
- Yes, tip vortices do not create induced drag.
However, the free stream air “sliding” next to the wing tip (where the tunnel wall would be) definitely interacts (pushes, gets pushed, blends, etc.) with:
The upper and lower flows (vortices) as it travels the chord of the wing.
The “combined” upper/lower flow (vortex sheet) at/after the trailing edge.
so the more the wing tip is “walled”, the effects of this interaction get weaker and consequently the induced drag gets weaker....
The explanations correctly say that wingtip vortices do not create induced drag but, at least that’s how I understand it, neglect the importance of the physical exposure of the wing tips to the air flow that causes the “interaction” described above.
Thanks to everyone for bearing with me, I’ve never written about aerodynamics in a forum…