I see that some members(Peter,JZYl..)claim that wall to wall airfoil in wind tunnel produce downwash. Can you explain how this is possible?

If air goes down behind wall to wall airfoil than above must be vacuum,because lower and upper walls dont allow air circulation to feel this place with new air!

Maybe only this happend if air up-behind airfoil is streched and air down-behind airfoil is compressed.

Can you explain what heppend behind airfoil in detail?

enter image description here

airflow behind this plane can not happend in wall to wall airfoil in closed wind tunnel.. enter image description here

DOWNWASH VIDEO: https://fyfluiddynamics.com/2014/04/newtons-third-law-says-that-forces-come-in-equal/

No downwash in this wall to wall airfoil video:

  • $\begingroup$ Might be simpler if you considered e.g. the vanes in aircondition ductwork. $\endgroup$ – jamesqf Aug 17 '20 at 18:15
  • $\begingroup$ If you look very carefully the "vacuum" is filled by air flowing from underneath. Air flows from High to Low. The air deflects down, then up, almost like a spring. $\endgroup$ – Robert DiGiovanni Aug 17 '20 at 23:46

There's no vacuum, there's just expanding and contracting streamtubes. At some point you have probably read (or worse: been told) something about wingtip vortices and lift which was at best misleading, at worst false, but has established itself as a great way to sound profound. Essentially the continuation of the unholy "the suction side is longer therefore the air must travel faster to get to its appointment" debacle, which still exists in school textbooks, to this day.

The issue with the video you've posted is that the test section is fairly narrow, which means that the influence of the walls is very strong, and the downwash from the profile is straightened out very quickly. You can of course see that directly on a profile at incidence, the flow is not parallel to the wall, so why would it always have to be strictly parallel behind it? Look at the video at 8:14. See the downwash?

Here's a few better pictures, probably taken in a somewhat larger tunnel:

streamlines on a 2D airfoil in a wind tunnel, at varyingangle ofattack

You can nicely observe several things:

  1. The stagnation streamline is diverted upwards, towards the profile
  2. On the profile, the streamlines follow the profile shape, except in the third picture, which is where it separates
  3. Behind the profile, due to preservation of momentum, the streamlines cannot just turn a sharp corner and become parallel to the walls -- they flow off roughly parallel to the profile trailing edge.
  4. The further above or below the profile you get, the more the streamline shapes become smoother and straighter. Once you reach the walls (which are not in the picture), they will be completely straight.

Let's focus on the midle picture because that's most representative of a wing profile in "normal" operation. On top of the wing, close to the tip, where the profile diverts air upwards, the stramlines are "squeezed" together. That's because they're accelerating over the profile, thus narrowing. This means the same mass goes through a smaller cross-section. The further up you go, the closer the spacing gets to "normal", which is because the influence of the profile is diminishing. Below the profile, close to the stagnation point, you can see the inverse: Streamtraces are spaced further apart. That's because they're slowing down. Again, moving from the profile to the wall, you can see the effect fading. Same thing at the trailing edge: On the top side, the flow is now slowing down, and you can see the spacing increasing, while it's acelerating at the bottom. That's partially because of the profile shape, but it's a little exagerated in a wind tunnel because the channel between the top side and the top wall is widening. If the top wall was not there, the streamlines would follow the profile a little closer, but the difference is not huge.

Now, behind the profile, you can clearly see the flow still having a downward component, and the stramlines below the profile still being squeezed a little further. This is because yes of course the profile has been turning the flow, and it can't just change direction like that, even though the wind tunnel walls of course do turn it back to parallel. But there's no need for vacuum to explain that, just streamtubes expanding and contracting, with matching changes to flow velocity and pressure.

aside I'm not happy with how few videos and pictures of more modern tests are freely available. See here for a picture of a more recent set-up, and notice how much larger the test section is than the wing profile. This is already much better, but many facilities actually use slotted walls, where some flow can exit and enter the test section, so the streamlines at the walls don't even have to be completely straight.

The maths for anyone who heard about potential flow, with words This whole thing can also be mathematically modelled, of course. I'm not going to explain the maths behind potential flow models here, although I suspect that the idea of downwash being impossible may have arisen from some attempt at using potential flow to explain lift. For that reason, and in case you have some idea about potential flows: If (in 2D) you overlay a dipole with a constant flow, you get a cilynder, and if you add a vortex at the center, it rotates, creating lift, including upwash in front and downwash behind itself. With enough dipoles and vortices, you can create any shape of airfoil, in case a cylinder doesn't look realistic enough. Now, if overlay any flow field with its mirror image on any particular plane, there is no flow across that plane, making it a wall. If you generate an infinite mirror cabinet, you can make two parallel wind tunnel walls parallel to the main flow, and your potential flow around the profile in the middle still has downwash -- it just decays towards the walls. No magic needed, no vacuum either.

  • $\begingroup$ +1 for the potential flow explanation of Zhukowski airfoils (Joukowsky? It seems to be spelt differently every time!) $\endgroup$ – Stuart Buckingham Aug 18 '20 at 15:15
  • $\begingroup$ The snark was my second favourite part of this answer :-( $\endgroup$ – Stuart Buckingham Aug 18 '20 at 16:12
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    $\begingroup$ I liked putting it in, too, but we're supposed to keep it civil, and most of the people who misunderstand this topic are not to blame for the misinformation they've been handed. So it's actually bad form to make fun of it -- as much as misreadings of superficial explanations get on everyone's nerves... $\endgroup$ – Zak Aug 23 '20 at 10:57

Streaklines from the smoke visualizations are very useful in explaining what is going on. If we assume that the flow is 2 dimensional (e.g. there is no flow into & out from the screen), then no flow goes through any of the streaklines. Therefore we can treat the flow between each streakline as a bound streamtube, where we can apply Bernoulli's equation along the streamtube.

Where the streaklines are close together, the cross-sectional area of the streamtubes is smaller. At a constant density, the flow speed is increased proportionally, e.g. if the distance between streaklines is halved, the velocity is doubled, and the pressure is decreased according to Bernoulli's principle. Conversely, if the streamtubes are wider, the velocity is lower, and the pressure is increased.

The walls that constrain the flow at the top and bottom mean that there will be more contraction between the streaklines at the suction peak, and then more spreading of the streaklines at the trailing edge on the suction side. This has the effect of creating more suction on the suction surface, as well as creating a larger pressure rise after the airfoil on the suction side. Of course, on the pressure side, the downwash is also constrained by the wall, and the streaklines will be closer together, and therefore create less pressure.

The overall effect of being in a closed section is that the airfoil and walls create blockage and that flow cannot be displaced as much as it would be if you were operating far from walls. There is never any vacuum formed, the image you posted does not accurately represent how the streaklines look for any airfoil in a closed section.

This diagram shows (greatly exaggurated!) the upwash infront and downwash behind an airfoil in a closed section. The red lines show the stagnation streamlines. Own diagram

  • $\begingroup$ ,"There is never any vacuum formed, the image you posted does not accurately represent how the streaklines look for any airfoil in a closed section."... For sure that is not vacuum in real life,this is just image what would happend IF air goes downward behind airfoil in closed section=which is IMPOSSIBLE...that is what I talking about all the time.. $\endgroup$ – user50657 Aug 17 '20 at 20:08
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    $\begingroup$ @NoahPrandtl Downwash doesn't mean that all of the flow gets turned downward...in the tunnel, if the top and bottom walls are very close together, then it will completely distort the flow field. $\endgroup$ – JZYL Aug 17 '20 at 20:15
  • $\begingroup$ @JZYL is correct. I have added a diagram to try illustrate that. $\endgroup$ – Stuart Buckingham Aug 17 '20 at 20:21
  • $\begingroup$ imagine you have air particle leaving your 2D airfoil trailing edge and then goes downstream at your diagram,this pariticle will never fall down /touch to bottom wall.In real 3d wing this air particle is going down and down...and it will fall down/touch bottom wall..that I call downwash.. $\endgroup$ – user50657 Aug 17 '20 at 20:36
  • $\begingroup$ Even if there was no wall below the airfoil, flow does not go down forever. For that to happen, you would have to turn all of the flow above and below the airfoil, which is not possible. Most simulations assume (very accurately) that the flow is straight again 10 ~50 chord lengths downstream. $\endgroup$ – Stuart Buckingham Aug 17 '20 at 20:46

In this diagram, above (and above/behind) the airfoil the air pressure is lower than elsewhere in the diagram, but not so low as to be zero PSI as suggested.

"Circulation" has nothing to do with it.

Intuitively, imagine this diagram with an airspeed of zero mph. Then pressure is the same everywhere. Now make it 1 mph. Immediately below the airfoil, pressure is slightly higher, and thus elsewhere it's slightly lower. Now make it 10 or 100 mph...

  • $\begingroup$ Can you tell where you see on this video that air behind airfoil going down?youtube.com/watch?v=rCpZpKZLz14 $\endgroup$ – user50657 Aug 17 '20 at 18:01
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    $\begingroup$ @NoahPrandtl In that video, every time the airfoil tilts up you can see the air stream going down. Sure, it turns around pretty fast to fill in the slightly-lower pressure zone in the wake of the airfoil, but it's definitely going down when it leaves the trailing edge. $\endgroup$ – HiddenWindshield Aug 17 '20 at 18:55

Downwash is most often used to describe the phenomenon of angle-of-attack decreasing towards the wingtips. This downwash is caused by the circulation around the wing being shedded towards the wingtips and forming wingtip vortices.

There are no wingtips in a wall-to-wall foil, no wingtip vortices and no downwash. There is however, as you can see in the pictures in the other answers, circulation producing faster lower pressure flow above the wing and slower higher pressure flow below it, together producing lift.

  • $\begingroup$ There is only one downwash field as far as 3D flow field is concerned (because there is only one velocity field). Lift cannot exist without downwash. With or without the walls at the wingtip, there will be downwash. $\endgroup$ – JZYL Aug 18 '20 at 15:54
  • $\begingroup$ @JZYL, airflow like this behind wing in video below, in wall to wall airfoil in closed section can never happend..fyfluiddynamics.com/2014/04/… $\endgroup$ – user50657 Aug 18 '20 at 19:17
  • $\begingroup$ @NoahPrandtl The vortex sheet roll-up would be much closer to the wing root. But what's your point? $\endgroup$ – JZYL Aug 18 '20 at 19:42
  • $\begingroup$ @JZYL,so you think that behind airfoil in wind tunnel will be two wake vortex?if that is case than central part of airfoil must has larger downwash than outboard,but why central part of airfoil will be producing more downwash? $\endgroup$ – user50657 Aug 18 '20 at 20:47
  • $\begingroup$ @NoahPrandtl The large vortex structure from your OP's photo is not a single vortex. There are an infinite number of vortices. It's more appropriate to be called a vortex sheet roll-up. The vortex sheet will exist with or without your bounding walls at the wing tips. $\endgroup$ – JZYL Aug 18 '20 at 20:53

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