# Why can't we see the downwash/wake roll-up behind 2D/wall-to-wall airfoil tests? [duplicate]

Why we can't see the downwash/wake roll-up behind any wall to wall airfoil wind tunnel video?

To make that happen, the air behind the center of the airfoil must go down and the air at the side walls must go up, producing two roll-up vortices. Could you upload video of that if it exists (which I doubt)?

DOWNWASH BEHIND WING VIDEO

here is how downwash and wake roll-up vortex looks at finite wing in wind tunnel:

• I am seriously tempted to close this question as a duplicate of your previous one Commented Aug 20, 2020 at 8:07
• If nobody answers to what you consider being the main question, you can comment on their answers, and edit your question to be phrased differently. asking the same question over and over is not really good practice. Commented Aug 20, 2020 at 8:23
• Downwash does exist all throughout the span. Again, what you see in the viz is just the roll-up. You don't see the downwash because they are not readily visible.
– JZYL
Commented Aug 20, 2020 at 11:37
• @NoahPrandtl No, I haven't seen any video like that. Again, I don't see the point of discussion here. If you want to prove us wrong, go do it, publish a paper and then you can convince us.
– JZYL
Commented Aug 20, 2020 at 13:13
• Does this answer your question? How can wall to wall airfoil in wind tunnel produce downwash? Commented Aug 20, 2020 at 14:57

Who says we can't? It just needs open eyes and an open mind.

To understand the picture below, keep these things in mind:

• If the streamlines are packed together, the flow speed is higher while pressure is lower than ambient.
• Conversely, when the streamlines are spaced wide apart, flow speed is lower but pressure is higher.
• Converging stream lines mean accelerating flow.
• Diverging streamlines mean the flow slows down.

Somehow, the air has to flow around the obstacle that the airfoil represents in a closed wind tunnel. It does so by speeding up (at least in subsonic flow). Conversely, when the airfoil tapers towards the trailing edge and an angle of attack makes the cross section downstream become wider, the air slows down in order to fill the available space. In the end, the mass flow near the suction peak where the cross section left open by the airfoil is narrowest is equal to that near the trailing edge when the cross section has become much larger. The same happens on the lower side: All the air passing below the stagnation line has to squeeze through the gap left between trailing edge and tunnel wall. This is only possible by a large change in speed.

Now have a look at stream lines in a windtunnel. Note that the lines near the upper and lower edge of the picture are almost straight and nearly follow the wall contour of the tunnel (picture source):

The downwash is the downward-pointing part of the streamlines over the rear part of the upper airfoil contour. Due to tunnel wall interference, the flow on the lower side near and especially past the trailing edge is very different from free flow and the air has to speed up to flow through the gap left between the airfoil and the tunnel wall.

You also see the smoke lines past the trailing edge fan out: The flow near the center section of the tunnel is still fast and stays near the bottom while the flow near the wall slows down and bends upwards, causing the lines to spread out. The lines near the wall even intersect the ones near the center! Clearly, this is a 3D effect which resembles the wake rollup past a wing in free flow, but crippled by the proximity of the tunnel wall.

While the air coming off a wing in free flight with attached flow has approximately the same speed over the whole height, here tunnel wall blockage means that the air coming off the lower side is much faster than air coming off the upper side. This is necessary to let the air shift downwards past the airfoil, as it does in the wake of a wing. Of course, the tunnel wall and friction will limit that downward movement, but by adjusting flow speed the tunnel airfoil is able to create a downwash, too.

Another way to look at it: The slow, high pressure air coming off the upper side of the airfoil squeezes the fast, low pressure air coming off the lower side down. Either way, downwash is the result.

The downwash is right there and hard to overlook: The streamlines coming off the airfoil clearly have a downward direction. Wake rollup is also happening, albeit near the right edge of the picture and less pronounced than in free flight. If that is not evidence enough, I don't know what ever will be.

The picture has to be sufficient. Sorry, no movie.

• A movie wouldn't improve the visualization of this steady-state case! Commented Aug 20, 2020 at 15:50
• @PeterKampf It seems in aerodynamcis there is lots of place for different opinios.Here AL Bowers explain there is no downwash in wall to wall wing at 2:25 youtube.com/watch?v=w-dk1NpVNNI&ab_channel=modelaircraft
– user53913
Commented Jan 14, 2021 at 17:35

Based on comments you've made to other people's answers, I think you're confusing the existence of a downwash with the continued existence of a downwash.

In a wind tunnel, the downwash exists, as depicted in the videos and pictures you and others have posted, because the air is moving in a general downward direction as it leaves the rear of the wing. Someone who's doing a wind tunnel test of a wing is typically only interested in the performance of that wing, and so doesn't care what happens to the air after that point. The downwash exists in the area of interest, and that's all that matters.

After that point, however, the downwash creates a high pressure area at the bottom of the wind tunnel, and a low pressure area at the top. This quickly overcomes the inertia of the downward-flowing air, which stops the downwash fairly quickly. In the real world, of course, that doesn't happen, so the downwash can continue to exist until friction disperses the energy.

You can see wingtip vortices in a wind tunnel, e.g. in this picture:

A wind tunnel model of a Cessna 182 showing a wingtip vortex. Tested in the RPI (Rensselaer Polytechnic Institute) Subsonic Wind Tunnel.

(source: Wikimedia)

The lines that you see here are created using helium bubbles:

A model Cessna with helium-filled bubbles showing pathlines of the wingtip vortices.

(Wikipedia: Flow Visualization)

You can only see the wingtip vortices when these lines are created close to the wingtip. If you are interested in the in the flow over the general airfoil, you would only create such lines over the central section of the wing and you won't see any wingtip vortices.

Also note that you need a full model of the entire aircraft to accurately get these wingtip vortices. If you only have the airfoil without attaching it to the body of an aircraft, the effect will be different. And if the airfoil goes all the way to the wall of the wind tunnel, there is obviously no wingtip resulting in no wingtip vortices.

That phenomenon is known as a wingtip vortex. It's produced by the airflow around the end of the wing. A wall-to-wall airfoil won't produce wingtip vortices because it has no wing tips.