Is circulation real?

That's right. But that doesn't mean that compressibility is the cause of the lift. In the text that comes with the video "Wing lift Holger Babinsky", I read

" When the video is paused, it's clear that the transit times above and below the wing are not equal: the air moves faster over the top surface and has already gone past the end of the wing by the time the flow below the aerofoil reaches the end of the lower surface. "What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes - lower pressure on the upper surface and higher pressure on the lower surface," clarified Holger."

Which is absolutely right also. The comment "In other words, it's the curvature that creates lift, not the distance." is appropriate. Here, the intend of the video is to demonstrate that the assertion that

""A wing lifts when the air pressure above it is lowered. It's often said that this happens because the airflow moving over the top, curved surface has a longer distance to travel and needs to go faster to have the same transit time as the air travelling along the lower, flat surface."

is wrong. I agree that more and more didactic information should be spread in people's head to fade away this misconception. The demonstration that this assertion is wrong has been made many times, but people's belief is hard to change.

Nevertheless, I don't see here any matter putting in question the role of the compressibility of the air, nor the air speed over the extrado being greater than over the intrado. I know ( and used ) the PIV technique to study flow fields, when studying at the ONERA (fr) Flow field survey and visualization https://www.onera.fr/en/windtunnel/flow-field-survey-and-visualization. This technique is very old, very common, and very acccepted. In the attached document from the DLR, which have made a great work with the ONERA, you will find some general informations about PIV. And also these two figures, showing an experimental study of the velocity field around an foil profile. Where it could seen that, as said in the comment of the the video "Wing lift Holger Babinsky" : "the air moves faster over the top surface ...". By looking at the norm of the velocity vectors.

 

I think you are focusing on velocities relative to undisturbed air whilst Babinsky is looking at local changes in velocity.
Take the line A - B, in the upper photo, at all points as you rise from B to A the line moves to the right.
However in the lower photo the line has become "S" shaped. The only way I can visualize that happening is for the bottom half of the air accelerating relative to the upper half thus creating the bulge.

 

In this frame from the video, it's clear that all the streamlines above the airfoil extend farther downstream than all of the ones below it.

The two streamlines closest to the upper surface seem to be shorter than expected, but the appearance is exaggerated due to the extra distance they have traveled along their more curved paths going around the leading edge.

It is perhaps too bad that the airfoil is shown at such a large angle of attack - something like 25 degrees by my measure. I don't know what the Reynolds number is, but I would expect there to be significant amounts of separated flow over the aft portion of the upper surface to complicate the flow description.

Regardless, I think this is a good demonstration of the differences in velocities above and below the airfoil.

 

@Doug Halsey So I'm confused about your reply.
Yes, we all agree that it's a "good demonstration of the differences in velocities above and below the airfoil." It was good enough for the professor of aeronautical engineering at Cambridge University to deem it good enough, and it is good enough for you and me.
That was never the issue under discussion yet your first and last paragraphs dwell exclusively on that issue.
Your two middle paras do address the issue, but your position is not clear.
Are you saying

1. the velocity of the air does increase but it's not apparent in the video, or
2. the AoA is too high or the Re too low to induce the velocity increase?

I should add Prof. Babinsky assured me in an email on Sept 3: " And yes, on my video the air pressure and velocity follows Bernoulli's equation."

Can I encourage you to consider the possibility that the low pressure above the wing is not being caused by an increase in the fluid velocity at all, but by the much simpler process that I have described.

 

Yes, absolutely. See my response above!

 

That is not my proposition.
My proposition is that there's no evidence in the video that the velocity of the air increases over the wing.
Your response does not address my post.

 

The video shows adjacent streamlines converging in a very small region near the leading edge. That is the region where the velocity is increasing. Aft of that region, the streamlines are diverging, so the velocity is decreasing. That's totally consistent with calculations such as the one I made today.

 

So are you saying that the velocity does increase but the video doesn't show it?

 

The video shows the streamlines converging over a small distance near the leading edge, which means that the velocity is increasing. Farther aft, it shows the streamlines diverging from the airfoil, which means the velocity is decreasing.

 

Please, are you saying that the velocity of the air over the wing does increase but the video of the experiment does not show that?

I'm sure I don't have to point out that by examining the progress of the smoke frame by frame, if in successive frames smoke in one streamline progresses further than the smoke in another streamline, then the air in the first streamline is moving faster than the air in the second.
It is quite clear from the video that the smoke in the streamlines at the top of the screen(the "freestream") progresses frame-by-frame further than the smoke in the streamlines close to the top of the foil.
In none of the frames is the smoke seen to travel faster than the freestream air.
This means that the video shows that air over the foil is actually moving slower than the free stream air for its entire progress over the top of the foil.
Your deduction that the spacing of the streamlines can be used to determine velocity is exactly that: a deduction, based on your years of experience in fluid dynamics.

The paradox that I am pointing out is that Babinsky's experiment clearly demonstrates that the the velocity of the air does not increase over the top of the wing.

Can I encourage you to consider the possibility that the low pressure above the wing is not being caused by an increase in the fluid velocity at all, but by the much simpler process that I have described.

 

I wouldn't conclude anything from what the streamlines at the top & bottom of the screen are doing. They could be very close to the ceiling & floor of the tunnel, which could be either solid or porous, with or without suction to control the boundary layer, and so on. The upstream edge is normally considered to be the undisturbed freestream.

In the early frames, before the pulses reach the airfoil, the streamlines closest to the airfoil are already lagging behind the ones farther away and this loss of distance will probably increase as they get closer (because of the proximity of the stagnation point). Whether or not they catch up is immaterial. Just pay attention to the streamlines closest to the airfoil (1 or 2 on each side). If you do that, I think things will make more sense to you.

If you want to use your method to explain it, go ahead and try, but last I heard, it was incapable of giving any quantitative predictions, so I don't see how it's of much use. And I see nothing wrong with the method that I used.

 

If the statement above is correct and the velocity of the air does not increase over the top of the wing then the basic principals of aerodynamics which have developed over more than a century based on both experimental results and mathematical modeling are fundamentally wrong.

I'll leave it to the reader to decide the probability of the basic principals of aerodynamics which have developed over more than a century based on both experimental results and mathematical modeling being fundamentally wrong.

 

But doesn't the evidence of Babinsky's video and the complete absence of any experimental evidence give you any pause to reflect... at all???
Not even a little bit?

I understand that contradiction.

The problem is that the complexities of fluid dynamics don't lead to a framework in which we as racing sailors can discuss or understand the process of trimming sails.
The best that sailing texts can do is to say that "a sail is like a wing", and leave it to the individuals to try and work out that means in practice.
I am trying to find an alternative explanation on which to develop that framework.

 

Please accept that Professor Babinsky knows how to perform wind tunnel experiments.
This experiment clearly shows the velocity of the airstream does not increase over the wing.
There is no experimental evidence that it does.
I know it's challenging, but maybe....?

 

Then see the post #406. Pr. Babinsky come to the same conclusion than the authors of the study given in post #361: I recall :

"the current result provides a convincing argument about the mechanism behind circulation development over the airfoil. It is simply a momentum conservation mechanism;

So, now that this is established, why cannot we use this corner stone to

?

More camber = more momentum = more lift / Too much camber = Too much momentum = No more lift
More incidence = more momentum = more lift / Too much incidence = Too much momentum = No more lift