Is circulation real?

I think you should review your high-school physics books. The distinction between solids, liquids and gases is not at all arbitrary. This is not the forum to be explaining the classical properties of matter.

That's exactly what Babinsky does in the video.

 

In fact, the classical definition is arbitrary. For example, glass is considered a solid, but it slowly flows if a force is applied to it for a long time. Creep, a well known phenomenon, is a behavior of solids where they deform permanently. That is not what the classical definition of a solid allows. High-school physics books usually oversimplify definitions. This is a diagram showing what a molecule is supposed to look like. It is a drawing that fails to teach that atoms and molecules are not neat geometrical shapes, but clouds of probable locations and states.

 

No, it doesn't . That's an urban myth.

 

Wow, you refute creep in materials.
Creep and flow of glasses: strain response linked to the spatial distribution of dynamical heterogeneities | Scientific Reports https://www.nature.com/articles/srep11884

 

We're talking air over sails and wings, not "amorphous solid-like materials"! (Yes, I scanned the document.)
Please, don't confuse the discussion with references to obscure and irrelevant issues.
Do you have an answer to the questions I posed in posts #430 and #433?

 

You brought up the issue of high-school physics. If it is irrelevant you got only yourself to blame.

 

OK.
Do you have any answers to the questions I posed in posts #430 and #433?

 

I find this work very didactic. In the end, you will found what Prantl has found himself : The computed speed using the circulation theory has a meaning only outside the boundary layer that is formed in the physical world, around an wing profile. You will also find that, without introducing any vorticity in your PHP script, you will not be able to obtain the same effect on the smoke trails. Without vorticity, and so momentum, nothing fly, no lift is created. In that sense, the explanation that 'lift is created because the flow is accelerated over a profile' may, indeed, be viewed as confusing. What acceleration are we talking about ? Is it an absolut value ? Is it a vector ? Where do we take this acceleration ? Surely not at the surface of the profile, because the speed is zero everywhere, according to Prantl. To clarify this, please let me show again experimental results obtained using the same PIV technique that I described in previous post. This study shows that the absolut value of the flow speed (norm of the velocity vector) is almost constant, where they have been plotted , and that the acceleration that we are talking about is not a linear acceleration, because the flow is shown to have vorticity. (down below extracts p. 60 and p.61, p.62 of the attached pdf document ) :





The flow momentum perturbation by a surface should be considered as the most direct and sensible explanation for the lift, skipping the very complicated explanations based on the boundary layer, skipping the lack of physical meaning of the circulation theory. A lift is created around a profile because the profile deviates the flow. So should you introduce some flow deviation in your PHP script to correctly represent what is happening in the physical world.

 

Well if that is the case then velocity distribution provided by @Doug Halsey must be wrong, or relate to a different situation.
The video is a record of an actual experiment not a simulation.
I am using simple applied maths to the velocity data supplied in the velocity distribution using xn = xn-1 + vn *t . The velocity distribution supplies the value for vn. No interpretation necessary.

 

Yes, an interpretation is necessary, if you wish to make any comparison between the velocity distribution provided by @Doug Halsey and the video from Babinsky.
Are you going to make your smoke trails run in straight line ? How far from the profile surface is the velocity given by @Doug Hasley ? Are you going to use a flat vertical velocity distribution ? If there is some differences between the animation you are working on and the video from Babinsky, are you going then going to conclude that the calculated velocity profile is wrong ? In that case, having made no interpretation at the beginning of your work only drive to one very predictable, but also false, conclusion.

A far nicer use of the great idea that you have in programming a "2D smoke trails chamber" would be to allow surface-to-trail interaction and trail-to-trail interaction, in the form of elastic collisions. This is what is used in game engine to "simulate" fluid flows.

 

Yes.

At the surface.

Nope. Just three values: free stream upper surface, lower surface.

Yup!

Statement by assertion, no supporting argument, worthless.

Not looking for anything "nicer"!
Just trying to find out what the published velocity distribution looks like.
Trying to get away from arguing about angels on the head of a pin.

 

This is indeed an hard task that you are making.

Nevertheless, I think that the velocity profile that you will obtain will be like this where velocity depends only on the position x on each half of the figure. It's what I meant by flat velocity profile. It neglects the "no slip" boundary conditions of Prantl. At the surface, the flow velocity is almost null. So, I keep some strong doubts about this animation demonstrating something related to any real world experiment. Anyway, your work reminds me my own work, that I've done on my own, during my studies, to understand the Navier-Stokes equations. I think you're in the good direction, even if the path may be a little long.

 

Whenever aero calculations and data are compared, it is crucial that the Reynolds numbers (Re) be the same (or nearly so), but so far no one is asking about the Re in that video.

My calculations were for an inviscid flow (essentially infinite Re). In such cases, there is no boundary layer and velocities are calculated on the airfoil surface itself. These should be quite a bit different than the velocities in the video, but by how much is uncertain.

The data you show are for an Re value of 10,000, which is too small by one or two orders of magnitude to be representative of typical cases for sails.

So neither one of these (my calculations or your data) will tell us much about Babinski's video.

I don't understand how Babinski's airfoil can be given such a large angle of attack without showing much stronger separated flow than it seems to have (assuming the modest Reynolds numbers typical of most universities' tunnels). There must be something about the tunnel or the model that hasn't been explained.

 

Yes, I know

Ah, so you noticed that as well!

Yes, based on the preceding discussions, I was surprised that you submitted that one.

If you would re-do your numbers, I will be able to simulate them (when I've got the bugs out my code... haven't had much time over the past couple of days)

No, I have assumined a 2 metre chord with a 10 m/s freestream velocity, gives an Re of 10^5.

No, not my data... I'm using the data you supplied in #412. All I am doing is modelling that data using using xn = xn-1 + vn *t
I think it will be quite revealing!

Well, as I said, after a spirited discussion with him, he was adamant that "the air pressure and velocity follows Bernoulli's equation".

And please, could I ask you to nominate the one text that in your opinion best explains the foundations of aerodynamics?

 

Yes, it is an interesting challenge. Nearly there.