Is circulation real?

[Sailor Al]

I used to think it was just a mathematical theory, but during the years I've learned through simulation that the physical flow around a lifting surface actually has a resemblance to what the mathematical model of circulation theory describes

OK, I asked which interpretation of circulation you were using and I see from your response that it's not clear. Unfortunately, unlike Lewis Carrol's character, it's not ok to make words mean exactly what you want them to mean. Circulation has a limited number of meanings, and when you use a term, you have to be clear which meaning you are using. In the "circulation theory of lift", that circulation is the one defined by vector calculus ("the line integral of the vector field around a closed circuit") and has no physical characteristics that can be observed.

The circulation that you may or may not be observing using simulation (CFD) is something else.

CFD is a tool to model the real world. When a CFD model starts to yield results that match the real world results, then you can gain confidence that the model is working, and that the physics and maths you put into the model correspond to the way the world actually works.

The model will only represent the logic of the modeller. Garbage in - garbage out.

When you use a CFD model to demonstrate a physical phenomenon that doesn't exist in nature, you are just demonstrating that the model is wrong.

 

[Mikko Brummer]

CFD is a tool to model the real world. When a CFD model starts to yield results that match the real world results, then you can gain confidence that the model is working, and that the physics and maths you put into the model correspond to the way the world actually works.

Quite so.

 

[Mikko Brummer]

When you use a CFD model to demonstrate a physical phenomenon that doesn't exist in nature, you are just demonstrating that the model is wrong.

Oh but this model demonstrates a physical phenomenon that does exist in nature - see the video at Movie 1 https://movie.biologists.com/video/10.1242/jeb.214809/video-1

This demonstrates Tawny owl the 3D theory of circulation, but nevertheless the same thing. The research paper is here High aerodynamic lift from the tail reduces drag in gliding raptors | Journal of Experimental Biology | The Company of Biologists https://journals.biologists.com/jeb/article/223/3/jeb214809/223686/High-aerodynamic-lift-from-the-tail-reduces-drag

 

[Mikko Brummer]

I don't have the Tawny owl, but this albatross shows the same thing, enough to convince me that the world's most respected CFD code is accurately reproducing reality. I could post the video so you can see the similarity of the tip vortices rolling & the downwash.

 

[Sailor Al]

Oh but this model demonstrates a physical phenomenon that does exist in nature - see the video at Movie 1

Now I know you're messing with me!
Nobody is denying the existence of wingtip vortices.
The wingtip vortex axis is parallel to the direction of flight and patently exists.
It's the contra-rotation around the aerofoil , with an axis parallel to the wing that is under discussion.

don't have the Tawny owl, but this albatross shows the same thing, enough to convince me that the world's most respected CFD code is accurately reproducing reality. I could post the video so you can see the similarity of the tip vortices rolling & the downwash.

I don't see any "circulation" around the albatross's wing, just wingtip vortices and some turbulence behind the trailing edge.
Now you're showing a CFD image that doesn't even contain the phenomenon to demonstrate a non-existent phenomenon!

 

[DCockey]

I suspect the word "circulation" in aerodynamics results in some folks mistakenly thinking it implies fluid particles orbiting the airfoil rather than the somewhat escoteric definition that aerodynamacists use.

To be clear:

1) Fluid particles do not orbit an airfoil when the airfoil is in a flow producing lift.

2) The path integral of velocity along a closed loop containing an airfoil producing lift is non-zero. This is the definition of circulation used in aerodynamics.

 

[patzefran]

I wouldn't say the definition is esoteric ! It is as obvious as the definition of work as integral of the force along a path !

 

[Mikko Brummer]

I don't see any "circulation" around the albatross's wing, just wingtip vortices and some turbulence behind the trailing edge.
Now you're showing a CFD image that doesn't even contain the phenomenon to demonstrate a non-existent phenomenon!

I'm starting to realize how all time spent speculating with you is time wasted. "When you use a CFD model to demonstrate a physical phenomenon that doesn't exist in nature, you are just demonstrating that the model is wrong." Just showing you that XFlow is quite capable of demonstrating natural phenomena.

Here's "circulation", well, "the flow pattern", demonstrated around the mid-body of the same albatross... I'm sure you'll find something offensive to comment about it, too.

 

[Sailor Al]

1) Fluid particles do not orbit an airfoil when the airfoil is in a flow producing lift.

Welcome back to the bear pit.
You know that, I know that but writers from Lanchester in 1910, Prandtl in the 20's, Gentry in the '70s, Marchaj in the '80's, and Voila last year, to @Mikko Brummer today appear to believe otherwise and continue to generate diagrams containing arrows pointing towards the oncoming airstream to demonstrate their theory.

 

[DCockey]

You know that, I know that but writers from Lanchester in 1910, Prandtl in the 20's, Gentry in the '70s, Marchaj in the '80's, and Voila last year, to @Mikko Brummer today appear to believe otherwise and continue to generate diagrams containing arrows pointing towards the oncoming airstream to demonstrate their theory.

Incorrect. You apparently do not know what Lanchester in 1910, Prandtl in the 20's, Gentry in the '70s, Marchaj in the '80's knew, and what Mikko's very good illustrations show.

You need to study the differences between total velocity and perturbation velocity. You need to study why a flow may be steady in one reference frame and unsteady in another. You also need to study the differences between streamlines, streaklines and pathlines.

Then you should be able to understand that the perturbation velocity can point upstream and why the perturbation velocity streamline does not represent the motion of a single fluid particle.

A solid understanding of the concept of perturbation velocity is essential to understanding technical discussions of aerodynamics.

 

[Sailor Al]

Oh dear, and just when I thought we were starting to get on.
Please, then in terms of simple classical physics, tell us what the arrows do mean! @Mikko Brummer explained them as: "Here's "circulation", well, "the flow pattern"".
Unfortunately the referenced authors are no longer around and Viola has withdrawn his explanation and isn't talking to me any more.

 

[DCockey]

Please, then in terms of simple classical physics, tell us what the arrows do mean!

The arrows in Mikko's plots show the direction of the perturbation velocity. The color shows the magnitude of the velocity, aka the speed. In other plots the arrows may show the perturbation velocity vectors.

You need to study the differences between total velocity and perturbation velocity. You need to study why a flow may be steady in one reference frame and unsteady in another. You also need to study the differences between streamlines, streaklines and pathlines.

There are numerous sources of information on the topics mentioned available online in various formats. If you seriously want to learn you can avail yourself of that information, which can be easily found with a simple search.

 

[Sailor Al]

The arrows in Mikko's plots show the direction of the perturbation velocity. The color shows the magnitude of the velocity, aka the speed. In other plots the arrows may show the perturbation velocity vectors.

I did ask for an explanation from "classical physics", in which "perturbation velocity" is unfortunately, not mentioned.
If it's just the velocity relative to a different frame of reference, please, which one are we talking about?

 

[TeddyDiver]

In other words, just take the airspeed (of an aeroplane or apparent wind speed on a sailboat) of and what's left is the circulation.. IMO the easy way to explain.

 

[Mikko Brummer]

@DCockey Correct me if I'm wrong, but the curved arrows in the albatross midbody screen #203 represent streamlines in the earth reference frame, as would be seen by an observer standing still looking at the albatross flying by on a calm day (no wind). And as such, the perturbation velocity?