Is circulation real?

[Remmlinger]

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 explained this a few days ago: Is circulation real? https://www.boatdesign.net/threads/is-circulation-real.46025/page-13#post-907115 but it was obviously "too long to read". To one of the forum members your teaching is like speaking into the wind.

 

[Earl Boebert]

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?

No one is obligated to explain the circulation model in terms of "classical physics," the movement of particles of pixie dust, or any universe of discourse other than the model itself. To demand otherwise is to demonstrate a profound misunderstanding of the nature and application of models.

 

[DCockey]

The arrows are the perturbation velocity because they do not include the "free stream" as seen by an observer traveling on the albatross. A streamline could be created using the arrows by by drawing a curve which is tangent to the arrows. This would be a perturbation velocity streamline or a streamline in the earth reference frame (assuming no ambient wind).

"Streamline" is another word that aerodynamacists use which can cause confusion, A streamline is a curve which is tangent to the local velocity everywhere at a intant in time. Streamlines differ depending on the reference freame. A pathline is the curve a fluid partcle follows with time. Streamlines and pathlines coincide only if the flow is steady in the reference frame and if the streamline is based on the total velocity.

Streaklines are a third type of curve used to illustrate fluid motion. A streakline is the curve of fluid particles which have all passed through a fixed point in the frame of reference. A streakline is different than a streamline and different than a pathline unless the flow is steady in the reference frame.

These terms are confusing. An introduction to fluid mechanics book should cover them.

 

[DCockey]

To one of the forum members your teaching is like speaking into the wind.

Unfortunately that appears to be true. But others appear to be receptive and I learn when I explain.

 

[Remmlinger]

I learn when I explain.

This is a very good point David! When I try to explain, I notice where my understanding is clear and where it is only fuzzy. So, even if the postings are sometimes annoying, it helps to clarify my thoughts. In the end, it is Mikkos thread and the pictures he has posted are fascinating. The linked video of the raptors was striking, I had never seen anything like that. Just for that, the visit to this forum is worth the time.
Regards, Uli

 

[DCockey]

In the end, it is Mikkos thread and the pictures he has posted are fascinating. The linked video of the raptors was striking, I had never seen anything like that. Just for that, the visit to this forum is worth the time.

Agree.

 

[Earl Boebert]

Agree.

Ditto.

Cheers,

Earl

 

[tspeer]

Sorry to be late to this discussion, but I think the explanation for the starting vortex is simple and the same as for the trailing vortices: conservation of mass.

Conservation of momentum says the lift is due to the wing imparting a velocity to the fluid that is at right angles to the free stream. Ideally, that wake wash velocity is uniform over the span of the wing. The wing has a finite span, so the deflected wake also has a finite span. If the lift has been produced for a finite time, then the wake also has a finite length.

It really is the same flow pattern one sees when a canoe paddle is drawn through the water. If you view the wing in the steamwise direction, you see the same flow pattern. The paddle displaces a slug of fluid sideways, and a strong vortex forms that wraps around the paddle, running down one edge, across the bottom, and back up the opposite edge. It is very clear the vortex forms because the water displaced on the front side has to go somewhere, and the water fills in what would otherwise be a void on the backside of the paddle. That's conservation of mass - the fluid is a continuum, and for the flows we're talking about it's negligently compressible. That means the entire flow has to form a consistent picture.

The starting vortex is the equivalent of the vortex at the bottom of the canoe paddle. Since the displaced fluid has a finite length, there must be a displacement and a replacement at the downstream end of the displaced fluid. It's not different from the displacement and replacement that occurs along the edges of the wake, forming the trailing vortices. And for that matter, the flow pattern must include the bound vortex at the upstream end of the displaced fluid. Conservation of mass is why vortices that are entirely in the fluid must form closed rings.

You don't need CFD or fancy math. Just observe the humble canoe paddle.

 

[CT249]

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.

Yep, but in some ways the issue may not be in laypeople "mistakenly thinking it implies fluid particles orbiting the airfoil", but in those who write texts aimed at laypeople but fail to mention that the aerodynamic definition is very esoteric. I remember reading the famous original Arvel Gentry article on circulation several time and thinking that if it was true, the telltales on one side of the sail would have to flow towards the bow. I don't think the article ever mentioned that he was using a specialist definition.

I was only a teenager and I did get the idea of what he was talking about, but it (and similar articles) are still problematic because any article written for laypeople in any subject should surely mention it when a common English term is used in a different and specialist way.

 

[patzefran]

Sorry to be late to this discussion, but I think the explanation for the starting vortex is simple and the same as for the trailing vortices: conservation of mass.

Conservation of momentum says the lift is due to the wing imparting a velocity to the fluid that is at right angles to the free stream. Ideally, that wake wash velocity is uniform over the span of the wing. The wing has a finite span, so the deflected wake also has a finite span. If the lift has been produced for a finite time, then the wake also has a finite length.

It really is the same flow pattern one sees when a canoe paddle is drawn through the water. If you view the wing in the steamwise direction, you see the same flow pattern. The paddle displaces a slug of fluid sideways, and a strong vortex forms that wraps around the paddle, running down one edge, across the bottom, and back up the opposite edge. It is very clear the vortex forms because the water displaced on the front side has to go somewhere, and the water fills in what would otherwise be a void on the backside of the paddle. That's conservation of mass - the fluid is a continuum, and for the flows we're talking about it's negligently compressible. That means the entire flow has to form a consistent picture.

The starting vortex is the equivalent of the vortex at the bottom of the canoe paddle. Since the displaced fluid has a finite length, there must be a displacement and a replacement at the downstream end of the displaced fluid. It's not different from the displacement and replacement that occurs along the edges of the wake, forming the trailing vortices. And for that matter, the flow pattern must include the bound vortex at the upstream end of the displaced fluid. Conservation of mass is why vortices that are entirely in the fluid must form closed rings.

You don't need CFD or fancy math. Just observe the humble canoe paddle.

So, you mean necessity of closed vortex ring is not the direct consequence of Angular momentum conservation ?

 

[jehardiman]

So, you mean necessity of closed vortex ring is not the direct consequence of Angular momentum conservation ?

"direct consequence'? Yes and no. Energy can only be transferred at the actual physical interface. How that energy is actually distributed through the rest of the fluid is the mystery. This is why the terms "near field" and "far field" are thrown about. Each hydrodynamicist needs to come to their own understanding about that because there is no absolute "correct" answer. It all depends....

 

[patzefran]

"direct consequence'? Yes and no. Energy can only be transferred at the actual physical interface. How that energy is actually distributed through the rest of the fluid is the mystery. This is why the terms "near field" and "far field" are thrown about. Each hydrodynamicist needs to come to their own understanding about that because there is no absolute "correct" answer. It all depends....

Yes and no. Theoretical physics models relies only on the accuracy of their experimental predictions. There is no absolute "correct" answer, but there are best correct answers !

 

[Rasmith]

I am curious Mikko Brummer and others, after 15 pages did you get your questions answered?

The circulation theory of lift is a mathematical explanation of lift and how it is generated. But the actual lift is a real world example of the math.

Yes, you would experience the wind shift as described in post # 3.

 

[jehardiman]

But the actual lift is a real world example of the math.

Nope, the Kutta–Joukowski condition is not real, but is needed in the math. Lift is just drag in the direction you want to go. No more, no less.

 

[DogCavalry]

I disagree with that statement, @Rasmith. Circulation is an observation made after the fact. The fact that all of the vectors inevitably imposed on the air stream by the presence of a lifting form perturbing the air flow happen to sum up to an approximate circle, that easily distracted folk (squirrel!) seize on as an explanation for lift is an unfortunate sideline. Circulation is not an explanation for lift, it is an emergent property in a system where lift is being made. But the fact that it isn't the source of lift can be proved by generating lift where there is no circulation.