Vortices as source of aerodynamic force?

[AlexanderSahlin]

The flow outside of the core of a point vortex is irrotational. The flow is only rotational at the core of a point vortex. However there is circulation around a point vortex.

A fundamental mistake made by many folks new to aerodynamics is to assume the flow around an isolated vortex is rotational, with "particles" of fluid rotating. It is not rotational. The particles of fluid around an isolated vortex are not rotating. They are only translating.

David, maybe Sailor Al is as familiar with vector analysis and concepts like irrotational flow-fields as you are. But I think also sailors, that are not so familiar with such concepts, deserve a physically correct explanation that you can grasp with just some understanding of concepts like forces, pressure and momentum. I think there is such an explanation of lift and lift-induced drag, based on change of momentum of a streamtube, very early in Hörner's book before he develops the rest of the theory. I think there should be something similar also in Marchaj's book. But maybe someone has a better explanation of aerodynamic lift?

 

[DCockey]

The concepts of vorticity and a point or line vortex, while useful in modeling and predicting aerodynamics, have little connection with understanding the physical causes of lift. My answer to the question in the title of the thread "Vortices as source of aerodynamic force?" is vortices and vorticity should should not be seen as a source of aerodynamic force.

Doug McLean in Understanding Aerodynamics - arguring from the real physics
attempts to provide a

a physically correct explanation that you can grasp with just some understanding of concepts like forces, pressure and momentum

No equations are used but it is twelve pages long. It is the best explaination I know of but it is not simple and certainly more than is needed to effectively sail a boat or design a sail rig.

McLean also discusses why there have been so many misconceptions about the origins and causes of lift:

Explaining lift in physcial terms is more difficult than most people realize, and the difficulty is inherent in the basic nature of fluid mechanics. We are dealing with countless little parcles of the fluid that move in coordination with their neighbors and exert forces on their neighbors, all while separately and sumultaneously obeying Newton's second law. It simply isn't possible to look at an airfoil and deduce, by mental effort alone,, what flow pattern satisfies the physical laws everywhere at once. There are two many simultaneous relationships to keep track of. And the difficulty is compounded by the circular nature of the cause-and-effect relationship between pressure and velocity.

In the next paragraph McLean describes using mathematics to model flow using the Naviar-Stokes equations and "lengthy calculations that are only practical on a high-speed computer". He concludes the paragraph with:

However, all they do is provide a simulation of what would happen in a real flow; they don't provide a physical explanation of how it happens.

McLean continues with:

Simplified theories have also been developed, such as potential flow theory, which ignores viscosity, and the Kutta-Joukowski theorem, which relates lift to a circulatory component of flow (circulation) and the airfoil, but these don't provide a direct physical explaination for lift either.

His final paragraph of the section is:

Faulty explainations often assume too little prior knowledge and then try to do more than is logically possible by mental effort alone. As a result, they tend to leave important things unexplained (such as what really causes the high velocity over the upper surface) and to resort to logical fallicies such as one-way causation.

 

[Remmlinger]

David, I watched the video that you posted and I don't understand your admiration for Doug McLean. He makes it unnecessarily difficult for students to understand the phenomenon of lift creation. To understand lift is basic knowledge that should be taught in a way as simple as possible.

When you teach physics, you must use a model, that obeys the laws and principles that are relevant for your purpose. To explain lift, a model of the potential flow in 2D is sufficient. You show the field of streamlines of the uniform flow around a cylinder and the picture with the streamlines around a flat plate that you get from conformal mapping of the cylinder into the flat plate. Then you show the same with an angle of attack and the students will see, that the rear stagnation point is on the upper surface of the plate. You show the students a picture from the windtunnel with the stagnation point on the trailing edge and ask, what can be done, to make the potental flow model consistent with the real flow. The answer is, you must superimpose a circulation around the cylinder to the translational flow. The conformal mapping of the superposition into the flat plate will show the students, that now the streamlines in the mathematical model correctly represent the streamlines in the windtunnel. You are done! Now you can use your mathematical model to calculate the forces and the students will realize, that the force will lift the airfoil up.

You can not create an understanding if your model is too complex. Discussing 3D-pressure patterns on the ground is absolutely unnecessary. Keep it simple and short !!!

 

[Sailor Al]

Applying a little rough statistics, in around 30 posts on this thread, from seven members, in answer to my question "Are they (Viola's theories) relevant to the layman sailor?", I see three members voting in the negative, four sitting on the fence, and none supporting the proposition. That gives me some confidence in my conclusion that he's off with the pixies. Thanks guys.

And yes, there is still a need for an understandable explanation without resorting to Kutter, Bernoulli, potential flow, stagnation points, vorticity and circulation and aeroplane wings.

 

[jehardiman]

But I think also sailors, that are not so familiar with such concepts, deserve a physically correct explanation that you can grasp with just some understanding of concepts like forces, pressure and momentum. .... But maybe someone has a better explanation of aerodynamic lift?

Ok, the first concept a sailor needs to grasp is that there is no "lift", i.e. moving "up" an energy flow. There is only drag. And both the sails and the hull only provide drag. It is the vector summation of these two drag forces on the vessel as a whole that provides motion that appears to go "up wind", but in the energy sense it doesn't. This is easily provable by direct experiment. A sailor needs to know that the sails are useless without the hull, and the hull limits absolute performance regardless of what type of sail is used.

 

[DCockey]

Re

When you teach physics, you must use a model, that obeys the laws and principles that are relevant for your purpose. To explain lift, a model of the potential flow in 2D is sufficient. You show the field of streamlines of the uniform flow around a cylinder and the picture with the streamlines around a flat plate that you get from conformal mapping of the cylinder into the flat plate. Then you show the same with an angle of attack and the students will see, that the rear stagnation point is on the upper surface of the plate. You show the students a picture from the windtunnel with the stagnation point on the trailing edge and ask, what can be done, to make the potental flow model consistent with the real flow. The answer is, you must superimpose a circulation around the cylinder to the translational flow. The conformal mapping of the superposition into the flat plate will show the students, that now the streamlines in the mathematical model correctly represent the streamlines in the windtunnel. You are done! Now you can use your mathematical model to calculate the forces and the students will realize, that the force will lift the airfoil up.

The classical mathematical physics approach to lift in 2D, dependent on the flow meeting the requirements to be modeled using a potential and a Kutta condition to determine circulation. An extension of the approach is to use a conformal mapping between the cylinder and an airfoil shape. Determine the governing equations and associated conditions which are sufficient to provide a unique solution, solve the equations and you are done, for many purposes.

But does it answer the question of why does the flow behaves that way?

 

[Sailor Al]

Ok, the first concept a sailor needs to grasp is that there is no "lift", i.e. moving "up" an energy flow. There is only drag. And both the sails and the hull only provide drag. It is the vector summation of these two drag forces on the vessel as a whole that provides motion that appears to go "up wind", but in the energy sense it doesn't. This is easily provable by direct experiment. A sailor needs to know that the sails are useless without the hull, and the hull limits absolute performance regardless of what type of sail is used.

I’m sorry, but that is just crazy talk that adds nothing to the conversation.

 

[Remmlinger]

But does it answer the question of why does the flow behaves that way?

A good characterisation of my post. With your last sentence you enter the realms of philosophy. Why are the planets rotating around the sun? Why is there a gravitational force? Why is the second law of thermodynamics valid? etc.......
Uli Remmlinger

 

[Doug Halsey]

Re

The classical mathematical physics approach to lift in 2D, dependent on the flow meeting the requirements to be modeled using a potential and a Kutta condition to determine circulation. An extension of the approach is to use a conformal mapping between the cylinder and an airfoil shape. Determine the governing equations and associated conditions which are sufficient to provide a unique solution, solve the equations and you are done, for many purposes.

But does it answer the question of why does the flow behaves that way?

A simple explanation for why is that, without the Kutta condition, potential flows predict infinite velocities at sharp trailing edges, and the Kutta condition is just a way of removing those unphysical results.

It doesn't explain what's going on physically, but that would be more how than why. Still, it's intuitively clear that, since potential flows are frictionless, the explanation involves friction somehow.

 

[jehardiman]

I’m sorry, but that is just crazy talk that adds nothing to the conversation.

A good characterisation of my post. With your last sentence you enter the realms of philosophy. Why are the planets rotating around the sun? Why is there a gravitational force? Why is the second law of thermodynamics valid? etc.......
Uli Remmlinger

Unfortunately, this lack of emphasis on philosophy is where many people like the OP, Sailor Al, fail themselves. The early "philosophers" in aero and hydrodynamics saddled us with terms and concepts that are patently incorrect or were misunderstood at the time. These errors/misrepresentations have been further compounded by being baked into the the subsequent texts. If Newton had said that "lift" was the fictional component of body force* perpendicular to the inflow vector, we would not be having this discussion. But philosophically, he did not concern himself with the internal body forces, and how they are formed, but with the external vector mass flow. Subsequent "philosophers" then added their own incomplete understanding and models. Even today, we do not have a complete understanding of how the body force is generated and how flow energy is actually transferred to the body. We can see it, we can make many philosophical models, but we cannot completely describe it. Unfortunately for Sailor Al and many others who have not had the benefit of broader philosophical discussions on the subject during their education and work experiences, they are caged in this limbo of incomplete understanding. Regardless of which "philosophical" model you want to use to get "lift", as I said in my first post all the models are wrong because 'lift' itself is non-corporal, it is only a concept...not a real thing in and unto itself.

If I had a half-decent secondary school physics lab with an air table and air track, I could easily show that there is no physical "lift", only a component of the body force in the direction you want to define it in. I'd have Sailor Al and the other students make all kinds of sails and "lifting" surfaces and we would test, and track, and analyze each and every one. We would get down to the "philosophy" of the physics of "lift". But I can't do that for people on the internet when they choose to keep their mind closed.

(Edit: * should be "net aero force on the sail", see Post #49)

 

[Doug Halsey]

Ok, the first concept a sailor needs to grasp is that there is no "lift", i.e. moving "up" an energy flow. There is only drag. And both the sails and the hull only provide drag. It is the vector summation of these two drag forces on the vessel as a whole that provides motion that appears to go "up wind", but in the energy sense it doesn't. This is easily provable by direct experiment. A sailor needs to know that the sails are useless without the hull, and the hull limits absolute performance regardless of what type of sail is used.

I've got to agree with Sailor Al that this is just crazy talk.

Why are you referring to lift as moving up an energy flow? It's at right angles to the flow. And if there were only drag, you couldn't sail upwind.

 

[jehardiman]

Why are you referring to lift as moving up an energy flow? It's at right angles to the flow. And if there were only drag, you couldn't sail upwind.

That's correct, you don't sail "upwind". That wind, and you ability to take energy from it, is gone...you stole energy from it, but you never sailed anywhere close to completely through it. You never exceeded it's original energy state, you only took work (i.e. a body force* vector) that never had a positive dot product. You drifted "downwind" the entire time...all you did was slow your downwind drift.
Ok, philosophy ...
Let us say I'm sailing into the wind @ 45 to the wind @ 10 knt over ground in a mass wind of 10 knots over ground. In one hour, I will sweep across a mass of wind 07.07 NM wide and 10 NM long. At the end of that hour I have extracted all the energy I can from that mass of air. If I start at coordinate 0,0, I end at coordinate 7.07, 7.07 making 7.07 along the wind vector. The mass of the wind started in the middle of of its block 3.53, 5 and ended at the middle of it's block 3.53, -5.
What is my change in position relative to the wind that provided the energy in the wind direction only ..... vector math: wind vector + vessel vector ...((wind end)-(wind start))+ ((vessel end)-(vessel start))
((-5)-(5))+((7.07)-(0)) = -10 + 7.07 = - 2.93
Yes, on the ground I made 7.07 NM "to weather". As far as the wind is concerned, I drifted downwind 2.93 NM.
(Edit: * should be "net aero force on the sail", see post #49)

 

[Doug Halsey]

It sounds like you don't believe that VMG can be greater than the wind speed. But it can be, whether you're going upwind or down.

 

[jehardiman]

It sounds like you don't believe that VMG can be greater than the wind speed. But it can be, whether you're going upwind or down.

Vmg is a most abused term because it is often not based upon true wind as well as most vessel being unable to actually measure true windspeed and direction. FWIW, while Vs/Vt may exceed 3, and maximum Vmg/Vt can higher, angle at maximum Vmg/Vt can never exceed 45 degrees and only then for a perfectly frictionless hull with infinite side force at any Vt above zero, a condition not achievable in the real world. This enforces my statement that sails are useless with a hull.
As I pointed out before, it is an energy problem, not a force or velocity one. Show me the math....But before you do that I would suggest you go to Marchaj and read what he says about Tornado, Monitor, Icarus, Crossbow, and a DN.

(edit to clarify and change to nomenclature consistent with Marchaj.)

 

[Doug Halsey]

Vmg is a most abused term because it is often not based upon true wind as well as most vessel being unable to actually measure true windspeed and direction. FWIW, while Vs/Vt may exceed 3, and maximum Vmg/Vt can higher, angle at maximum Vmg/Vt can never exceed 45 degrees and only then for a perfectly frictionless hull with infinite side force at any Vt above zero, a condition not achievable in the real world. This enforces my statement that sails are useless with a hull.
As I pointed out before, it is an energy problem, not a force or velocity one. Show me the math....But before you do that I would suggest you go to Marchaj and read what he says about Tornado, Monitor, Icarus, Crossbow, and a DN.

(edit to clarify and change to nomenclature consistent with Marchaj.)

Since you're referencing Marchaj, here's what he says about VMG:

Marchaj_VMGmax

• Doug Halsey
• Apr 2, 2021

Doesn't any VMG > Vt disprove your argument?

This analysis of ideal iceboats doesn't require any discussion of energy. Just forces & angles. And the only thing that's infinite is the "hydrodynamic" L/D, but that's because D=0, not L=infinity.