gallois wrote:
But is there really such a thing a still air outside space and a vacuum?
Good question. Sticking to fluid mechanics it is, but going further into de details of quantum mechanics and it all becomes particles, except perhaps Bose-Einstein Condensate. which is utterly weird quantum stuff, but still a fluid AFAIK
My point was that there is a huge conceptual difference in observing air moving over a wing and a wing moving through the air. Only when observing a wing moving through the air (observing the air, not the wing) will you start grasping the “secrets” of what lift really is. Then it becomes very clear that lift is nothing but circulation. That is what Prandtl, Kutta and others did more than 100 years ago. This original movie by Prandtl shows the starting and stopping vortices when an airfoil starts moving and stops moving. Try explaining those vortices by “Bernoulli and Newton”
Interesting video and interesting.to see the amount of stick (particles sticking to) the upper side of the wing (extrados).
Has anybody quoted McLean’s book “Understanding Aerodynamics” yet?
As a follow up, Feynman’s interview. There’s always a limit to physics explanations.
I believe humanity doesn’t really understand the four basic interactions fully, so we haven’t “really” understood anything yet:
When people ask me why airplanes fly I tell them:
Imagine any kind of odd shaped object (a potato for instance) moving through the air, you can “intuitively” understand that the flow of air around that object will cause “some” sort of force on that object, this is due to the interactions of the fluid on the surface of the body. Now decompose that resulting force into two: one component perpendicular and one parallel to the direction of travel. The perpendicular component we call lift and the parallel component we call drag. Now the interesting question here is making lift much larger than drag, this is accomplished by shaping the potato in clever ways which aeronautical engineers have figured out over time, a lot of it involving trial and error. And then you equip the potato with some sort of powerplant to overcome that drag component and be able to sustain flight over a longer period of time. You also want your potato to fly more or less “smoothly” which is accomplished with the addition of stabilising surfaces (like the feathers on an arrow); and finally it’s desirable to be able to control the direction of travel somehow and that’s why we add “control surfaces” such as ailerons, elevators and rudders. Then you call it an airplane instead of a potato and there you have it!
But the fact that lift exists at all shouldn’t surprise anyone! What should surprise you is that we managed to generate lift at reasonable prices of drag in exchange for that lift, and that the resulting flying contraption is also stable and controllable and can carry a useful amount of payload from A to B.
LeSving wrote:
lift is nothing but circulation
The problem I have with circulation is that it’s a mathematical concept, not a physical one. It doesn’t “explain” lift per se, it describes it in mathematical terms.
Circulation is a result of viscosity, and that is a physical idea. So I’d rather say lift has a lot to do with viscosity and without viscosity there would be no lift rather than speaking about circulation which is a line integral around a closed loop which is correct but doesn’t help in understanding anything.
Let’s recall:
Wright Brothers practical bicycle engineers that used crude wind tunnels
Navier-Stokes can describe flow quite well but can’t calculate flow separation and turbulence, hence the Case $1million Mathematic prize is still outstanding at Balliol for the worthy ATPL QB basher to complete the equations, apparently a prompt engineer would be eligible ig AI arrives at the solution
Jutta-Zukowski I understand posited circulation theory, which physics would not recognise, as a mathematical ‘solution’
Basically on puddle-jumpers it is around 80% Newtonian mechanics, and 20% Bernoulli’s Hoover in the sky above the wing
It is 100% Newtonian mechanics
The only thing above (or below) NM is quantum effects, relativity, etc
Alpha_Floor wrote:
Circulation is a result of viscosity, and that is a physical idea. So I’d rather say lift has a lot to do with viscosity and without viscosity there would be no lift rather than speaking about circulation which is a line integral around a closed loop which is correct but doesn’t help in understanding anything.
Sorry to say, but every thing in those sentence are wrong Circulation is not a “physical idea”, it’s real. It’s what tornadoes are made from. It’s what a tornado IS. But I guess some people “don’t believe” in tornadoes as well Just a mathematical curiosity? The same goes for wingtip vortices. Circulation is weird and often counter intuitive, it creates strange effects and has some peculiar properties, particularly in 3D, which is perhaps the reason it is down played in all layman’s explanations of lift. But, the fact remains. There is absolutely no way to understand lift without understanding circulation. It will all be just bits and pieces with no coherence.
Lift has very little to do with viscosity. Nothing in fact. There have been lots of speculation of what exactly causes the Kutta condition, which seems to be some kind of odd ad hoc law of nature. Is it viscosity? is it the Coanda effect? Is it momentum effects. This has been solved (see further “up” in this thread) by variational principles, which is in essence the same principle underlying F = m*a for instance, or conservation of mass. That video by Prandtl is a fundamental key. Why these vortices when the airfoil starts and stops? What happens in between? Are those vortices not circulation? If you understand those vortices, you understand lift.
Prandtl is considered the father of modern fluid dynamics. He was probably the first person on the face of the earth that understood lift. Not just foils, but full 3D, induced drag, wing tip vortices, planforms. The optimum planform for a wing is an elliptical shape. Why? The Spitfire is the direct result of Prandtl’s work, and the only reason for this elliptic shape is minimizing induced drag by optimizing circulation over the entire wing.
Circulation doesn’t so much explain lift, as it is more correct to say that circulation IS lift. So what is circulation? Just a mathematical concept? The whole Navier Stokes equations are nothing but a mathematical concept, but so is also F=m*a (Newton), which is implicit in the Navier Stokes equations. Circulation is the ability of a fluid to swirl around itself. Where a rigid body has translational and rotational motion only, which can be described by F = m*a (Newton), a fluid is much more, well fluid. It can do translational and rotational motion, but it also sheer and bend, squeez and stretch. Circulation is a combination of rotation, often translation, and sheer, squeez and stretch if you want.
Circulation is not a mathematical concept, it is a natural way of motion for a fluid. It’s as natural for a fluid as translation and rotation is for a rigid body. Mathematics can be used to describe it of course, in the same manner as mathematics can be used to describe the motion of a rigid body.
LeSving wrote:
Lift has very little to do with viscosity. Nothing in fact.
Well sorry to say but this is not correct :)
LeSving wrote:
Circulation is not a mathematical concept
Circulation is defined as a line integral around a closed loop. It doesn’t get more mathematical than that…
Peter wrote:
It is 100% Newtonian mechanics
Is it? Bose Einstein Condensate is 100% a quantum state, yet it can be described by Navier Stokes equations Think about that for a minute or two.
It’s a bit appalling that intelligent aviators self restrict their understanding of matter to Newtons laws for rigid bodies. Anyone with an IQ of 30+ understands that air is not a rigid body.
But you are correct of course, it’s 100% Newtonian mechanics. You only have to specify: Newtonian mechanics of a fluid, also known as fluid mechanics
Alpha_Floor wrote:
Well sorry to say but this is not correct :)
You can believe whatever you want, and that issue has been debated for a long time, so it’s not a clear cut thing. Today there is no debate over it however, other than by students at universities perhaps. The issue has settled.
Alpha_Floor wrote:
Circulation is defined as a line integral around a closed loop. It doesn’t get more mathematical than that…
Well, that’s the mathematical definition of circulation, and is used to calculate it for any given flow field with known velocity vectors. Force is defined as acceleration times mass, it’s equally mathematical. Everything in physics are mathematical concepts. You can dive into the philosophy of mathematics vs reality of course, but that’s a rabbit hole with no ending, or start for that matter. It leads nowhere.
Now please, explain those vortices by Prandtl please, preferably in a non-circulation way
