Timeline for How complete is our understanding of lift?
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22 events
| when toggle format | what | by | license | comment | |
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| Jan 20, 2022 at 23:06 | comment | added | Charles Bretana | At the most basic level, (the exchange of momentum when two particles approach and rebound due to their electromagnetic repulsion), we understand that completely (although what exactly causes electromagnetic force is still a mystery). Every other theory of lift (INCLUDING NAVIER STOKES), simply involves calculating aggregate values as a result of the trillions of molecular collisions that are the source of the force. When we define a new abstract "force" to be useful in a particular scenario, or to solve a specific problem, we sometimes do not understand how to calculate it. | |
| Jan 20, 2022 at 23:01 | comment | added | Charles Bretana | Be aware that at the molecular level, ALL forces are just the force exerted by the electromagnetic interaction between the electrons in the outer shells of the molecules interacting with each other when they close to each other. There are only four forces in the universe. Electromagnetic force (the strongest), The "strong" force, responsible for interactions within the nucleus of an atom, the "weak" force, responsible for the stability of the quarks within protons, neutrons and other hadrons, finally, the weakest force, gravity. Every other theory of Lift is derived from that. | |
| Oct 8, 2020 at 7:06 | comment | added | Peter Kämpf | @DKNguyen: Unsteady flow simulation is computationally demanding. That not more has been done has to do with the limited budgets and access of biologists for high performance computing. What I find (link, link, however, shows good agreement between experiment and computations. We do understand what is happening, at least when you read more recent research. | |
| Oct 7, 2020 at 22:13 | comment | added | DKNguyen | @PeterKämpf My understanding is that we do not model the unsteady state flow accurately involved with insects, nor really understand what is happening. | |
| Oct 7, 2020 at 22:12 | comment | added | Peter Kämpf | @DKNguyen What trouble in particular? | |
| Oct 7, 2020 at 22:02 | comment | added | DKNguyen | I'm surprised there are so many answers saying we understand it very well when we are having trouble with things at smaller scales such as humminbirds and especially insects. | |
| Jan 13, 2020 at 21:18 | comment | added | Orbit | @PeterKämpf I think it is easier to understand when superimposed, at the lower side the flow is slowed down and at the upper side it speeds up. It makes sense that this creates lift. Thanks for taking the time to respond, it's nice to finally understand the concept, even though it is 15 years after my exam. I think I will leave it at this and not dig in to the complex derivation of the theorem at this point. | |
| Jan 13, 2020 at 16:32 | comment | added | Peter Kämpf | @Orbit: Rotation is only part of it. Superimpose it on translation and you get the flow bending I am talking about. But it is harder to grasp that way, even if the superimposition is linear. Kutta gives you the right amount of rotation to superimpose on that translation. | |
| Jan 12, 2020 at 22:43 | comment | added | Orbit | @PeterKämpf I wasn't aiming at the trailing, more the general idea that lift is related to rotation. I thought maybe there are still some things there that are not completely understood(maybe because I found it difficult to understand). I looked in to it a bit again, and I was mistaken, lift is not really caused by rotation, rotation is just a way to calculate it more easily. | |
| Jan 12, 2020 at 21:05 | comment | added | Peter Kämpf | @Orbit: I did not want to use theories but make the explanation descriptive and graphic. Adding that the flow does not go around the trailing edge because of some arcane theory is not in that spirit, I thought. I have to add that Kutta is instrumental if you need to find the right vorticity for a potential flow code but again, explaining lift with sources, sinks, doublets and vortices is a poor way to make it graphic and easily graspable, however well it works mathematically. | |
| Jan 12, 2020 at 11:04 | comment | added | Orbit | The answer does not cover the Kutta–Joukowski theorem, would that be an interesting addition to it? | |
| Jun 26, 2018 at 13:39 | comment | added | yshavit | Thanks! That's the one bit that's always confused me -- the notion that lift is generated more on the top of the wing than on the bottom. I think it makes sense now. | |
| Jun 26, 2018 at 13:24 | comment | added | Peter Kämpf | @yshavit: Yes. The wing pulls the air above itself down as much as it pushes the air below itself down, too. Now, on the molecular level, you are right to say that the air above the wing is only pushed down by pressure from above because the wing creates a barrier to pressure from below. But that is suction, only in other words. | |
| Jun 25, 2018 at 21:39 | comment | added | yshavit | Hm, okay. The reason I asked is that at the molecular level, "suction" is not a force. The only way an air molecule could suck the wing molecule up is if you had an attractive electrostatic force between them. So if the mental model is pressure, then it's really a case of the wing being pushed up from the bottom, not pulled up from the top. But it sounds like you're saying that's a valid way to think of it? | |
| Jun 25, 2018 at 6:55 | comment | added | Peter Kämpf | @yshavit: Yes, suction is just less pressure on one side. Now it depends what you see as the direct source of lift. You can 1) either vote for suction, or 2) for impulse exchange, or 3) for pressure. All three views are equally defensible - it depends on your point of view. | |
| Jun 25, 2018 at 6:28 | comment | added | yshavit | I read somewhere that most of the lift is produced not just at the front, but front of the upper surface. Is that correct? If so, it doesn't seem consistent with your explanation of lift at the molecular level (that it's due to more molecules bouncing on the lower surface than the upper). That would suggest that the lower surface is what produces most of the lift, since that's where the net-positive bouncing happens. | |
| Apr 13, 2017 at 12:59 | history | edited | CommunityBot |
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| Oct 27, 2015 at 15:43 | vote | accept | Dan | ||
| Oct 7, 2015 at 5:40 | history | edited | Peter Kämpf | CC BY-SA 3.0 |
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| Oct 6, 2015 at 22:35 | history | edited | Peter Kämpf | CC BY-SA 3.0 |
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| Oct 6, 2015 at 22:29 | history | edited | Peter Kämpf | CC BY-SA 3.0 |
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| Oct 6, 2015 at 22:04 | history | answered | Peter Kämpf | CC BY-SA 3.0 |