Timeline for Is it possible to statically generate lift with the difference in pressure like wings?
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22 events
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| Jan 24, 2023 at 20:28 | comment | added | Robbie Goodwin | Could you please get a colleague to provide a better translation of the Question, as well as the exposition? Neither is seriously wrong in ordinary English but sadly, neither is clear in technical terms. | |
| Jan 24, 2023 at 17:00 | comment | added | Ivan Nepomnyashchikh | @VladimirFГероямслава, to elaborate a bit more on your Bernoully statement. Consider a cylinder in a uniform flow, it doesn't have a net force (i.e. lift force). Lift force can only happen in a non-uniform flow. To get a non-uniform flow and still be able to use Bernoully for inviscid flow, we use a trick. We put a vortex into the cylinder. The vortex curves the stream lines, making velocity nonuniform across the cylinder's diameter. Then we write Bernoulli for that curved flow and get Joukovsky formula. The physical explanation for nonuniformity is viscosity. | |
| Jan 24, 2023 at 16:57 | comment | added | Ivan Nepomnyashchikh | @VladimirFГероямслава, first, I was going to say it in the previous comment, слава Украине! Second, you need to have a look how lift is introduced in an intermediate fluid mechanics course. I can't give you the full derivation in the comment. It's true there are other ways to have velocity gradient in a flow, but we imply classical velocity gradient in a viscous flow when derive lift force equation the way I talk about. Joukovskiy analysis - being for potential flow - relies on the velocity gradient of the upstream flow which, in turn, is due to viscosity. | |
| Jan 24, 2023 at 16:39 | comment | added | Vladimir F Героям слава | @IvanNepomnyashchikh In that case I do not understand or agree at all. You can have pressure gradients in inviscid fluids just fine - even in stationary flow. It just means that some other force have to balance it, most often the inertial force (the fluid accelerates along the pressure gradient). The lift explanation due to higher/lower velocity and the Bernoulli principle holds fine in inviscid flow. We only need viscosity to explain the Kutta condition. The Zhukovsky analysis is for potential inviscid flow. | |
| Jan 24, 2023 at 16:07 | comment | added | Ivan Nepomnyashchikh | @VladimirFГероямслава I was a bit general with my explanation. If you have a look at the simplest possible way to introduce lift force - which I believe was developed by Nicolai Egorovich Joukovskiy,- you'll see that we rely on velocity gradient across a cylinder's diameter (the cylinder is conformally mapped to a wing later). What I tried to say was that you can have velocity gradient if the fluid is viscous. I think I mentioned that in case the author asks why we have velocity gradient in a flow. With regards to your "good results": they can be "good", but it doesn't matter in this question. | |
| Jan 24, 2023 at 11:46 | answer | added | anaximander | timeline score: 5 | |
| Jan 24, 2023 at 10:15 | comment | added | Vladimir F Героям слава | @IvanNepomnyashchikh I do not really understand what you mean by "Difference in velocity is the result of air being viscous" as one gets very good results even from ideal fluid computations or from solutions with the Euler equations. Do you just mean the physical reason for the Kutta condition? | |
| Jan 23, 2023 at 19:52 | comment | added | Vladimir F Героям слава | The high speed/low pressure vs. low speed / high pressure in the picture is correct. However, the distance is almost irrelevant and the flow cannot be parallel to the ground both before and after the airfoil. The streamlines are more like this - note the airfoil is symmetric. For pictures with wind coming horizontally from left, try hyperphysics.phy-astr.gsu.edu/hbase/Fluids/airfoil.html Note how it turns down after the airfoil. | |
| Jan 23, 2023 at 19:32 | answer | added | Michael Lorton | timeline score: 1 | |
| Jan 23, 2023 at 17:51 | comment | added | Ivan Nepomnyashchikh | You don't understand it correctly. The shape of the wing doesn't generate "lift/thrust with the difference in pressure" (whatever that means). Difference in pressure is the result of difference in velocity across the wing. Difference in velocity is the result of air being viscous. Even if you don't have a wing, there is still difference in pressure across the air FLOW - the stagnant air doesn't flow, thus there is no pressure difference in it (neglecting hydrostatics). That all comes from the Navier-Stoke's. In order to understand all that, you have to take an advanced fluid mechanics course. | |
| Jan 23, 2023 at 11:19 | history | edited | Qmechanic♦ |
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| Jan 23, 2023 at 8:44 | comment | added | Vorbis | This section on the Wikipedia article about lift en.wikipedia.org/wiki/… explains why lift isn't generated the way you think it is. | |
| Jan 23, 2023 at 8:44 | comment | added | Maja Piechotka | Other option would be hydraulic system. You have a high pressure and low pressure areas and high pressure area pushes on piston which separates them. This continue until the high pressure area expands enough to equilize the pressure. If they are used to lift an object the piston 'levitates' in static way (without requiring external input of energy or any movement). | |
| Jan 23, 2023 at 8:17 | comment | added | Agnius Vasiliauskas | The closest thing to a "static lift" would be a balloon with a heated air raising due to buoyancy force. But you have said "no" to a balloons, so... | |
| Jan 22, 2023 at 16:40 | vote | accept | Fulano | ||
| Jan 22, 2023 at 12:00 | history | tweeted | twitter.com/StackPhysics/status/1617129974925778945 | ||
| Jan 22, 2023 at 5:52 | comment | added | Tanner Swett | For what it's worth, that picture is totally inaccurate. | |
| Jan 22, 2023 at 5:32 | history | became hot network question | |||
| Jan 21, 2023 at 23:56 | answer | added | Cort Ammon | timeline score: 10 | |
| Jan 21, 2023 at 23:13 | answer | added | Pioneer_11 | timeline score: 22 | |
| Jan 21, 2023 at 22:36 | answer | added | John Doty | timeline score: 29 | |
| Jan 21, 2023 at 21:32 | history | asked | Fulano | CC BY-SA 4.0 |