4 ms·

"the explanation of lift generation is both inseparable from and dependent on vortex formation" Are you sure about this? There are of course high lift devices

by vlehto 8y ago

"the explanation of lift generation is both inseparable from and dependent on vortex formation" Are you sure about this? There are of course high lift devices and wingtip vortices are unavoidable. But I'm pretty sure you can (at least theoretically) generate lift with an airfoil in a wind tunnel while having completely laminar flow.

mannykannot 8y ago
I am referring to the Kutta–Joukowski theorem [1]. I don't pretend to understand it in detail, but here it says "the fluid flow in the presence of the airfoil can be considered to be the superposition of a translational flow and a rotating flow." Your reference to a wind tunnel might have something to do with extending an airfoil from one side to the other of a wind tunnel, thereby approximating a foil of infinite span, but I don't think it is relevant to free flight. The intensity of wingtip vortices can be reduced by making the span longer, all else being equal, but then the volume of air involved in the circulation increases. [1] https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem (I have seen another version that I think offers a more intuitive, though less rigorous, presentation, and if I can find it I will add a link.)
- icegreentea2 8y ago
  The rotating flow / vortex in that sentence is probably referring to the 'circulation' of the air flow, which has some mapping to the lay idea of vorticity, but isn't fully it. It's basically a measurement of deflection through a closed boundary.
- vlehto 8y ago
  The wingtip vortices are of course part of the flight, but they are unavoidable side effect. I think I've seen the Kutta-Joukowski visualization too somewhere. If I recall correctly, it was more about visualization/memorization of pressure distribution than actual airflow.
  - mannykannot 8y ago
    Due to the superposition of the translational flow on the circulation, the airflow does not wrap around the wing, but the circulation is nonetheless there: http://www.onemetre.net/Design/Downwash/Circul/Circul.htm http://www.onemetre.net/Design/Downwash/Circul/Circul.htm http://www.onemetre.net/Design/Downwash/LiftLine/Liftline.htm http://www.onemetre.net/Design/Downwash/LiftLine/Liftline.ht... "Circulation is conserved, like energy or momentum. The circulation around a wing cannot simply appear by itself. A circulation vortex must be a closed system, and the two ends of the circulation must meet up. For some time, this seemed just a theoretical issue, but careful investigation demonstrated how this happens. As the wing moves forward, it sheds a "starting" vortex, equal to but opposite in circulation to the wing vortex, called the "bound" vortex, bound as it is to the wing. As the wing moves forward, tip vortices develop, with the result that a "closed" vortex loop develops. If we think about an aircraft taking off, it leaves its starting vortex at the beginning of the runway, and the tip vortices then build up down the length of the runway and into the air as the aircraft climbs. For a large aircraft, these vortices take several minutes to dissipate. See also my other reply to your earlier post, and its bathtub experiment.
mannykannot 8y ago
Here's the alternative presentation that I was thinking of. While it is ultimately about boat sails, note that its author was, at the time, a Research Supervisor at Boeing's Aerodynamics Research Department. Take a look at figures 14 through 17, and their description. http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail%20Theory.pdf http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail...

by vlehto 8y ago

mannykannot 8y ago

I am referring to the Kutta–Joukowski theorem [1]. I don't pretend to understand it in detail, but here it says "the fluid flow in the presence of the airfoil can be considered to be the superposition of a translational flow and a rotating flow." Your reference to a wind tunnel might have something to do with extending an airfoil from one side to the other of a wind tunnel, thereby approximating a foil of infinite span, but I don't think it is relevant to free flight. The intensity of wingtip vortices can be reduced by making the span longer, all else being equal, but then the volume of air involved in the circulation increases. [1] https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem (I have seen another version that I think offers a more intuitive, though less rigorous, presentation, and if I can find it I will add a link.)

icegreentea2 8y ago
The rotating flow / vortex in that sentence is probably referring to the 'circulation' of the air flow, which has some mapping to the lay idea of vorticity, but isn't fully it. It's basically a measurement of deflection through a closed boundary.
vlehto 8y ago
The wingtip vortices are of course part of the flight, but they are unavoidable side effect. I think I've seen the Kutta-Joukowski visualization too somewhere. If I recall correctly, it was more about visualization/memorization of pressure distribution than actual airflow.
- mannykannot 8y ago
  Due to the superposition of the translational flow on the circulation, the airflow does not wrap around the wing, but the circulation is nonetheless there: http://www.onemetre.net/Design/Downwash/Circul/Circul.htm http://www.onemetre.net/Design/Downwash/Circul/Circul.htm http://www.onemetre.net/Design/Downwash/LiftLine/Liftline.htm http://www.onemetre.net/Design/Downwash/LiftLine/Liftline.ht... "Circulation is conserved, like energy or momentum. The circulation around a wing cannot simply appear by itself. A circulation vortex must be a closed system, and the two ends of the circulation must meet up. For some time, this seemed just a theoretical issue, but careful investigation demonstrated how this happens. As the wing moves forward, it sheds a "starting" vortex, equal to but opposite in circulation to the wing vortex, called the "bound" vortex, bound as it is to the wing. As the wing moves forward, tip vortices develop, with the result that a "closed" vortex loop develops. If we think about an aircraft taking off, it leaves its starting vortex at the beginning of the runway, and the tip vortices then build up down the length of the runway and into the air as the aircraft climbs. For a large aircraft, these vortices take several minutes to dissipate. See also my other reply to your earlier post, and its bathtub experiment.

mannykannot 8y ago

Here's the alternative presentation that I was thinking of. While it is ultimately about boat sails, note that its author was, at the time, a Research Supervisor at Boeing's Aerodynamics Research Department. Take a look at figures 14 through 17, and their description. http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail%20Theory.pdf http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail...