-3

I think it will generate lift, because downwash air flow still exists. But I'm not sure if I'm right.

enbin
  • 1,972
  • This isn't exactly a duplicate question, but the answer of mine on there will answer this as well: https://physics.stackexchange.com/questions/46131/does-a-wing-in-a-potential-flow-have-lift/46134#46134 – tpg2114 Nov 17 '19 at 22:43
  • @tpg2114 What is potential flow? – enbin Nov 17 '19 at 22:46
  • Potential flow is an approximation where you assume the flow is inviscid and irrotational -- which is the same situation basically as what you are proposing here (perfectly smooth wing that creates no friction). – tpg2114 Nov 17 '19 at 22:49
  • @tpg2114 What does potential mean? Gravity? – enbin Nov 17 '19 at 22:53
  • 1
    If you assume the flow is inviscid and irrotational, you can introduce a scalar potential field that fully describes the velocity vector field. If you want to learn more about potential flows, search around a bit and then ask specific questions if needed. There's entire textbooks dedicated to it, so it's not something I can answer in comments. – tpg2114 Nov 17 '19 at 22:54
  • @tpg2114The fact that the wing has no friction does not deny that the fluid is viscous. – enbin Nov 17 '19 at 23:02
  • @tpg2114 Although the wing has no friction, but the wing has angle of attack, so there is downwash air flow, so there will be lift. – enbin Nov 17 '19 at 23:05
  • @tpg2114 The leading edge of the wing makes the air flow move upward, while the wing is inclined downward, so the air flow tends to move away from the wing in the normal direction, so the top of the wing produces low pressure. Right? – enbin Nov 23 '19 at 12:49

1 Answers1

0

The wing surface itself does not generate friction (though surface roughness does matter). It's the viscosity of the fluid that generates friction. Even a perfectly smooth boundary will experience friction due to no-slip condition. Therefore, you can't have a viscous fluid and a "zero friction surface" simultaneously.

If you assume flow is inviscid, on the other hand, then lift cannot be generated by itself, since an irrotational flow remains irrotational for all time. If you assume flow tangency condition, you will get a unique non-lifting flow solution.

We are able to derive lifting solution in potential flow because we impose an extra Kutta condition at the trailing edge. But the Kutta condition is completely external to potential flow. The actual physical process that results in the Kutta condition is viscosity itself.

JZYL
  • 137
  • Without viscosity, there will be low pressure on the top of the wing, right? In a container, the air pressure will decrease with the increase of the volume of the container, which has nothing to do with the viscosity of the air, right? – enbin Nov 23 '19 at 03:06
  • @enbin A fixed volume isn't a flow. It has nothing to do with a wing. Google non-lifting cylinder flow. While the upper surface will have low pressure, the bottom surface will have equal low pressure; no net lift. – JZYL Nov 23 '19 at 03:20
  • The leading edge of the wing makes the air flow move upward, while the wing is inclined downward, so the air flow tends to move away from the wing in the normal direction, so the top of the wing produces low pressure. Right? – enbin Nov 23 '19 at 12:48