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Will this aerofoil generate lift, Bernoulli

  1. Jan 9, 2013 #1
    I essentially want to understand weather or not an aerofoil with a zero attack angle will generate lift, i.e. using only Bernoulli.

    I have attached an image to illustrate my question, will the orange block lift if airflow is passed horizontally over the top of it?

    Many thanks
     

    Attached Files:

  2. jcsd
  3. Jan 9, 2013 #2
    I say, Yes, you can demonstrate this with a sheet of paper.
    Roll the edge in your fingers and blow only on the top of the paper.
    The paper will rise up.
     
  4. Jan 9, 2013 #3
    That seems like a slightly different case as the speeding up of the air in that case is done by your lips and so is not reacting against the front of the aerofoil. I could be wrong but I think this is a different problem?
     
  5. Jan 9, 2013 #4

    Low-Q

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    Yes. The airfoil will generate lift. There is less pressure above the airfoil than under it. Similar cases are demonstrated with balloons in front of a small fan. If the ballon is hit by the air flow just on one side, it will be sucked into the airflow and stay there.

    Vidar
     
  6. Jan 9, 2013 #5

    rcgldr

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    That's mostly due to coanda effect, the air tries to follow the curved shape of the paper, and it's mostly the acceleration of air perpendicular to the surface of the paper that results in lift. The component of acceleration perpendicular to the flow doesn't directly change the speed of the air, but does reduce pressure of the air due to "negative" work being performed the paper, which violates Bernoulli (work is done). The surrounding air at ambient pressure will be accelerated towards the low pressure area in compliance with Bernoulli.

    I'm not sure about the case in the diagram. Normally airfoils generate lift by accelerating air downwards (diversion of relative flow), or by creation of a pressure differential if in ground effect (air "strectched" or "squeezed" from a surface above or below an airfoil). If the free stream is diverted downwards while over the airfoil (to be later diverted back upwards by the block aft of the airfoil), then lift can be generated.
     
  7. Jan 9, 2013 #6

    CWatters

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    I believe so, however I suspect there will also be vertical forces on the white blocks so it might be the right answer to the wrong question.
     
  8. Jan 9, 2013 #7

    cjl

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    The orange block will generate lift, yes, although there will also be a net downward force on the white blocks.
     
  9. Jan 10, 2013 #8

    boneh3ad

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    I disagree that it would generate lift by default. For one, the pressure on the bottom face is on a tiny area, so even if the pressure there is higher, it would be extraordinarily high. That would mean if the air was fast enough then you could lift the orange portion. I would agree that there would also be a downward force on the white block.
     
    Last edited: Jan 10, 2013
  10. Jan 10, 2013 #9

    sophiecentaur

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    Newton's third law always applies, whether it's that block or the wing of an aircraft. If there's a force 'lifting' the aircraft, there must also be a force pushing the air down. This point seems to be missed in many 'explanations' of how an aircraft stays up. Any lifting force on the orange block must be compensated for by an increase in pressure on the white area before and / or after it.
     
  11. Jan 10, 2013 #10

    boneh3ad

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    Which several posters have already mentioned by noting a downward force on the white block.
     
  12. Jan 11, 2013 #11

    sophiecentaur

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    Interesting comment but I can't find the word "Newton" anywhere on this thread except when I have supplied it. The guy's name is surely worth introducing for people who may not have made the connection with the basics of Physics.
     
  13. Jan 11, 2013 #12

    CWatters

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    The problem is it normally leads people to ask (and argue over) is it Newton or Bernoulli that makes the lift. It's a false dichotomy.
     
  14. Jan 11, 2013 #13
    Bernoulli can be used to calculate the lift force but it cannot be used to explain the lift. Bernoulli is only a statement of conservation of energy. Using Bernoulli as an explanation essentially creates a circular argument.

    The flow speeds up so the pressure decreases.
    Why does the flow speed up?
    Well, it speeds up because the pressure drops.

    Rcgld was mostly right. The object (airfoil) forces the flow to curve which results in a centrifugal force on the fluid elements. The fluid has inertia and wants to continue moving in a straight line. This centrifugal force is balanced by a pressure gradient pointing towards the center of curvature. So the pressure increases as you move away from the surface. This is the reason for the lower pressure on the surface of the airfoil, and the cause of the flow accelerating.

    So the airfoil in the drawing will experience a force, but as boneh3ad pointed out it may not be enough to actually lift the airfoil but that all depends on the weight of the airfoil and the pressure difference.
     
  15. Jan 11, 2013 #14
    I disagree that there will be a force on the white blocks in the drawings. The force on the orange airfoil is due to the momentum transferred to it by air molecules. When the pressure drops on the upper surface there are less air molecules per area contacting the surface resulting in less momentum transfer. On the lower surface where the pressure is higher there are more molecules per area contacting the surface. This results in the lifting force. The only thing that would happen for the white blocks is that with no airflow they would be supporting the weight of the airfoil and then with flow they would be supporting less weight due to the force imparted to the airfoil by the air.
     
  16. Jan 11, 2013 #15

    boneh3ad

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    The problem with your reasoning, RandomGuy88, is that in order for an upward force to be generated on the orange portion of the diagram, there would have to be a net downward airflow generated by the object to satisfy a force balance in the absence of the white support. If the shape drawn was essentially a symmetric airfoil missing its bottom half, that would probably generate some lift since it is essentially a cambered airfoil now.

    The case here is akin to setting said half-airfoil down on a surface. The shape would still tend to want to create a downwash, but it wouldn't because there is a surface blocking it. The surface has to turn that downward angled flow upward back to parallel, therefore there is a downward force on the surface.

    At any rate, as drawn, assuming that the orange part isn't floating and that there is no gap between the orange piece and the walls of the hole it is in, whether or not there is any lift on it would be determined by how large that hole in the bottom is and therefore how large an area the pressure at the hole acts on (I am assuming it is atmospheric pressure). No matter how low the air above gets in pressure, it still has to push with a harder total force on its surface than does the higher pressure below. There would be some critical diameter of the hole that allows this.

    Assuming instead that air can enter the cracks between the orange and the white and therefore act on those surfaces, then I suppose a lift force would have to be generated provided that there is a downward force on the white block.
     
  17. Jan 11, 2013 #16

    AlephZero

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    Let's try another of Newton's Laws. Force = rate of change of momentum.

    Assuming the incoming air velocity is horizontal, to get a lift force the exit air velocity must have a downward component. I can't see where that is coming from.

    You will certainly get some local forces (a.k.a. pressure changes) on different parts of the red curved surface, and some of them will give an local upwards force, but that doesm't mean there will be a global lift force on the whole red block.
     
  18. Jan 11, 2013 #17

    boneh3ad

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    You are overlooking the fact that there needn't be a downward component provided that the lift force is balanced by something else. In this case, the downward velocity that would result is turned further by the white block, meaning you would, in that case, have a lift force on the orange piece which is balanced by an opposite force on the white piece as opposed to a direction change in the flow.
     
  19. Jan 13, 2013 #18

    CWatters

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    That's one of the most easily undertood explanations I've seen. Thanks.
     
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