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Can you 'increase pressure' by restricting fluid flow?

  1. Sep 13, 2013 #1
    I just read this, from a site dedicated to popularizing science:

    " ...think of air as being like water. If you have a garden hose and put your thumb over the end, the pressure increases and water sprays out. Same thing with air. Except the increased pressure is under a plane's wings and keeps it up."

    Surely this has it all backwards? Bernoulli's principle, etc...
     
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  3. Sep 13, 2013 #2

    jfizzix

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    Well, it would be only a relative increase of pressure. I expect the pressure under the wing remains the same while the pressure over the wing is reduced because it is moving faster (Bernoulli's principle). This gives the net upward force of lift.
     
  4. Sep 13, 2013 #3

    boneh3ad

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    Yeah that article clearly doesn't have any idea what it is talking about. The phenomena leading to the generation of lift is largely different than anything you can demonstrate with a garden hose other than simply the relationship of pressure and velocity, which the article also apparently gets wrong. Up against your thumb there is certainly a pressure increase since the flow is stagnating against your thumb. The pressure in the moving stream of water decreases as it accelerates around your thumb, however, so I don't really know what the author here is talking about. Further, the pressure under an airfoil that holds it up is static pressure, not stagnation pressure, so it is an entirely different phenomenon from what you feel on your thumb in the example.

    The pressure under a wing absolutely does not remain the same as ambient. It is often slightly higher than ambient pressure, but can under circumstances be lower than ambient pressure or roughly the same. There is nothing saying it has to be the same. The pressure over the upper surface is substantially less than ambient, however.
     
  5. Sep 14, 2013 #4

    rcgldr

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    First in the case the garden hose, the source of the pressure in the hose comes from the water company that pumps water into your household. The decrease in pressure (called head loss) over a length of hose is related to the speed^2 of the flow. Restricting the flow reduces the velocity of the flow and reduces the decrease in pressure. Wiki articles:

    http://en.wikipedia.org/wiki/Hydraulic_head#Head_loss

    http://en.wikipedia.org/wiki/Darcy–Weisbach_equation

    This is not how wings function. A wing generates lift by diverting the relative (to the wing) air flow downwards. Some common laws of physics are involved. Netwon's third law pair of forces: the wing exerts a downwards force onto the air, coexistant with the air exerting an equal in magnitude and upwards force on the wing. Newton's second law, force equals mass time acceleration. In level flight, the downwards force exerted by the wing onto the air results in downwards acceleration of the air. Bernoulli principle, air accelerates from higher pressure areas towards lower pressure areas, and during the transition, if no external (to the air) forces are involved, then Bernoulli equation relates the increase in velocity to the decrease in pressure as the air accelerates from a higher pressure area to a lower pressure area. Bernoulli doesn't explain how those higher and lower pressure areas are maintained, but those require external (to the air) forces.

    A conventional sub-sonic wing reduces pressure above a wing more than it increases pressure below, but using the air as a frame of reference, a wing increases the total mechanical energy of the air, which violates Bernoulli. Generally at the trailing edge of a wing, where the flows above and below a wing merge, there's a net increase in pressure that translates into increased downwards acceleration of the air until it's pressure returns to ambient. How much of an increase in energy of the air versus the amount of lift generated is related to the efficiency of a wing.
     
  6. Sep 14, 2013 #5
    The example is awful but I'd also suggest people avoid using Bernoulli's principle in what is clearly a viscous application of a pipe flow driven by a pressure gradient. Putting your thumb over the end will increase the pressure just upstream of the obstruction, and then lead to a steeper pressure gradient through the pinch point. Flows in pipes have substantial viscous effects, and you cannot apply the standard Bernoulli "constant upstream velocity" if you're going to put your thumb over the end.
     
  7. Sep 14, 2013 #6

    boneh3ad

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    Sure you can... carefully. I agree with you overall, particularly in the fact that putting your thumb over part of the end will increase the pressure and it is not in violation of Bernoulli's principle, and I agree that Bernoulli's principle doesn't really apply over the length of the hose since it is a highly viscous situation. That said, the reason the pressure is going to increase in the vicinity of your thumb is not related to viscosity, but stagnation flow, and very locally along a streamline the viscous losses are going to be negligible, so Bernoulli's equation would give a very good approximation along that streamline and is still reasonably useful, especially conceptually.
     
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