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Explanation about air flows in tubes

  1. Oct 28, 2012 #1
    Hi people, I'm new here, so, if there's anything wrong with my post, please tell-me =)

    So, if we have a recipient, with a cavity and a small hole, and imagine that we have a sudden expansion of the gas inside the recipient(we assume the volume increase was ΔV, and the duration of the expantion is negligible (Δt→0)). We will have an air flow through the hole. If we put our hands from a distance "d" in front of the hole, we'll feel the air flow.
    Now, imagine again the recipient in the initial condition. This time, we'll have a compression in stead of an expantion. In the compression, we have the same variation of the volume(ΔV), with the same duration(Δt→0). We put our hand in front of the hole, from the same distance("d"), but, we don't feel the same intensity of the first flow. I mean, the same mass of gas that was expelled from the recipient in the first experiment, was sucked in the second one. If both occured with the same conditions, why the first flow, leaving the recipient, is stronger than the second flow in the same distance from the hole, which get into the recipient?
    I thing it's because in the expansion, the gas hurled in only one direction, and in the suction we have air from all directions getting into the recipient by the hole... but, if it's correct, why does this occur?
    A superficial explanation is enough, or even a source where I can read about it(I've already looked for, but with no success.).

    Sorry if i wasn't clear. And, it's not a homework, but a personal question.

    Thank you for any answer!
  2. jcsd
  3. Oct 28, 2012 #2


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    Your explanation is right, but I don't know how to answer why it occurs other than to ask why not? What bothers you about it? Do you see it as asymmetric? The underlying asymmetry is that entry to the hole is driven by a pressure difference, whereas exit from the hole is driven by momentum (overcoming an opposing pressure difference).
  4. Oct 31, 2012 #3
    First of all, thanks for the answer! I'm investigating this because I'm solving one of the IYPT problems for 2013, the Helmholtz Carousel, and I think it is an important thing to think about. I'm looking for the answer following your suggestion about the symmetry, and it seems to be the right way!

    I'll keep looking! Thanks!
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