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Homework Help: Fluid Dynamics/Air flow

  1. Apr 20, 2004 #1
    We are studying fluid dynamics in class right now...

    Here is a problem that came up on my homework that im not sure how to solve:

    When a person inhales, air moves down the bronchus(windpipe) at 10.4 cm/s. The average flow speed of the air doubles through a constriction in the bronchus. Use (density of air) = 1.29 kg/m^3. Assuming an incompressible flow, determine the pressure drop in the constriction. Answer in units of Pa.
    When i first looked at the problem i took "The average flow speed of the air doubles through a constriction" to mean that it goes from 10.4 cm/s to 20.8 cm/s.

    Then i just plugged the values into Bernoulli's equation and then plugged the following into my calculator:

    x + (1/2)(1.29)(.104) = (x-y) + (1/2)(1.29)(.208).... only solving for y, which would be the pressure drop. i got .020929 Pa as my answer, and this is not correct.

    So, unless i made some mistake in the above equations, i assume that "The average flow speed of the air doubles through a constriction" means something else... but im not sure where to start.

    Any help is appreciated.
  2. jcsd
  3. Apr 21, 2004 #2


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    Aside from too much precision in your answer, it looks right to me (you missed the squares typing it in, but you didn't in your calculations).

    How do you know it's not correct? If it's entered into a computer, it may not like the 5 sig figs, or it may be looking for the change in pressure, and not the pressure drop (switch negative sign). It's possible I'm missing something too, but I don't think I am...
    Last edited: Apr 21, 2004
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