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I Does air flowing through a tube cause a pressure change?

  1. Jun 23, 2017 #1
    I have been involved in a fairly furious debate about a fluid dynamics experiment regarding the pressure of air passing through a tube attached to a moving car. It is similar in concept to the issue of whether or not an opening in an aeroplane fuselage would suck people out, where the opening did not interfere with the airflow.

    The situation is in the accompanying diagram.

    So the controversial issue is whether or not the pressure monitor would show a pressure which varies with the velocity of the car and whether the pressure it reads would be ambient pressure.

    This is more tricky problem than appears, and many many people who ought to get the answer right and are versed in physics seem to get it wrong initially. The issue to consider is whether Bernoulli's equation applies etc etc. I feel very strongly for one position but I thought I might ask for opinions on this first, hopefully which would confirm my position.

    I have a friend who is proposing to test this in the field with a car and some plastic tubing of about 2cm in diameter. If anyone can suggest potential problems with the test ( such as the difficulty of aligning and maintaining alignment of the T piece ) I would appreciate it.
     

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    Last edited: Jun 23, 2017
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  3. Jun 23, 2017 #2

    anorlunda

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  4. Jun 23, 2017 #3
    Thank you for the welcome :)

    I am and pitot tubes are not relevant because they are blocked tubes, there is effectively no airflow through the tube as there is in this experiment.
     
    Last edited: Jun 23, 2017
  5. Jun 23, 2017 #4

    f95toli

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    Another thing worth googling would be differential pressure devices. These are e.g. used to measure the flow rate in water pipes (this is why flow rates are sometimes given in the unit of "pd" )
     
  6. Jun 23, 2017 #5

    russ_watters

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    He said "pitot static", not just pitot....
     
  7. Jun 23, 2017 #6
    Oooooooops! He did :D

    I will investigate.
     
  8. Jun 23, 2017 #7
    And a pitot static system is based around a pitot tube, which is effectively a blocked tube, which is not relevant to my diagram.
     
  9. Jun 23, 2017 #8
    Such devices are interesting, but they rely on the venturi effect, constricting the fluid flow, and that is not happening in my experiment.

    But if you think it is relevant, please suggest an effect and a conclusion as to whether a pressure difference dependent on velocity would occur.
     
  10. Jun 23, 2017 #9

    f95toli

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    Why not? If the tube is not in any way affecting the flow, what does it do?
     
  11. Jun 23, 2017 #10
    Well that is the question!

    Do you believe from the diagram it is doing something or nothing?

    If it is doing something, presumably this would manifest itself in a pressure change detectable by the pressure monitor, if it is doing nothing, then presumably the pressure monitor would not detect any change from ambient, or possibly there is another alternative.
     
  12. Jun 23, 2017 #11

    anorlunda

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    No it is not necessarily blocked. Even if there was a hole in the diaphragm allowing a through flow, there would still be a pressure difference. Airspeed is proportional to the pressure difference between the two ports.

    On your diagram, as long as there is a flow resistance in the pipe, there will be a pressure drop.

    Another analogy, a ramjet engine.



    ASI-operation-FAA.png
     
  13. Jun 23, 2017 #12
    In your diagram the tube is effectively blocked. They have to be otherwise the pitot tube would not pick up all of the dynamic pressure.

    Well I did say not to assume viscous forces in the diagram.

    The diagram you offer shows a very severely constricted tube, which is quite different to my diagram.

    I don't quite know how reasonable it is to assume a lack of viscous force, I was proposing 2cm tubing, and was hoping this would be enough to be able to discount the viscous forces.
     
  14. Jun 23, 2017 #13
    It is definitely not similar to people being sucked out of a hole in the fuselage. In a plane at cruising altitude, the atmospheric pressure outside the cabin is only a small fraction of the cabin pressure. This is not the case in the system you describe.
     
  15. Jun 23, 2017 #14
    I should have mentioned in describing my plane analogy that it was an unpressurised plane.
     
  16. Jun 23, 2017 #15
    Even so, are you aware that the atmospheric pressure at cruising altitude is less than half of that at ground level?
     
  17. Jun 23, 2017 #16
    I am, for the cruising height of a jet aeroplane, the atmospheric pressure is indeed less than half that of ground level.

    My experiment is concerned with pressure effects or lack of them due to relative air velocities. Which is also valid for aeroplane scenarios and openings in the fuselage, providing the aircraft are unpressurised.
     
  18. Jun 23, 2017 #17
    A slightly improved diagram of the scenario
     

    Attached Files:

  19. Jun 23, 2017 #18

    russ_watters

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    Not really, no: the pitot port and static port are separate pressure ports on separate tubes that just happen to be concentric. You can measure both pressures together to get the difference or you can measure them separately. The behavior of the static port is exactly what you are asking about. And the answer is, as the name implies, it reads the static pressure.

    However, while you have expressed that this is in the context of getting sucked out of an airplane, the scenario is not the same as your setup because of the influence of the fuselage.
     
  20. Jun 23, 2017 #19
    Yes it is.

    The pitot tube measures the total pressure which is a combination of dynamic pressure and static pressure. It does so using what is effectively a blocked tube ( pressure chamber in the wikipedia diagram). The static pressure is also measured in this system using a SEPARATE port. The difference between the two gives the DYNAMIC pressure which is related to windspeed, then this is used to give indicated air speed.

    (A pitot-static TUBE ( not system) which is another alternative incorporates the static intake from another SEPARATE ISOLATED part of the structure which holds both the static and the pitot elements. )

    The scenario I have described is exactly the same as having an opening on the side of the fuselage ( where it is straight and away from wings etc which accelerate the air) of an unpressurised aeroplane. Where the cross section of the fuselage does not change with distance along the fuselage, the air is not accelerated and so the fuselage should not affect airflow or pressure. Where the fuselage does change cross section, e.g. near the front, near the wings, and near the end then airflow will be affected. My contention is that where the cross section of the fuselage is constant with distance along the length of the fuselage the same effect will be had on airflow as in the tube in the instrument in my experiment. Similarly a train where it has a constant cross section with distance along the train, will not cause airflow changes, except due to viscosity issues and boundary layer issues, which I have tried to ignore in this experiment.
     
    Last edited: Jun 23, 2017
  21. Jun 23, 2017 #20
    This is a pretty straightforward problem to model (analytically). Do you have any idea how to do it? Are you aware that the problem you have described is the same as the car and T being stationary, and the wind (in the far field) blowing past it at constant far field velocity v?
     
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