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Calculating the force air has on an object

  1. Jan 29, 2015 #1
    I am trying to calculate the force that compressed air has on a cylinder.

    These cylinders are pushed in a production line for transportation using air. I would like to calculate the force such air puts on the cylinder.

    I have the velocity of the air, the área of the cylinder.

    Will using the equation of quantity of motion (F=rho*Q*deltaV) work? I have not seem to find a difference in the speed of air in any part of the trajectory.

    Please, help, and thank you for your time.
  2. jcsd
  3. Jan 29, 2015 #2
    Exactly how does the compressed air apply a force on the cylinder? As in on which area? If I understood right, the cylinders are pushed in by the air, so the air would have to act on the base of the cylinder
  4. Jan 29, 2015 #3
    They are actually bottles, that are transpoted from one area to another through guides and propulsed by air. Air flows around the tip of these bottles and it pushes them forward. I need to calculate the force that air, which comes horizontally and pushes them horizontally too. There are guides that avoid that these bottles fall.
  5. Jan 29, 2015 #4
    Ok. I'm not too sure, but as I see it, the pressure energy of the compressed air will be converted to kinetic energy when the air comes out of the storage tank. This will lead to a fall in pressure, which will depend on the velocity. Calculate the fall in pressure an in turn, the pressure acting on the bottles. Knowing the pressure and area, you will get the force.
  6. Jan 29, 2015 #5
    I am sorry if I did not explain myself very well. I am not a native English speaker.

    The air does not come from a storage tank. It comes from a compressor which sends air through some vanes. The air flows along a guide and through that guide, cylinders travel. They are pushed by air that flows from the walls of this guide. I can determine the velocity of such air, and I have the área of the cylinder that is in touch with the air.
  7. Jan 29, 2015 #6

    jack action

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    The force you can get is the pressure times the area.

    Your compressor can produce a known maximum total pressure P0 (for example, when you are filling tank). Total means that the velocity is zero.

    If that fluid starts moving at a certain velocity v, the pressure will drop to a value P. According to Bernoulli's equation, the following relation will be true:

    P0 = P +½ρv²

    Where ρ is the density of the fluid.

    What you need to find out is that total pressure P0 and multiply this value by the area to find out the force. This will represents the effect of the static pressure (P) plus the effect of the dynamic pressure (½ρv²).

    The previous equation can be used with compressible fluid as long as the velocity is below Mach 0.3. Otherwise, the compression of the fluid has to be considered.

    More info on Wikipedia.
  8. Jan 29, 2015 #7
    Thank you, guys. You have been really helpful.
  9. Feb 6, 2015 #8


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