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Opposing force of pushing air through a duct vs outside it?

  1. Oct 16, 2015 #1
    I'm an average joe here, trying to figure something out. Lets say I have a tube and a disk that travels from top to bottom pushing air kinda like a pump. Both tube ends are open so as air is pushed out of the bottom end, more air flows in through the top.
    Here is my question: Does pushing air through a tube produce more opposing force on that disk, than it would if that disk was doing the same motion outside the tube? If so, what could be a good estimate percentage wise?
    Also, would a couple of millimeters gap between the cylinder and the tube wall, negate any gains in performance?
  2. jcsd
  3. Oct 17, 2015 #2

    Andrew Mason

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    The net force on the disk is the difference in pressure from one side to the other multiplied by the area of the disk. The ambient pressure is atmospheric pressure - about 14.7 pounds force/square inch or 10,000 kgf/m2 (100,000 N/m2 or Pascals). If you cover the top of the tube and pump (or blow) air into it and if there is a seal between the disk and the inside of the tube, the added air has no where to go so pressure increases significantly above the disk. The pressure on the bottom remains the ambient pressure. So you end up with a significant pressure difference and net downward force on the disk. But if you don't confine the air, it is not so easy to create a significant pressure difference.

    Last edited: Oct 20, 2015
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