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How does dynamic pressure affect static pressure?

  1. Feb 9, 2012 #1
    I have attached another diagram, and in the diagram I show a round piston with a 1 sq ft surface area pushing water through the pipe out into to open atmosphere at 14.7 psi at a constant velocity of 75 ft/sec. There is a vacuum behind the piston, and I have calculated a 37.9 psi dynamic pressure being generated by the flow.

    My question is: Based upon the dynamic pressure generated by the flow, the atmospheric outside the pipe, and the vacuum behind the piston plate, (ignoring the viscosity of the flow and the pipe walls) what will the net acting force be on the piston plate?

    Will it simply be 2116.8 lbs, which is the pressure difference between the vacuum and the outside atmospheric pressure, or does the dynamic pressure or momentum of the flow have any affect?

    Thank you,


    Attached Files:

  2. jcsd
  3. Feb 13, 2012 #2
    Any Thoughts would be greatly appreciated, thanks!
  4. Feb 13, 2012 #3


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    The dynamic pressure will not have an effect on the net force acting on the piston. Dynamic pressure effects only occur when the velocity of a gas or fluid is changed. If the velocity is constant, then dynamic pressure is just a number relative to some frame of reference. This is ingoring issues like friction with the walls of the pipe and viscosity in the water. In the real world, the pressure at the piston would have to be slightly greater than 1 atm, and pressure would decrease slightly with distance the water travels in the pipe due to friction and viscosity.
  5. Feb 13, 2012 #4
    Does this mean that the net acting force on the piston would be equal to the 2116.8 lb atmospheric pressure difference?


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