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Effects of press on a submerged chamber w/complex sidewall

  1. Jan 16, 2015 #1
    I would like to show the details of the situation before asking the specific question so:


    The attached sketch shows a situation that is slightly after the start, since the balloon is shown with air already inside. The question that I will ask is about a situation where the balloon is completely empty to start with. Note: the pressure required to fill the balloon is not a factor.

    The chamber is circular with an accordion fold sidewall that will not deform or twist due to the water pressure against it, but still acts like an accordion and the chamber is fixed to the bottom of the tank.

    The top plate of the chamber has zero buoyancy and is attached to the bottom plate by the sidewall so that the chamber is airtight within the tank.


    The question is what volume of air would be needed to supply enough buoyancy in the balloon to create suction in the inlet pipe protruding from the bottom of the chamber to atmospheric pressure?

    Attached Files:

  2. jcsd
  3. Jan 16, 2015 #2


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    Nice drawing! :-)

    What is the context of the question? What does this mechanical setup do?
  4. Jan 16, 2015 #3


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    I don't get it either:
    1. Is the whole apparatus underwater?
    2. Where does the water come from to fill the "chamber"? By that, I mean, what is the pressure?
  5. Jan 16, 2015 #4
    It appears as though the balloon is inside a 3x15 ft. sealed box with an "accordian" bottom end. Right?

    If that is the case, adding volume to the ballon will push the bottom down due to the increased water pressure in the box.
    So, no suction. You will have discharge instead.
  6. Jan 16, 2015 #5


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    Note that the accordion top is the part that is 3 ft wide(radius?). The size of the tank is unspecified, so (I think) it can be made arbitrarily large to accommodate a balloon of a much larger diameter than the accordion.
  7. Jan 16, 2015 #6

    Oh! I see that now. So it is a bouyant force vs force due to pressure issue?

    No... Intuitively I think chamber vs. accordian size doesn't matter. If the volume of the balloon increases and the chamber is sealed the bottom must move down.

    But we don't know if it IS sealed. Do we? Or am I missing it?
  8. Jan 16, 2015 #7


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    There is a different line weight/shade at the top, consistent with an open top, but it isn't unambiguous.
  9. Jan 16, 2015 #8
    This sounds suspiciously like a homework problem. Is it? If so, it should have been posted in the homework forum, using the required template. Also, if it is a homework problem, it should have an exact problem statement that you can provide word for word. You are wasting people's time keeping them guessing about the problem statement. I expect you to provide an exact problem statement.

  10. Jan 16, 2015 #9


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    I was going to guess perpetual motion. Want to bet one "prop"?
  11. Jan 17, 2015 #10
    Hi berkeman,

    The drawing is derived from a concept that I am working on and the mechanical setup by itself has no purpose. It is just a creation that allows me to clearly ask the question.

    What I am trying to understand is how the folds in the sidewall change the amount of force required to expand the chamber. I think a discussion of the details surrounding this question would be interesting in general and will certainly help me out.

  12. Jan 17, 2015 #11
    The chamber and the balloon are 15 ft below the surface of the tank, and the question is asked when the tank is already full. The pressure at 15 ft is approximately 6.48 PSI
  13. Jan 17, 2015 #12


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    The only force you'll be getting is from the "spring constant of the bellows." Google that first.
  14. Jan 17, 2015 #13
    Sorry for the confusion montoyas7940,

    The tank diameter is not defined and the tank is open at the top. The chamber consists of a 3 ft dia top and bottom plate (crosshatched in the sketch). Are you saying that there is no amount of air (assuming the tank cant overflow) that will lift the top plate. Also assuming that the balloon will never touch the sides of the tank, again sorry for the poorly described situation.
  15. Jan 17, 2015 #14
    Thanks Bandersnatch,

    Due to the poorly described situation that I presented I need all the help I can get.
  16. Jan 17, 2015 #15
    Sorry Chestermiller,

    This has nothing to do with homework I assure you, and I apologize for my Inability to succinctly state the question but it was my intention to do so. I attempting to create a discussion about how the accordion fold sides effect the force required to lift the top plate etc. By the way I haven't seen the inside of a classroom for of 30 years.
  17. Jan 17, 2015 #16
    Is the tank also submerged? Partially, completely? How does the water level inside compare to the level outside? Equal due to the accordian - I would think.
  18. Jan 17, 2015 #17
    The bottom of the chamber is attached to the bottom of the tank and the chamber is sealed against the contents of the tank but open to atmosphere through the bottom of the tank. So are you saying that the top plate will move down? Also since the accordion fold side offer a surface to the ambient pressure it seems it would have to be considered.
  19. Jan 17, 2015 #18
    Sorry, I should have added that the tank is not submerged in my last response.
  20. Jan 17, 2015 #19
    Are you saying, assuming that the chamber and the balloon start out fully collapsed, that no amount of air pushed into the balloon will move the top plate?

    This may seem simple to you but I am having a hard time understanding it.
  21. Jan 17, 2015 #20


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    You'll be able to lift the top plate with a large enough balloon. I meant only that the sidewall will have no effect beyond the spring constant of the bellows.
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