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Why does water spin in funnel?

  1. Jan 12, 2008 #1
    As water is pulled into an opening by gravity, it begins to spin. Why does it spin?
     
  2. jcsd
  3. Jan 12, 2008 #2

    DaveC426913

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    Because angular momentum from the initial state of the water is preserved. It's the same thing that a skater uses to start in an open, slow spin and pull their arms in to go into a closed tight spin.
     
  4. Jan 13, 2008 #3
    Thanks DaveC426913,

    Actually, there are two regimes of spinning with two different speeds of spinning: immediately after opening a hole in the bath and about a minute later.

    Immediately after opening the hole, conservation of angular momentum already works and one may see very slow spinning far from hole and faster spinning close to hole.

    A minute later, the spinning becomes many times faster than in the very beginning. So, what is the reason of the fast spinning? Or what is the reason of increasing of the speed of spinning a minute later?
     
  5. Jan 13, 2008 #4
    Angular momentum for a spinning object is mass times velocity times radius. As momentum is preserved and as the radius decreases (because of the water going out), the velocity must increase.
     
  6. Jan 13, 2008 #5
    That's corect, but that was an answer to a different question.

    Consider the numerical exapmle.
    R = 30 cm, r = 3 cm.

    Immediately after opening the hole in the bath, we have:
    v(R) = 1 cm/sec, v(r) = 10 cm/sec

    A minute later, BOTH of the speeds, the speed far from the funnel and the speed close to funnel becomes much larger,
    for example
    v(R) = 12 cm/sec, v(r) = 120 cm/sec

    The question is:
    Why a minute later the speed at the distance 30 cm from funnel increased from 1 cm/sec to 12 cm/sec? Why a minute later the speed at the distance 3 cm from funnel increased from 10 cm/sec to 120 cm/sec?
     
  7. Jan 13, 2008 #6
    i probably think that in this case what is decreased is the fluid mass, so it start to spinn faster... L=mvr. thats it.
     
  8. Jan 13, 2008 #7

    Dale

    Staff: Mentor

    [nitpick]In the case of water down a drain angular momentum is not precisely conserved. The tub (and maybe gravity?) does exert torque on the water.[/nitpick]Accounting for that amount of torque the rest of what has been said about angular momentum is correct.

    In addition to the conservation of angular momentum there is also conservation of energy. As the water moves down into the drain there is some loss of PE. By conservation of energy you can also get an overall increase in KE in the tub depending on the KE of the water going down the drain. [nitpick]Of course, accounting for energy lost to viscous heating etc.[/nitpick]
     
    Last edited: Jan 13, 2008
  9. Jan 13, 2008 #8
    There is a large bath, about 100 gallons of water and a small hole, about 1 inch diameter. A minute later there is still about 95 galons of water. Decrease of the mass of water is only 5%, but increase of rotation speed of the whole funnel is about 1200%.
     
  10. Jan 13, 2008 #9

    DaveC426913

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    I feel like I've been hustled. :rolleyes: Your OP belied the depth of your knowledge on the subject.
     
  11. Jan 13, 2008 #10

    DaveC426913

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    But I don't think the whole volume participates at that point. Due to inertia and friction I imagine you can consider the dynamics of a smaller volume of only a few gallons surrounding the drain.
     
  12. Jan 13, 2008 #11
    I am not sure about gravity, but the tub, actually bottom of it near the hole, exert friction. So it should reduce the angular momentum. But actually, the angular momentum increases a minute after beginning of the process.

    Yes. There are two mechanisms of the speed increase as the water approaching the hole. The first one is that water goes closer to vertical ax, momentum conservation and so on... The second one is that water goes to a lower level, PE => KE and so on... But the question was not about speed increase as the water approaching the hole, but about increase of the speed of the funnel as whole a minute after beginning the process.
     
    Last edited: Jan 13, 2008
  13. Jan 13, 2008 #12
    I am not satisfied with my knowledge of the subject... what I actually want is to find any effective measures against tornadoes and tropical storms that are too annoying in my lovely Florida. But in order to find something, I need deep understanding of rotation phenomena. :rolleyes:

    So, I am not satisfied with the hurricanes in Florida and not satisfied with the present knowledge of the subject... :cry:
     
  14. Jan 13, 2008 #13

    Dale

    Staff: Mentor

    You cannot consider part of the water in isolation to the rest. Viscous forces "connect" the water approaching the hole to the rest of the water in the funnel. The viscous forces are small, but not negligible. That is why, as you observed, it takes a rather large amount of time.
     
    Last edited: Jan 13, 2008
  15. Jan 13, 2008 #14
    Consider only FIVE gallons of water surrounding the drain.
    At t = 0 (or t = 10 sec), the funnel spins slowly.

    At t = 1 min, the first five gallons are gone. There are another five gallons of water surrounding the drain. The funnel spins quickly. Why behavior of the next five gallons, which forms quickly spinning funnel is different from behavior of the first five gallons, which formed slowly spinning funnel?
     
  16. Jan 13, 2008 #15
    That is exactly what I was thinking about, but I needed an independent opinion... thanks :smile:
     
  17. Jan 13, 2008 #16

    Dale

    Staff: Mentor

    The five gallons of water surrounding the drain is a very poor system to choose. It is not an isolated system and the boundaries and interactions are very difficult to define. You are much better off considering all of the water in the tub. That makes the boundaries much easier to define as well as the interactions with the surroundings.
    Different initial conditions.
     
  18. Jan 13, 2008 #17
    You are absolutely right!

    And different boundary conditions!
     
  19. Jan 13, 2008 #18
    i might be wrong but i think as the rotation progresses, the viscous resistance decreases, so as letting the velocity increases. just might be

    but one thing that kicks me is the fact, it rotates. why does it rotate at all?? i have a large tank, full of water, i punch a hole in it, water, a lil after, drops below forming a vortex. why does it happen?? i asked this question, all through my course, but didnt get any answer OR i am ultra stupid;))
     
  20. Jan 13, 2008 #19
    What exactly do you mean? The coefficient of viscosity decreases or the viscous resistance as a global phenomenon decreases at constant coefficient of viscosity?

    I believe that answer to the question why does it rotate at all?, is the same as the answer to the question "why it rotates faster and faster as the rotation progresses?".

    So, there is a mechanism exists that accelerates spinning the funnel as whole. In such a situation an initial fluctuations of angular momentum are enough to develop global spinning until nonlinearity restricts it at some reasonable level.
     
    Last edited: Jan 13, 2008
  21. Jan 13, 2008 #20

    Dale

    Staff: Mentor

    This question has got me thinking. The Wikipedia page (http://en.wikipedia.org/wiki/Vortex) mentions that for a free vortex "The tangential velocity v varies inversely as the distance r from the center of rotation, so the angular momentum, rv, is constant". I believe that this is constant as a function of r, not as a function of t.

    As you indicate the whole thing can start spinning faster, so something must be exerting a net torque on the fluid in the same direction as the angular momentum. The viscous shear forces should exert a net torque in the opposite direction, the normal forces in a symmetric vessel should not exert a net torque, and I can't see how gravity would exert a torque about a vertical axis.

    Where's the torque?

    EDIT: I cannot reproduce the "points far away start spinning faster" thing in my sink even though anecdotally I think I have seen such occurences. The drain plug may be interfering. The situation you described above, was that just hypothetical, or have you done such an experiment?
     
    Last edited: Jan 13, 2008
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