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Why do smaller holes shoot higher jets of water?

  1. Oct 23, 2008 #1
    So I know that F = P*A

    When you put a nozzle on a hose the area is smaller so does the pressure increase and the force stay the same, does the pressure stay the same and the force increase, or do they both change?

    Also if you poke a hole in the bottom of a bucket full of water will the velocity of the water out of the hole (i.e. how far it goes) be related to the size of the hole? Would water shoot out further from a smaller hole than a larger hole?

    Thanks
     
  2. jcsd
  3. Oct 23, 2008 #2

    rcgldr

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    The pressure never increases. A hole reduces the resistance to flow, which reduces the pressure. A smaller hole reduces the pressure less than a larger hole.

    Pressure would be a function of how high the water is above the hole. The higher the water, the higher the pressure at the hole. Because of viscosity, if the hole is too small the stream will be restricted. As the hole gets larger the stream would approach some terminal velocity based on the pressure difference between water at the hole and the air pressure outside.
     
  4. Oct 23, 2008 #3

    russ_watters

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    I'm not sure I like that explanation...

    This issue is all about eliminating dynamic losses in the pipes. When the garden hose is open, the pressure at the outlet is near zero, while the pressure at the main can be up to around 50 psi. The flow comes into equilibrium when the pressure gradient at a particular flow rate "uses up" all of the available 50 psi. By putting your finger over the end of the hose, you reduce the flow rate and effectively eliminate the dynamic losses, making the entire 50 psi available at the end of the hose.

    Then you can take that pressure and use Bernoulli's equations to figure out the maximum velocity and height of the stream.
     
  5. Oct 23, 2008 #4
    I'm a complete and total layman, but... I think I know what you're talking about. It has to do with how much resistance there is- the higher resist. of a small hole reduces pressure less, but lets less actual water out (normally), it just goes farther.
     
  6. Oct 23, 2008 #5

    rcgldr

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    That's also because it's accelerating towards an ambient pressure environment beyond the hole. The length and diameter of the hose and flow rate are factors in the amount of dynamic pressure loss. I probably should have explained that better before.

    Not quite, if there's any flow at all, then there are some dynamic losses in the hose.
     
  7. Oct 23, 2008 #6
    If the flux is constant, then the water will have to move faster through a smaller hole in order to achieve the same flux.
     
  8. Oct 23, 2008 #7

    russ_watters

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    By "effectively eliminated", I mean that since the losses are a square function of flow rate, if you cut down the flow rate enough, you reduce the losses enough to ignore them. Ie, if you have 50 psi available and you cut the flow by a factor of 4, you reduce the losses by a factor of 16, giving you almost 47 psi. Cut the flow rate by 90% and you get 49.5 psi available.
     
  9. Oct 23, 2008 #8

    russ_watters

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    The flux isn't constant.
     
  10. Oct 23, 2008 #9
    What type of system are we talking about? A tank draining through a hose?
     
  11. Oct 23, 2008 #10

    russ_watters

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    Whether it is a tank draining through the hose or a household water system with the water coming from a main in the street, the flow will be a function of pressure, area, and loss. In order to make the flux (mass flow rate) constant with a change in area, you'd need to have a way to vary (and regulate) the pressure of the system providing the water.
     
  12. Oct 24, 2008 #11
    Wouldn't a tank draining through a hose be governed by Torricelli (as a first approximation)? He says that the velocity of the jet is proportional to the square root of the height of the water above the jet. Changing the size of the aperture should not affect the velocity of the jet.
     
  13. Oct 24, 2008 #12

    russ_watters

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    Well you said "flux", which is mass flow rate, not velocity, but in any case, that approximation is true only when there is no loss in the system. Here, the loss is the key to why the velocity changes with the orifice size.
     
  14. Oct 25, 2008 #13
    By `loss' I think you mean an ideal fluid. What do Euler's equations of motion say about this phenomenon? I believe Bernoulli just gives the same as Torricelli.
     
  15. Oct 25, 2008 #14

    russ_watters

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    I don't know what you mean by "ideal" fluid, but perhaps you are talking about viscocity. Euler's equations deal with inviscid flow (and yes, they do give the same as Bernoulli's* when applied to the whole system). Navier-Stokes deals with viscous flow. Thinking about it a little more, the friction loss and viscocity loss are probably the same thing: with water in a pipe, the velocity at the pipe wall is zero, so the loss is modeled strictly as a viscous friction issue.

    *In fluid dynamics, it is critical to know what assumptions you can use and how those assumptions determine what equations you can use. When the garden hose is just open, you cannot simply apply Bernoulli's equation to the water pressure at the main to calculate how fast the water will be moving at the outlet of the hose. Bernoulli's equation requires inviscid flow and conservation of energy and you don't have them in that case. My point in post #7 was that if you can reduce the velocity through the pipe enough to all but eliminate the losses, then Bernoulli's equation can be applied, treating the piping system as a pressure vessel with a single orifice outlet and nothing between them to eat up your pressure.
     
    Last edited: Oct 25, 2008
  16. Oct 25, 2008 #15
    I agree that a transparent list of assumptions is critical to any study of fluid dynamics. What assumptions can we make in this case?

    Certainly we have incompressibility. Also, the fluid is baratropic, meaning it is under the influence of a conservative potential (gravity). Can we assume an invisid fluid? If so then we could also assume irrotational motion. If not then I think we are stuck with Navier-Stokes.

    I think I am still unclear what type of system we are trying to model.
     
  17. Oct 25, 2008 #16
    Russ is correct. You can see the effect by filling a drum (say 55 gal) after putting a small nozzle in the bottom. The water "spurt" will no longer depend on whether you put your finger over the nozzle, almost closing it.
     
  18. Oct 25, 2008 #17

    russ_watters

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    We certainly cannot assume inviscid flow.
    When someone says "garden hose" it almost certainly means a real-world garden hose: one connected to a domestic water piping system.
     
  19. Oct 26, 2008 #18
    Do domestic water piping systems drain under gravity, or is pressure added to the system?
     
  20. Oct 26, 2008 #19

    russ_watters

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    Most (but not all) domestic water systems are pumped and regulated to maintain a constant pressure at the street. This actually makes them act pretty similar to gravity systems.
     
  21. Oct 26, 2008 #20
    I think I am beginning to understand the phenomenon at work here. In response to the OP's questions, I think it depends (as always).

    If you are draining a large enough tank through small enough holes, then I do not think the velocity will be appreciably affected by the size of the holes, only on the depth of the hole beneath the surface. However, as the tank and hole size become similar sizes, the situation gets much more complicated and difficult to predict.

    I am still unclear on how domestic water supplies work. Wikipedia's article on `globe valves' offered some insight. As stated before, is seems that a fairly constant pressure is provided to homes. To turn the water on in your kitchen, you must open a valve that causes the water to follow some crazy path that I think reduces the kinetic energy of the fluid from frictional losses. The valve's geometry is such that when the valve is more open, there is less frictional loss, and the water flows faster. I suppose this is an example of how two holes of different sizes can produce jets of different velocities.

    I am still struggling with how placing your thumb over the end of hose increases the flow's velocity. My only guess is that there is no appreciable frictional losses due to your finger, and so the flux (mass flux or velocity flux, they are the same in an incompressible fluid) remains constant, requiring an increase in velocity through a smaller aperature. Perhaps russ you could elaborate on your explanation involving pressure?
     
    Last edited: Oct 26, 2008
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