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Water Jet Impact Force

  1. Apr 20, 2015 #1
    Hi,

    I've got this water circulation system. Diagram attached:

    http://postimg.org/image/wmf2hql6p/

    In the upper container jets of water come out from the pipe due to the small holes I have drilled into the sides. I would like to know how I can work out the impact pressure on surfaces that may be struck by these jets. What do I need to calculate this?
     
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  3. Apr 20, 2015 #2

    Simon Bridge

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    You need the mass flow rate - force is the change in momentum over change in time.
     
  4. Apr 20, 2015 #3
    Okay, I will go and measure the mass flow rate. When I know this, what else do I need to work out impact force and how it varies with distance?
     
  5. Apr 20, 2015 #4
    I believe you need to know the following
    A the height of the surface above the hole. In your case you can consider it as distance between the top and the position of the hole.
    B the area of the hole.
    A would give you the pressure
    B would give you the area
    Multiplying you will get the force.
    Remember the hole must be small enough to consider uniform pressure
     
  6. Apr 20, 2015 #5

    Simon Bridge

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    Well, in the diagram there is a pump - so that will maintain a head pressure.
    The jets will roughly follow a ballistic arc (in the absence of air resistance ... i.e. if the setup is small) ... looking at it, you'd probably be better measuring the impact rather than calculating it. What is it you need the impact force for?

    Your main complication is that their is air in the container - this breaks the jets up so the impacts are lots of droplets. This could be modelled as a series of random small impulses and you take the time average to get a force. It gets arbitrarily complicated.
     
  7. Apr 21, 2015 #6
    Thanks for your reply :)

    What would be the best way of 'measuring' the impact? I am wanting to investigate cleaning effectiveness at various impacts.
     
  8. Apr 22, 2015 #7
    Anybody know the simplest way of measuring impact for my application?
     
  9. Apr 23, 2015 #8

    Simon Bridge

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    The measurement would normally be done these days with a force transducer.
     
  10. Apr 23, 2015 #9
    I've had a look online and they are rather expensive considering I only really need to do a couple of measurements. I literally need it for one day and then I will never need it again. Can you think of a way of doing this, other than buying one and returning one immediately after?
     
  11. Apr 23, 2015 #10

    Simon Bridge

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    Basically you have to put something in the way of the jet that responds in a known way to a force acting on it ... a balance of some kind should do the trick: I used to do this sort of thing by directing a flow onto a parallel beam balance though that was splashy and not in a good way. There's lots of other things that could tell you force - a spring? A water-wheel that lifts a weight (power-work-energy theorem) etc. Use your understanding of the physics of force to help you improvise.

    Note: If all you care about is the total force resulting from all the jets impacting the floor of the upper container, then probably the weight of the water in that container would end up more important more quickly ... it's down to what you need the measurement to tell you.
     
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