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Wind generated waves

  1. Aug 23, 2007 #1

    mck

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    (I'm not sure if I posted this topic in the appropriate category. The project I'm discussing is for my school, but seems bigger and different than a homework question. If it's not the right section I'm sorry)

    I want to measure the relationship between various aspects of wind generated waves (speed of wind, height and speed of waves, density of the fluid, etc.).

    Initially I wanted to use a vacuum-cleaner set to a reverse mode to generate wind and couple of long flower-pots connected to each other as the fluid container. Than I wanted to take pictures of the side of the flower-pot where the shadow casted by water is clearly visible. It would be easy to calculate everything than.

    The problem I encountered is the connection of flower-pots. It is really hard for me (if not impossible) to connect it in a way that it is straight and the water does not leak too much. I wanted to buy one long flower-pot (I think at least 2.5 meters for the waves to clarify) but was told the longest they produce is 1 meter in 3 big gardening stores.

    I was wondering if you maybe had any ideas how to carry this experiment in a different way?

    (I know it's bad that my first post on this forum is a help request, but I am really desperate)
     
    Last edited: Aug 23, 2007
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  3. Aug 23, 2007 #2

    mgb_phys

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    For a quick and easy wavetank I would try the guttering used for catching rainwater at the edge of a roof, it is either semicircle or square section and should be available in 2.5-3.5m lengths.
    Taking pictures of the entire length might be tricky, you would either need multiple cameras or such a wide field of view that you wouldn't see much detail and have to allow for perspective effects at the ends.
    Good luck and please post back for more help!
     
  4. Aug 23, 2007 #3

    mck

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    Hey thanks for a quick response.

    I don't need to take a photo of the whole thing, I have two cameras and that would be enough.

    The only problem is how to measure the height of the wave. Is the guttering thin enough for me to take a photo of it's side and see the waves shadow?
     
  5. Aug 23, 2007 #4

    mgb_phys

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    Usually about 4" (100mm) wide.
    Generally for wave tanks you make the sides transparent so you can see the full depth of the wave. You could make one with sheets of Perspex/Plexiglass but that starts getting complicated and expensive.
     
  6. Aug 23, 2007 #5

    mck

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    This is a part of my final High School mark, so I am not going to be too cheap on it. The plexiglass tank, however, seems a bit hard to build.

    How would you measure the height of the wave in my place (if I used the guttering)?
     
  7. Aug 23, 2007 #6

    mgb_phys

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    Draw a series of parallel black lines along the length on the inside
    Fill it with half way with water.
    Create waves, photograph them, see which lines they reach.

    or, Fill to a certain depth, make waves, see if the waves come over the top.
    Change water depth, repeat!

    It might be useful to cover the surface of the water in some light floating material (dust) so that it is easier to see the surface or dye the water with KMnO4 or food dye.
     
  8. Aug 23, 2007 #7

    mck

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    Man, the lines idea is super cool. I haven't had a slightest idea of that sort, you have to be really good at experiments.

    Thank you, thank you, thank you!
     
  9. Aug 23, 2007 #8

    mgb_phys

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    If you wanted to get clever you could put two thin strips of aluminium foil vertically up the side of the trough a few mm apart.
    Then use saltwater you could measure the resistance between the strips which would show you how high and fast the surface of the water had risen.
    You would need something like a digital oscilloscope or a data logger interface to a computer (http://www.picotech.com/educational.html)
     
  10. Aug 23, 2007 #9

    mck

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    I live in Poland and it is pretty hard to get these kind of things around here, but this would really make the process of data collecting much faster. I have to think about that.

    I would also have to come up with some smart equation to calculate the height from the resistance.
     
  11. Aug 23, 2007 #10

    mgb_phys

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    Do you have access to an old home computer? Apple II, BBC micro, Commodore 64?
    It can actually be easier to do real time measurements on those sort of machiens rather than on modern windows ones!
    Another alternative if you know some electronics or someone can help is to build a circuit to connect to the microphone input of a sound card. A sound card is just an analogue-digital converter. The sound level would give the wave height.

    It would just be linear with the height, but an experimental physicist would just fill the tank to different levels and record the resistance reading in whatever arbitrary units you wanted.
     
  12. Aug 23, 2007 #11

    mck

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    Maybe I will be able to do that myself. Got to think about this a little bit. Thanks for your help.
     
  13. Aug 26, 2007 #12
    A standard automotive gas gauge might help with this idea. They work on resistance.

    Also if you have access to building materials you could build the tank with strips of plywood,
    and leave a long rectangular hole down the length of one side of the tank. Then caulk and staple a piece of clear plastic to the inside of the tank, covering the hole, creating a viewing window where you can draw lines and take pictures, ect.
     
  14. Aug 27, 2007 #13

    mck

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    So I tried this resistance thing. I just took some microphone cable, divided it into two parts and connected them to two coins. I plugged the cable into my microphone port on my laptop and used a voice recording program. When I put the coins into the water the program started recording sound.

    It was really cool - like recording the sounds of electricity. I just have one question, is the resistance depicted as volume in this program?
     
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