Dry tank testing. Can there be such a material?

Discussion in 'Sailboats' started by Omeron, Jan 30, 2008.

  1. Omeron
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    Omeron Senior Member

    For a scale model of 1:10, would it be possible to manufacture a fine powdered material which would mimic water?
    I guess it is all about similarity of dimensions and densities.
    For a scale model, water is probably too dense to give actual readings.
    What would be the density of a hypothetical material, liquid or dry to equal water, and behave like water at this scale ratio?
    Is the result such that there is no such material which is as little dense as required.
     
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  2. Pericles
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    Pericles Senior Member

  3. Omeron
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    Omeron Senior Member

    Interesting reading. Any reference to the subject above?
    Density is even greater than water. I would have thought it should go the other way.
     
  4. If you wanted to try and scale the fluid you are testing in it's the viscocity you'd want to scale to get the same reynolds numbers, but you can just trip the flow to turbulent to simlate the flow situations.
     
  5. Omeron
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    Omeron Senior Member

    Hmm... Thats interesting too.. A quick search of viscosity of substances
    reveals that at a scale of 1:10 the only liquid which would resemble water
    is liquid nitrogen. Not a very friendly material to handle for this purpose.
     
  6. yeah thats why its not done yhou can only satisfy the Froude scaling, with if you trip the flow on the model to turbulents at aknown length you can simulate real Rn numbers.
    Alternativley you can test at full scale in a wind tunnell, they do this with keel/bulb configurations and keel/hull interactions.
     
  7. Wolczko
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    Wolczko Navarc Student

    Another thing you have to consider is that solids, even in a very fine powder, will not deform the way a liquid will. Any resistance encountered as an effect of hullform or hydrodynamics would be different than what one would expect to see with water. It seems to me that one would be unnecessarily reinventing the wheel by using some powdered solid to try and imitate a test fluid that could be used anyway. Why wouldn't you just use water? There is certainly a good deal of it within easy reach.
     
  8. nico
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    nico Senior Member

    For a correct model test (of a model travelling at the water-air interface), you need to keep the froude number and the reynolds constant. This cannot be done if water is used at model and full scale (in this case, only the froude number is kept constant).
    Writing both conditions (you can try it); the following result can be found:
    (kinematic viscosity)@model_scale = (kinematic viscosity)@full_scale * sqrt(scale).

    water is at around 1.14e-6; so with a scale at 1/10; we are looking for a fluid at 0.3e-6. (mercury is at 0.1e-6).
     
  9. Pericles
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    Pericles Senior Member

    Sand can be made to act like a liquid by passing high pressure air through it, so that the grains are dancing. If a rubber duck were placed at the bottom of the sand bed and a cannon ball placed on the surface of the sand, the cannon ball would sink and the rubber duck would bob to the surface as the air supply is turned on. http://www.patentstorm.us/patents/5911201.html

    Actually, how this helps I haven't a clue, unless talc replaced the sand.

    Pericles
     
  10. Omeron
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    Omeron Senior Member

    nico, you are on to something. But having gone that far you can perhaps let us know what comes as close as 0.3e-6. Surely mercury is way out.
    Are you implying that nothing comes close to this figure.
     
  11. water addict
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    water addict Naval Architect

    That's a pretty big model scale, water is the most likely medium in which to test. You will get good results at 1:10 scale, and eliminate a HUGE host of complications trying to use another testing medium. I work at David Taylor Model Basin (navy's testing tanks). Most of the models we test are lucky to be in the 1:25 range for scale factor. 1:10 - definitely use water!
     
  12. Omeron
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    Omeron Senior Member

    Water may be the easiest and cheapest liquid, and it is fair enough to
    look no further. But in reality you need tons and tons of it alongside millions of dollars worth of equipment to make it work. Or you need to pay a high price to rent an existing facility.So at the end of the day the fact that it is free does not mean much.
    What i was wondering was whether a small scale test tank could be made,
    (no bigger than a domestic acquarium)where the model is kept stationary, and a controlled circulating flow imposed upon it.
    I guess water is useless at this scale as you circulate and pump water it would be totally turbulent.
    Sand particles mixed with pressurised air sounds interesting but i doubt it would work in practice.
    Water Addict i agree with you that 1:10 is a very large scale for a naval vessel of any type. But my interest is for sailboats no bigger than 40 ft.
    So at most i am talking about 3-4 ft models.
    Can anybody advise whether a small test tank with a stationary model can be made with readily available materials such as a glass container, few feets of tubing, a pump etc. Or is this a futile project because of flow characteristics?
    Woludnt it be nice to have this simple setup at your home, and try several things just by plugging it into mains and video results without spending thusands each time...
     
  13. Omeron, there's no point in your setup, not to put it down, it could be usefull, if you let the water 'settle' after the pump, to get a general idea of the boats trim etc, but for accurate force and moment measurements therefore resistance estimates it would not be realistic due to the 'swirl' and associated turbulent characteristecs that would be imparted onto the hull and appendages. Americas Cup teams usuall test at 1:9 to get the general setup and then refine at 1:3 scale, yes their models are as big as some nice 'real' boats!
    Also to satisfy both Reynolds and Froude scaling, needed to get completely accurate results, you would need something denser than mercury and the speed of the model would have to be very fast, which is not practicle in most tanks as they need roughly 10m to get to const speed and 10m to slow down so in say a 30m tank you only get 10m of useable data and at a large speed (for complete scaling satisfaction at 1:25 scale, model would have to be 25times fater than full scale for const. Fn, say 12knot full scale = 300 knots at model scale!!!) this is not enough at all!
    Current tank testing here, at Southampton Uni, is satisfactorily accurate, Americas Cup test acchieve 0.5% accuracy compared to full scale trials! The smaller details are of more importance than a whole new regeime, such as water temperature variations and time between runs to allow currents created due to test to dissipate....
    If you want a cheap(ish) estimation of resistance... go for CFD, in the right hands you can get 8-15% accuracy for fully appended yacht...
     
  14. water addict
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    water addict Naval Architect

    You can pump in a circulating channel and create a mostly laminar test section by shaping the water channel. This is common in wind and water tunnels for flow testing, as we have both here at Carderock. If you are looking to create your own test facility, may I be so bold as to ask why?
     

  15. Omeron
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    Omeron Senior Member

    Well, as i said before, if you spend a few hundred dollars to have a small setup at your garage, i think it would be fun to experiment with ideas, and also help friends/amateurs.
    It is one thing to take your model to a professional facility and pay for it each time, and it is completely another thing to mess around with your own setup without worrying about the cost and hassle of taking it to a facility.
    And İ dont think test labs are as abundant as indoor swimming pools in your neighborhood.
    Apart from the cost it would take some amount of travelling to and from.
    And probably you would feel uneasy going back and forth every weekend
    for weeks just to see how minor changes affects your design.
    As for shaping the water channel, do you mean using some sort of strainers, or tubing to streamline the flow after exiting the pump?
    Would layers of honeycomb sheets do that?
     
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