Small Footprint Hull Resistance Testing Tank

Discussion in 'Hydrodynamics and Aerodynamics' started by Barry, Dec 28, 2018.

  1. Barry
    Joined: Mar 2002
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    Barry Senior Member

    I am considering designing and building a small unit to enable me to test various hull bottoms for resistance. I understand that a towing tank is the best platform due to the fact that the water before the tow is stagnant and offers a repeatable start point.

    As space is a problem as well as the complexity of a towing tank, I was considering an open channel water flow, straighteners to reduce the size of vortices within the stream. A controllable water flow with speed sensors. The hull then would be placed within the stream with an electronics package capable of providing resistance values. While the turbulence will create slightly variable readings, the average would give me an indication if I am on the correct path to determine how different lift strake profiles could help reduce resistance at various speeds. ( planing mono hulls)

    The space that I would be prepared to set aside for this project is about 5 meters long by 1 1/2 meters wide and at a height so the channel is a meter ( 20 feet by 5 feet wide by 4 feet high prox) and a bit off the floor just to make set up easy. The water flow at this time will be a loop, water of the end back into a reservoir, back into a pump and so on.

    Another question, when we were doing some hydraulic labs, eons ago, we introduced a different small diameter color stream to observe streamlines in the flow. Perhaps a glycerine and water solution. But I do not remember if this mixture dissolved in the water or had to be separated. Any ideas here?

    Again, I am aware that this will not be 100% accurate but will show me if my ideas are on the right track.
     
  2. Heimfried
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    Heimfried Senior Member

  3. Dolfiman
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    Dolfiman Senior Member

    You should know that the main feature of such water circulation tanks, when compared with a standard towing tank, is the speed limitation, usually in the range of 2 m/s instead of 5m/s or more for a towing tank.
    An example is the the "wave-current flume tank" of Ifremer :
    18m long, 4m width, speed max 2,2 m/s , waterdepth up to 2,2 m, turbulence adjustable ratio from 5% to 25%
    Main features :
    Caractéristiques https://wwz.ifremer.fr/manchemerdunord/Technologie-marine/Caracteristiques
    Instrumentation involved :
    Instrumentation https://wwz.ifremer.fr/manchemerdunord/Technologie-marine/Instrumentation
    Example : a study of 2 bows (conventional, inverted) for a research vessel, and the influence on the ratio of air bubbles on the sonar active faces (the les being the best of course) :
    Étude du phénomène de bullage sur les carènes de navires océanographiques https://wwz.ifremer.fr/manchemerdunord/Technologie-marine/Actualites/2014/Etude-du-phenomene-de-bullage-sur-les-carenes-de-navires-oceanographiques
    You can have more info on such device by contacting : gregory.germain@ifremer.fr
     
  4. Remmlinger
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    Remmlinger engineer

  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    I was not considering a flume as the prop tester had built and when I said a loop, I was probably not clear. The water would come out of say a pump, or a reservoir through a valve to control the speed of the water into a channel, through straighteners to reduce the size of vortices to minimize their influence on data, the water would then go off the end of the channel and back into
    the reservoir by pump.

    Perhaps this is called a flume. I had considered getting the water up to speed from an inclined channel then a turn to horizontal then realized that there would be a possibility
    of varying pressure due to hydraulic jump. (but perhaps with proper channel design this might not be a problem)
     
    Last edited: Dec 28, 2018

  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Your biggest issue will be scaling.

    Notwithstanding the comments already noted above, re: Flume, to obtain results where you can establish real sensitivity analysis (as that is what i see you are after) the scale of the models will be very small.
    This begs the question of the accuracy and the inherent systematic accuracies of the models, and then the ability to calibrate and measure the instruments sufficient for said scale.

    I have built a 20m tank in my back garden:
    upload_2018-12-29_8-41-44.png

    Gets iced up in the winter, and can't use it for many months:
    upload_2018-12-29_8-41-9.png

    And even at 20m, the scale of the models are not huge. Dictated by the speed of the model runs. Since this is dictated by the water depth I use, to avoid interference of shallow water effects.

    Thus, I admire the objective.
    But the reality of such a small tank, is problematic, in terms of delivering consistent repeatable results at such a small scale that provides confidence in the sensitivity analysis.
     

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