optimization of a twinkeel boat with openfoam

Discussion in 'Hydrodynamics and Aerodynamics' started by Jochen, Oct 23, 2011.

  1. Jochen
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    Jochen Junior Member

    Hello everyone. I'm new here and new to Linux and Openfoam.

    I bought a custom made sailing boat with twinkeels in March, motored and sailed it during the summer and it went quite good until 5.5 knots. From then it was going like having tightened brakes. It is 11.5m long, so it should go up to 8 knots.

    My suggestion is, that the 2m long keels are with 0.8m so close together, that beginning at a certain speed the turbulences between the keels will slow down the boat.

    So I constructed the existing hull and an alternative one with keels in the max. possible distance of 1.2m with Freeship´s freeware, let the software calculate the resistance of the hulls and found no significant differencies among the results, I think, because the sofware does'nt consider turbulences.

    Because it´s a lot of work, to change the positions of the keels, I looked into the web, found openfoam and the nice collection caelinux and it took one week to go. Many thanks to Your patiency up to here. Now my question:

    Have I to learn all that heavy stuff with case dirs and so on or are'nt there a lot of folks, that did already the definitions of a watertank, where I can simply put my different hulls in?

    Hoping my english is understandable, Jochen
     
  2. lumberjack_jeff
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    lumberjack_jeff Sawdust sweeper

  3. MikeJohns
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    MikeJohns Senior Member

    Resistance prediction in programs like Freeship are simplified general models that estimate the simplified hull only drag. They are not CFD packages. But CFD is not going to be very useful in this case.

    I think you are wasting your time doing anything other than model testing to really refine the keel positions. Even then you need to consider heel and leeway.

    One much simpler solution would be to remove the 2 keels and replace them with one.

    Otherwise just follow some rules of thumb for twin keels or copy a successful design. forget open foam.
     
  4. Jochen
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    Jochen Junior Member

    Thanks fpr Your quick answer, Mike. Why should I forget Openfoam? Do You definitely know, that it cannot produce results, which could give me hints concerning my question?

    By the way and as a joke: the simpliest solution would be, to cut off the keels and go without it. But indeed, because I'm sailing the baltic sea with a lot of shallow waters and because the boat has a big beam and therefore has a great initial stability a centerboard could be an interesting alternative.

    But first I will try to get an answer for my question.
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It will take you a few months, maybe a year or more to be good enough with OpenFoam to produce (possibly) useful results. If you do not have good programming skills, you might never get there.

    Do you want to invest that much time and effort?
     
  6. MikeJohns
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    MikeJohns Senior Member

    You need a background in hydrodynamics and FEM before you can start getting sensible results at all, and that's presuming as Leo said that you've spent the time learning OpenFoam to get up to speed.

    But there are many considerations that alter the flow field around the hull for a real life vessel even in calm water that you would need to identify and correctly reproduce to define the problem correctly.

    [I'd better add] In studying FEM/CFD you would begin to understand why it's not the tool you presume it is.
     
  7. Jochen
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    Jochen Junior Member

    Ok Mike, I think I have to accept that and Thanks for You explanation. Maybe I hoped, someone would say, ok, send me the .stl of Your hulls and I will do it for You.
     
  8. Tim B
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    Tim B Senior Member

    Not a problem, how much money do you have? (I'm joking)

    Seriously, to do CFD well requires a lot of experience, and a big computing resource. These do not come cheap. That's why most people stick to the empirical methods.

    You could also search the web for data on biplane configuration, which will give you an indication of trends. There is also data on the web regarding the wave-creating effects of near surface foils, which is probably what you're seeing.

    Cheers,

    Tim B.
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That might be be even worse!
    More than one person could produce results (varying by more than 500%) for you. Which one would you then choose? :)
     
  10. Jochen
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    Jochen Junior Member

    Thanks Tim and Leo. I read a lot in the web concerning this issues and begin to understand Your hints. I thought CFD is an assured science and it is, but only in a tiny small window of a certain condition at a certain moment computed with one of the belonging theories.

    I think, building of two models an mesure their resistance with a sensitive horizontal spring could give me the results, I need.
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I hope you do get good results, but even if you do not, you deserve high praise for just "doing the experiment"!

    Leo.
     
  12. MikeJohns
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    MikeJohns Senior Member

    You already have one full sized model that you can get a lot of information from.

    A video or picture or two of your crafts attitude and wave generation at a range of Froude numbers in the trim heel and leeway conditions could be quite illuminating.
     
  13. Jochen
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    Jochen Junior Member

    Maybe I don´t have to earn the praise, Leo. I found an interesting video in the web: "Flow Separation - CFD Unsteady Simulation", which shows from laminate to turbulent flow two foils in a media. I don´t know, is it water or a gas, the foils are not parallel, not symetric like keels, not in line with the fluid and I even do not know the speed of the fluid, because I´m not aible, to read the scientific notations, but nevertheless it gives helpful hints.

    First: the beam of the turbulences reaches minimum the half of the lengsth of the foils, so I think the distance between my keels should be minimum the half of their lengsth

    Second: My keels should be naca-profiled to keep the flow laminar as long as possible and I remember, they aren´t at their sharp beginning (maybe this is more important than the distance between them)

    Third: I have one saildrive in the middle at the end of the keels. Looks like the propeller is working in chaotic conditions from zero-speed-water to high-speed water.
     
  14. Jochen
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    Jochen Junior Member

    Thanks Mike, but to understand Your advice, I first have to google, what a froud number is. I´m an architect, but not a naval one.
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Just remember to pronounce it "Frood" not "Frowd" or you will never be invited to wild and exciting hydrodynamics parties!
     
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