Predict flow lines along hull

Discussion in 'Hydrodynamics and Aerodynamics' started by Janne Enlund, Mar 28, 2015.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Like little strings attached to the hull, and see which way they stream ? Something like a go-pro camera hung over the side, bright day, clear water, reference points marked on hull beforehand.
     
  2. Emerson White
    Joined: Aug 2012
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    Location: Nordland, WA, USA

    Emerson White Junior Member

    Yes, though string might be a bit too flexible, maybe a scrap of thin aluminum stock bent around something. I don't know what the hull is made of, so I'm not quite sure how best to attach it. Whatever is done care should be taken not to disturb the normal flow. For the go pro, building an armature to hang it a ways away from the hull out of some scraps of angle might be the trick.
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    We're laughing with you, not at you. :)

    I was (cryptically) suggesting that you might need more than one computer
    and a lot of time to get reasonable, converged estimates of squat,
    resistance and wave patterns of an actual hull using OpenFOAM or any other
    CFD code. More so for catamarans and SES.

    I have quite a few hulls that you could use to try out your new CFD friend.
    See the "Validation" reports at
    http://www.cyberiad.net/flotilla/flotilla_mono.htm

    Those reports contain experimental data and references to other work on the
    same hull series. For example, the Delft372 demihull has been investigated
    by Prof. Fred Stern and his student using URANS.

    Another more severe test (and one that very few CFD vendors and users try)
    is to estimate resistance, squat and wave patterns before you see the
    experimental data.

    You have many months of hard, interesting work ahead of you.
    Have fun!
     
  4. Rastapop
    Joined: Mar 2014
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    Rastapop Naval Architect

    Ah, I see. You are undeniably the resident expert on resistance and wave patterns etc, around here Leo, and I think it's safe to assume that if I ever disagree with you I'll be the one who's misunderstanding something, but I'm wondering why you say it'd take so long to get resistance estimates with CFD? I've done them before on pretty crappy university computers and it's only taken me a day or two...

    Not me, I'm not the OP!
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'm only interested in simple, non-CFD predictions, for thin ships , so I'm hardly
    an expert.
    A day or two for a resistance and/or wave pattern prediction is far too slow
    for me. Inverse problems, e.g. finding a ship that makes a given wave
    pattern, could take several years!
     
  6. Rastapop
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    Rastapop Naval Architect

    Yes, for my thesis I wrote a program that generates hull shapes using genetic algorithms, and I was planning to ask you about using your software (the name slips my mind) to get better resistance estimates for the fitness function, but I ran out of time before I got a chance.
     
  7. cmckesson
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    cmckesson Naval Architect

    For full-scale testing, how about paint smear?
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    My program is called Godzilla.
    If you were at UNSW, Lawry Doctors might have been a better person to consult.
     
  9. Rastapop
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    Rastapop Naval Architect

    He gave me copies of some of his related papers.
    It was just that I knew you had written programs that quickly estimated resistance better than anything else I had heard of, so I wanted to call your program from mine to estimate resistance for each generated hull in the population. My plans for a single year of work ended up being a bit over-ambitious though :p
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Godzilla doesn't just estimate resistance, but it is itself a genetic (more
    correctly memetic) algorithm.
    Good to hear that Lawry is still dipping his oar in, despite having retired about 10 years ago!
    I've held back releasing a SES version of my program Flotilla until he
    completes his book on ship hydrodynamics in case there are some instructive
    features I can add based on his work over the last 40 years.
     
  11. Rastapop
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    Rastapop Naval Architect

    I just had a look at Godzilla - really fascinating! My program shares some similarities with it, although mine doesn't do multiple hulls.

    It isn't the one I was thinking of though, I just wanted the resistance from Michlet.
     
  12. Nick_D
    Joined: Jan 2015
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    Nick_D Junior Member

    Common sense prevails!
    It works on hulls, keels, props, lots of stuff!
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    How is that carried out ?
     
  14. cmckesson
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    cmckesson Naval Architect

    Paint smear

    Stripes are painted transversely on the hull, like station lines, using an oil-based paint. The hull is then run at the desired speed for long enough to cause the pain t to smear in the direction of flow. Immediately remove the hull from the water and photograph the smears.

    Inequalities in the smear - same as rain running down your windowpane - will result in each smear line showing many little fingers pointing locally-downstream. This provides a very good map of the streamlines over the hull.
     

  15. cmckesson
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    cmckesson Naval Architect

    I am sorry that my instructions can't be more specific, because my personal method for conducting paint smear tests is to write in the work order: "Towing tank shall conduct paint smear tests at the speed(s) provided by client" and then the tank personnel work out the details. :)
     
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