theoretical displacement hull shape for min drag

Discussion in 'Software' started by Padava, Jun 21, 2009.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No argument there. That paper was from 1996 and is, admittedly, for an idealised shape. Since then I have written other programs (e.g. Michlet, Flotilla, Flotsm, Godzilla) that can take real hulls and pressure distributions and do the same thing. I can account for sinkage and trim, reduce waves made by the ship, estimate bottom pressure signatures, and (for monohulls at least) estimate their performance in waves.

    There will always be a gap between the predictions made, the "optimal" forms found, and what happens out in the real world. I have never tried to hide from that fact. Nor do I think that an automated design code will ever replace the common sense of an experienced naval architect. I just write tools for other tools ;-)

    Leo.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Leo

    What are the parameters of the "hull-form" inputs that 'define' the applicablity?

    Is it applicable across a wide range of length/displacement ratios and froude numbers, say from 0.1 to 2.0 (Fn in the classic sense)? for example.

    How does it treat the waves, is it mere a function of amplitude caused by the pressure distribution or an energy based function?

    "..There will always be a gap between the predictions made, the "optimal" forms found, and what happens out in the real world. I have never tried to hide from that fact. Nor do I think that an automated design code will ever replace the common sense of an experienced naval architect. I just write tools for other tools ;-).."

    Nicely said, sadly too few appreciate the simplicity of your statement
    Unfortunately there are many who use your programs and seem to be under the impression it provides the holy grail, since they are not as you put it, experienced naval architects!

    PS..i co-authored a paper on hydrodynamics some 10 years ago, generally on higher Fn's...we found some very interesting results. If you PM me your email addy, i would be happy to send you a copy, if you're interested.
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No, they are generally for thin ships, say L/B > 6 or so.

    Energy-based, but I have started looking at ship routing problems, e.g. "optimal" paths around storm systems etc. It's a hobby interest (like most of my work) so I haven't got far yet.

    Schmucks, like the poor, will always be with us.

    my surname (in lower case) at gmail.com will find me.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    if i pressed all the right buttons in the right places...a copy should squirted down the line to you as i type...i hope!
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I got your paper. Thanks!
    I'm more interested in mathematical approaches and in physics-based methods rather than the empirical approaches that you use. That said, I also have to resort to empirical models for some transom calculations, estimating boundary layer effects etc. so I'm not being critical. It's purely a matter of taste.

    It was good to see that you were interested in wash effects all those years ago. I also had a look at the effects of viscosity on wave decay in a recent work. See:
    http://www.cyberiad.net/leo.htm

    I'm not sure how well the theory will work in the real world. Measuring wave decay rates kilometres from a ship is not an easy thing!

    Regards,
    Leo.
     
  6. Ad Hoc
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    Leo

    Non taken. You're approach is from a 'numbers' based environment, whereas mine is from a 'real world' environment, based upon 'numbers'. Each are unique but also require each others input for validation and understanding. Since numbers are just..well,...numbers without validation. And shapes are just...shapes, without know what affects them.

    The tank testing we did of various hull forms though, in regards to shape, cannot be ignored - has been proven by many since, theoretically too!

    You should the research report 457 by MCA in the UK..very good stuff. Also the obvious stuff by Dand and Whittaker, excellent work on wash. Leaders in the field I would say.
     
  7. Ad Hoc
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    Leo

    "..I can account for sinkage and trim, reduce waves made by the ship, estimate bottom pressure signatures, and (for monohulls at least) estimate their performance in waves..."

    I'm interested how you do this. What methods do you use and what do you use to validate the results, and over what range is the validation applicable?

    Also, I'm not aware of any reliable methods that can take into account the Stern transoms at both low and high speeds, how do you account for this, and what method of validation do you use for these over a speed range?

    I read many papers either as personal interest or asked to critique, so I'm wondering about your method of validations and their applicability.
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I know of Whittaker's work. His student Max Osterried used an old version of Michlet in his Master's Thesis of 2002.

    Recent work work on wakes by Macfarlane and Renilson, and Doctors and Day might be of interest to you. The first pair use a lot of experimental data; the second pair use a more theoretical approach.

    A more interesting problem for me is: given a wave pattern, what is the shape of the ship (or fleet of ships) that made that pattern? Godzilla can "solve" a simple form of the problem. Other approaches have been tried by some excellent hydrodynamicists, including Art Reed, Carl Scragg, and Jerome Milgram.

    I'd love to get some data that gives the wave elevations over a fairly large patch of otherwise calm water, but I haven't been able find anything that goes back far enough from the ship. Know of anything like that?
     
  9. Leo Lazauskas
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    I have no way of validating many results because that is not my field.

    I have a couple of methods to estimate bottom pressure signatures. Some use line sources, some use assemblies of Havelock sources.

    Transom sterns are, as you note, very difficult.
    If you want to see how CFD performs, look up papers by Kevin Maki, L.J. Doctors and Robert Beck. As you said, there is nothing reliable at very low speeds or very high speeds, however there are some empirical methods and a couple of theoretical results (that I know of) that can be used to get reasonable agreement with experiments. Sometimes.

    Doctors and Day did a lot of work on transoms, and haven't found anything that I consider consistent yet. I have dozens of variations on their theme and others and I always feel that I am shoe-horning results into experimental data by tweaking a variety of parameters.
     
  10. Ad Hoc
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    Leo

    I know Renilson well, we have worked together before. Doctors work is interesting, but his work has been, at times, too much theoretical work. By that i mean just research for the sake of research and has no real application, save for a thesis or a paper presentation. But there is some good stuff in there, just ahve to wade through the unusable stuff, from a practicing naval architecture point of view, not a academics/theoreticians.

    I have given some advice recently to a post-doc research student looking at weather systems ie sea spectrum's in relation to parametric rolling in terms of weather routing models...a possible link up with your far field waves?

    The MCA 457/ work by Dand have measurements up to 600m from the vessel track. Not perfect, for your work/area of interest, but it is a start!

    "...I have no way of validating many results because that is not my field..."
    Without any validation, i could only take the results with a pinch of salt!
    This is not to say it is totally meaningless, but just "on hold" so to speak until the results/theories are validated.

    PS..Molland and Hudson i recall have some some interesting work on transoms and wake fields, i know them too and could contact them to see what they have been up too recently, unless you already know?
     
  11. Leo Lazauskas
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    Sorry, I meant verify. I don't have access to facilities to compare my results to experiments in every case.

    Doctors work is often fairly theoretical, but in nearly every paper I have read he does try to compare results to experiments. I find that alone useful.

    I prefer the theoretical and mathematical because it can lead to unexpected places, whereas experiments with a hull, or series of hulls, just gives you info about, not surprisingly, more hulls.
    The work I started on in identifying ships from their wave patterns morphed into all sorts of weird stuff and, for example, prompted my colleague Ernie Tuck to use the techniques to investigate the Riemann Zeta function. That's a very long way from ship hydro!
     
  12. Ad Hoc
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    That's the beauty of "pure" research, takes you into unknown and unplanned territory.

    But research is only any good if the results can be validated and verified qualitatively. Otherwise it just remains 'numbers' for the sake of numbers sake. There is plenty of that here in Japan...most of it excellent work, but has no application and/or next to impossible to verify.

    "...Sorry, I meant verify. I don't have access to facilities to compare my results to experiments in every case..."

    So the work/results you obtain from your research and your programs, without verification, how do you know where the limitations are and hence its own applicability for use in modelling the real world environment?
     
  13. Ad Hoc
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    Just thought about this one, nice idea.

    "...A more interesting problem for me is: given a wave pattern, what is the shape of the ship (or fleet of ships) that made that pattern?.."

    What you're after is a 'hydrodynamic' signature that relates to a unique 3D immersed body moving at the water-air interface. Hmmm...given the fact that waves, especially far field waves, from various hull forms at varying speeds and varying depths, whilst "appearing" unique, all follow pretty much 'set patterns'; despite great pains to model such "simple" behaviour, especially in terms of wash. How you get a signature out of divergent, transverse and solitary waves, when most of this is still in its infancy anyway, very tricky...but interesting!

    You're probably better looking down the quantum physics route, just as those studying rogue waves, have found very similar correlations.
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I try to compare results to experiments whenever they are available. Even when they are available they are not always useful. For example, the NPL series of hulls were studied in great detail by Molland Couser and Wellicome. However all the hulls were only 1.6m long which raises a number of issues to do with viscous effects. Shame, it's one of the best pieces of experimental work on catamarans there is.

    I read through your paper last night. There is a huge amount of good work summarised in there! The only criticism I have is that there are no error bars on any of the experiments. You aren't alone there - hardly anyone does that any more but it makes verifying predictions all the more difficult.

    Cheers,
    Leo.
     

  15. Ad Hoc
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    Leo

    "We", my colleague and I, were often approached by universities for verification/validation of their work, with the boats we designed, at our previous company. We ended up with a good working relationship with one in particular. May be worth approaching a shipyard, or even as we had once, a ship operator, and ask if you could do some 'collaborative' work, which benefits both parties. Downside is that almost always it is confidential data that you obtain, hence limited to what you can demonstrate publicly.

    But without verification, it will be hard to "punt" your theories/research to anyone other researchers and academics. I have to justify and verify everything I do when designing, if i can't it comes down to experience and judgement...if a software program looks great, but has not been verified, wont use it...unless for just simple sensitivity analysis.

    Thanks for the comments, we got very favourable reviews when i presented it. Prof Molland actually said he'll make it essential reading for his 1st year students!..praise indeed.

    Yes, we didn't put error bars on just the trend lines, since the paper is not a real 'academic' paper as such, would have bored the audiance too!
     
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