Flotilla 6.2 Released

Discussion in 'Software' started by Leo Lazauskas, Apr 8, 2014.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Uli:
    I'm not sure what might have gone wrong in your residuary resistance
    calculations. Attached are my comparisons for the specific resistance
    (R/D) and resistance coefficients using Michlet and Flotilla 6.2.
    I used your in.mlt file and your offsets for Michlet.
    They look reasonably similar, with Flotilla doing a little better at
    predicting the total resistance. I can't see where the 33% difference in
    residuary would arise from these.

    Beautiful lines drawing, by the way.

    Just a naive question: why do you need the residuary at all?
    Isn't it easier to just use Michlet or Flotilla with a full size
    vessel and not bother with messy scalings?
     

    Attached Files:

    Last edited: Apr 23, 2014
  2. Remmlinger
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    Remmlinger engineer

    Thank you Leo fot the diagrams, that helped.
    I had an error in my Flotilla input-files. The correct ones are attached.
    With this correction I get lower values than from Michlet. See attached diagram.
    The difference between residuary resistance and wave resistance at Fn=0.45 is now down to 19%.
    Can you explain why Flotilla gives lower values than Michlet? Is it the extension of the water line at the bow by the bow wave?
    Are you asking this tongue in cheek? You know that viscous and wave resistance scale differently. Even if the total resistance at model size agrees with the measured value, that does not mean that the value for the full size ship is correct. This is only the case, if the viscous and wave resistance for the model are both correctly predicted. You might remember that my interest is mainly the resistance of sailing yachts at low speeds. Unfortunately thin ship theory can not be used and I am stuck with the Delft-regression. I am struggling for several months now to come up with a new regression for the residuary resistance. The design-space of the Delft models is rather small and I would like to extend the database by including e.g. the Inui-hull and the Delft-372. It would also help if I can create slightly "tweaked" hull-forms with Flotilla or Michlet.

    The viscous resistance coefficient is crucial. The values at Fn = 0.45 and for a water temperature of 15 deg. C are
    your diagram.............0.00314
    Grigson's line.............0.00315
    Grigson*form-factor...0.00332 form-factor from your equation (3)
    above + pin drag........0.00354

    using the last value and subtracting it from the measured total drag gives a residuary drag that is 19% lower than the value computed by Flotilla. I think this is not bad looking at the large radius of the waterline. The humps and hollows are predicted at the right Fn.
     

    Attached Files:

  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Sorry Uli, I wrote without considering what you were actually trying
    to do with the Delft series yachts. I was talking about just predicting
    full-scale results, not scaling up from model-sized results, or
    validating model-scale measurements.

    Flotilla is a little more accurate than Michlet at calculating some
    quantities; it is not due to the extension of the effective waterline
    length - that would shift the humps and hollows.

    You are correct that the large slope at the stern is troublesome for
    thin-ship theory. However, the URANS code predictions of the total
    resistance are not that good either, as I showed in the report for
    this hull.

    My form factor method is also non-standard in that I take into account
    the wave at the stern, so it is not just a constant multiplier of the
    skin-friction at each speed. I agree with Grigson about form factors:
    they are "pure concoction".

    If you leave out pin drag, and my form factor, the Flotilla results
    for the residuary look quite good.
     
  4. Remmlinger
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    Remmlinger engineer

    If I am nagging you, just tell me and I stop.
    On the other hand, I find this discussion helpful.
    I think the form-factor is necessary. Grigson developed his line for the flat plate.
    The potential flow around the hull increases the velocity by a factor of 1.023 (calculated)
    The dynamic pressure is therefore increased by the square = 1.047.
    In addition the pressure recovery at the stern is reduced by the presence of the slow and thick boundary layer that reduces the curvature of the stream lines. Your equation yields a value of 1.052 for the form-factor, a very reasonable value. My complex b.l.-calculation results in a viscous resistance coefficient that is identical to the value Grigson*form-factor.

    Broglia (2011) shows two diagrams, one for the drag as measured and one for the corrected drag where he deducted the pin drag. The corrected values agree much better with the test results of the 3 other facilities.

    For me the question is what is right and what just looks right.
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No problem. Nag away!

    That is a very gratifying agreement.

    I've just had a closer look at Fig. 7 and 8 of the FAST 2011 paper and I see
    what you mean. The differences are quite large for Fr < 0.4 for the
    uncorrected case. It's a little unsettling to see negative values of the
    residuary at very low Fr, but that can happen with the ITTC line.

    You should be aware that the Bulgarian towing tank is fairly shallow (1.5m)
    so the results for their larger (L=3.63m) model could be a little suspect.

    I just realised that another reason Michlet might be predicting higher wave
    resistance is if you are including the BL displacement thickness. Flotilla
    doesn't include BL thickness.
     
    Last edited: Apr 25, 2014
  6. Remmlinger
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    Remmlinger engineer

    This is true, see attached diagram

    Is there an explanation for the difference between Michlet w/out BL and Flotilla at low speeds?
     

    Attached Files:

    Last edited: Apr 26, 2014
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Flotilla calculates the integrals a little more accurately, especially near the
    waterline.

    Of course, a better comparison would be between measured wave pattern
    resistance and Flotilla (or Michlet) predictions as I did in the NPL report bundled
    with Flotilla. Unfortunately, there are not many published reports that show
    measured wave pattern resistance. Even then, measurements are not always
    very accurate because of problems with short or narrow towing tanks, or
    difficulties close to the critical depth-based Froude number.
     
  8. xuanyz
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    xuanyz Junior Member

    about Flotilla Demo 2.07

    I am a newer leaner of Flotilla.When I read the file named flo62_manual_mono ,I found that something intersting in Flotilla Demo 2.07,but now I can not downlload the old verson Demo 2.07.So could someone kind-hearted upload the Flotilla Demo 2.07,thanks a lot.
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I no longer support version 2.07, and I don't like the old way of calculating
    squat and transom stern resistance.

    I will release 6.30 soon.
    It will handle catamarans and it will also calculate perturbation velocities so
    you can make nice plots like the attached.
    The first plot shows the wave elevations.
    The second plot, which shows the transverse perturbation velocities, is a bit
    more interesting. There is also a strange beauty (IMO) to the antisymmetry
    around each demihull and to the entire flow field.
     

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  10. xuanyz
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    xuanyz Junior Member



    Thank you very much for your reply .I am waiting for the new vision.

    I also have another question about the waveplot .I wonder the theory of waveplot working,in other words ,how does waveplot work ? Because I run the monexwp.bat which can caculate wave patterns of Wigley Hull,and checked the file named ship_waves1.csv.I found that the plotted area of the hull in the waveplot picture was also caculated . I wonder how waveplot plot the wave elevations and the hull together in only one picture.
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The definition of the hull used by waveplot is in the wpgeo.gpl file which is
    created by flomono when it finishes calculating the wave pattern.
     
  12. xuanyz
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    xuanyz Junior Member

    thank you very much
    Do you mean that waveplot can plot the wave elevations and the hll together in only one picture when the length the beam and the waterplane offset are offered ? But how does it deal with the common region about the hull and the wave elevations? Because I want to plot both the hull and the wave pattern with a 3D tool .I do not think I can do it with the same parameters.
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Yes. waveplot reads both files.
    There are some useless parameters in the wpgeo.gpl file that I have removed in the next version. For example, waveplot does not use the submergence depth, draft, and beam of the hull.

    Sorry, but I don't know much about 3D plotting programs.
    Which tool are you going to use?
    Can you treat the region inside the hull like a hole in the surface, and then
    put your 3D version inside that hole?
     
  14. xuanyz
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    xuanyz Junior Member

    thank you very much .
    Do you mean that treating the region inside the hull like a hole in the surface conforms with the boundary conditions between the hull and fluid ?
    Mlatlab can do this ,but the picture is not very beautiful.I want to use the 3dmax.
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Yes. From version 6.2.
    In early versions (up to and including 6.1) it did not conform to the exact hull surface. I will make it even more accurate in version 6.3

    For example, the wave cuts in versions > 6.2 are better than earlier versions and other linear codes like SWPE.
    See Fig. 6 of the Deflt372 short report.
    http://www.cyberiad.net/flotilla/delft372_mono_10may14.pdf

    Gnuplot can also do it because it has 3d capabilities, but I haven't tried that myself.
    I don't know matlab or 3dmax: I am trying to make flotilla work with free programs so everyone can use them.

    Good luck!
     
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