Added Resistance in Waves

Discussion in 'Hydrodynamics and Aerodynamics' started by quequen, Jan 30, 2014.

  1. Mikko Brummer
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    Mikko Brummer Senior Member

    ... and I did sign in :), happy to support you with your good work.
     
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  2. Dolfiman
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    Dolfiman Senior Member

    I did a tentative here under, sailboat performance and seakeeping when sailing upwind on irregular seas, with the help of good friends more expert than me in hydrodynamics. The core of the approach is to assume relevant the use of the drift forces transfert functions FD/a2(w) which are the second order results of the computation obtained by pressure integration (following the Pinkster method), where FD is proportional to the square value of « a » the wave amplitude. I introduced in the VPP not only the FD1 (i.e. the resistance due to waves) but also FD3 (i.e. a lift force changing the displacement) and FD4 (i.e. heeling moment to add to the aero one).
    Sailboat performance and seakeeping evaluation when sailing upwind on irregular seas https://www.boatdesign.net/threads/sailboat-performance-and-seakeeping-evaluation-when-sailing-upwind-on-irregular-seas.61057/#post-839597
     
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  3. Pablo Sopelana
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    Pablo Sopelana Junior Member

    Thank you very much Mikko, for your encouragement and nice words. You are at the top of my list of the Beta Version first users. I can't wait either!
     
  4. Pablo Sopelana
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    Pablo Sopelana Junior Member

    Thank you very much!

    Very interesting performance and seakeeping evaluation. I have enjoyed it a lot reading it.

    I have evaluated the Dolfi 37 as well but now using Navalapp tools. I attach the results in a pdf file.

    Some points to take into account of my evaluation made with Navalapp are:

    - Navalapp uses a single RAO (Response Amplitude Operator), which considers the combined effects of all the degrees of freedom, especially heave and pitch (as it has been seen, these two are the ones with more weight in the added resistance in waves). I found that the Navalapp calculated RAO (the curve for Kyy = 0.25 Lwl which is the closest to Dolfi) approximates very well the RAO FD2/a2 of your evaluation regarding the peak values. However, there exists a difference in the frequency at which the peaks are reached.

    - I have modeled the sea state you use in your evaluation with a JONSWAP spectrum (H1/3 = 1.2m and T0=4.488s, and H1/3 = 0.6m and T0=3.491s), for deep water.

    - I show the added resistance (RAW) results for a wave direction of 140 degrees (40 degrees from the bow) and Fn=0.35.

    - Navalapp's results on % of weight refer to % of the hull's weight. The result for H1/3 = 1.2m shows 0.93% of hull weight. But it becomes 0.84% considering the total weight light, and 0.80% when considering the total weight heavy. Similarly, the result for H1/3 = 0.6m shows 0.54% of hull weight. But it becomes 0.49% considering the total weight light, and 0.45% when considering the total weight heavy. Here too, the results correlate better with the RAO FD2/a2 of your evaluation.
     

    Attached Files:

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  5. Dolfiman
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    Dolfiman Senior Member

    Wow, you reacted quickly !!! I'm going to look at your evaluation with NavalApp and comparison, give me some time, I must go back myself into the details of this study made 3 years ago :)
     
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  6. Dolfiman
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    Dolfiman Senior Member

    I am back as promised for this comparison for the Dolfi 37 case :

    I first completed the Keuning coefficients file, made the corrections that Pablo detected, added the coefficients for a = 135° by interpolation with a 140° and a 120°, and finally added a sheet that can calculate the coefficients with any Fn and Kyy/Lw in input data for this a = 135° case , i.e. waves at 45° as done with the « Pinkster » solver for a sailing upwind.

    Then, I can copy/special paste the coefficients so computed on another file to do all the computations 1) to compare the Raw/a2 with the ones extracted from « Pinkster » solver with exactly the same Dolfi 37 data and 2) to compute the Raw themselves with various Jonswap spectra used for the Dolfi 37 speed prediction (Hs = 0,2 m, 0,4 m, 0,6m, 0,8m, 1,0 m)

    These 2 files are attached + the results of the comparison in the pdf.

    Raw/a2 concordance are good for Vb 4 Knots (Fn 0,224), quite good for Vb 6 knots (Fn 0,336, the Keuning peak is a bit higher), bad for Vb 7 Knots (Fn 0,392) due to the « Pinkster » solver which apparently no longer gives the right order of magnitude for high speed. There is another difference, also a « Pinkster » issue, for λ/Lw < 0,7 : the Raw/a2 drops to zero or even negative values when speed Fn > 0,2 ( >>> we have erased to zero these values before integration with a Jonswap)

    Raw concordance is good to quite good for Fn range 0,224 - 0,336 which is the operational one for a sailboat sailing upwind.

    Conclusions (in my opinion) :
    About the use of such “Pinkster” solver, the comparison shows two weak points :
    • the Raw/a2 values for very small waves such as λ/Lw < 0.7 and Fn > 0,2
    • the Raw/a2 peak value when the Froude becomes too high, Fn > 0,34
    Fortunately, the first point gives small difference for Raw except on very small waves (Hs 0,2 m) and the second point is not a drawback for our purpose because Fn < 0,34 is usual range for a sailboat sailing upwind on waves, where the agreement between the 2 approaches is good to quite good.

    About the use of formulation "Keuning 2006" for Raw/a2, with just Kyy, Lw, Bw, Tc, Dc and Cp as input data : the comparison shows a very good agreement (in the useful range Fn 0,20 to 0,34) with an independent direct computation carried out with a sailboat (Dolfi 37) of shape and characteristics different from the DSYHS models having been used for its establishment.
     

    Attached Files:

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  7. Pablo Sopelana
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    Pablo Sopelana Junior Member

    Thank you very much @Dolfiman for this comparison study. I have gone through your report and found it fascinating!

    It is reassuring you have found that the concordance is good to quite good for the operational Fn of a sailing boat sailing upwind, especially when considering the calculations have been carried out for a boat much different from the models used at Delft.

    I wonder whether you would be interested in making an article out of your report. If so, I will publish it on Navalapp's website under your name. You could just add a small introduction to Dolfi 37, Pinkster's method, and Keuning's 2006. Feel free to answer this proposal either here or directly by email.
     
  8. Dolfiman
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    Dolfiman Senior Member

    Thanks Pablo for your kind message and proposal, ok, I will contact you directly for this article.
     
  9. JotM
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    JotM Junior Member

    Please excuse me for the delay, but I guess that would be Lex Keuning a.o.. He has gone into retirement, but is still active behind the scenes for the ORC technical committee as far as I know. I'll try to find contact details for you.
    You might drop Lars Larsson an email at Chalmers. I'm certain he'll be able to contact Lex Keuning.
     
  10. Pablo Sopelana
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    Pablo Sopelana Junior Member

    Thanks a lot @JotM ! On my side, I will also try to contact Lars Larsson as you suggested. I will post here any updates.
     
  11. Pablo Sopelana
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    Pablo Sopelana Junior Member

  12. Pablo Sopelana
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    Pablo Sopelana Junior Member

    Hi,

    Small update on this issue.

    Users of the Delft Systematic Yacht Hull Series Database website (DSYHS Database http://dsyhs.tudelft.nl/) should have by now received an email from the database administrator confirming that the excel file "2006 - MDY - Added Resistance in Waves - CoefficientsRAW.xls" did contain the errors I had pointed out to them.

    As per his own words:

    "In short the following parts of the matrix with coefficients are wrong:
    kyy = 0.25 L, Fn = 0.25, a = 120 and
    kyy = 0.25 L, Fn = 0.35, a = 100"

    The corrected Excel sheet with RAW coefficients is now in the Library of the website under the name "2006 - MDY - Added Resistance in Waves - CoefficientsRAW (corrected).xls".

    However, I have checked the file, and there are no new coefficients for those cases, but they have been instead completely removed (instead of the coefficients, it can be read: "sorry, this part was incorrect and therefore deleted").

    If somebody still wants to calculate the RAO for those specific cases, my suggestion is that the interpolation of the coefficients of the adjacent cases could be a good approximation.

    I attach again the pdf where the errors are pointed out in case somebody is interested in it.
     

    Attached Files:

  13. Pablo Sopelana
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    Pablo Sopelana Junior Member

    Hi!!!

    Have just received a message from Boatdesign.net about a new comment posted here.

    The message says:
    <<Attention raffle, do not miss, details here ...... >>

    And it adds a link at the end.

    DO NOT CLICK ON THE LINK! IT IS MALWARE!!!

    I cannot see the post here anymore, so I guess it has been removed by the admins.
     
  14. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

    That post has been removed and that forum user has been banned.
     
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  15. Pablo Sopelana
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    Pablo Sopelana Junior Member

    Great! Quick reaction from your side! Thanks for that!
     
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