Resistance prediction tool for arbitrary displacement hulls (modified Michell's theory)

Discussion in 'Software' started by jbasic, Oct 9, 2018.

  1. jbasic
    Joined: May 2010
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    Location: Split, Croatia

    jbasic Junior Member

    Hi all!

    Since a lot of things today are web-based, let me present a new online tool/method to predict ship resistance. The resistance prediction tool is intended to be a part of Prelimina - a toolbox for the acceleration of the preliminary design (work in progress). For the moment, this new method can predict resistance of arbitrary displacement monohull forms.

    Short introduction
    During some research, I (accidentally) discovered a physical connection between a modified potential-flow boundary condition and the boundary layer (PDF). It turned out that the modification can make panel methods yield realistic pressure fields, where e.g. even RANS has troubles. I implemented Michell's thin-ship theory along with this modification that accounts for the hull boudary layer (and some fixes for the phase). The results were suspiciously good - aft wavemaking is better and can handle transom sterns. Just recently the calculation method for the viscous pressure resistance was added, so I decided to let you know about the tool. Soon multihulls and dynamic trim/sinkage are coming (so wait a bit if your transom is just touching the WL), and form optimisation... also, the Rhino plug-in if there is interest.

    Validation
    The results for the Kriso containership (KCS) model are corresponding to the experimental data and RANS methods. Holtrop's method is dangerously smoothing out the wave-resistance hump.
    [​IMG]
    Series-60 (with CB = 0.6), where Holtrop's method underpredicts resistance for Fn > 0.3:
    [​IMG]
    And, obligatory, the Wigley hull with a slight resistance bump properly captured (tho it's not really a hull for Holtrop's method):
    [​IMG]
    Using the introduced enhancements that account for viscous and non-linear effects, more accurate results are obtained, as compared to the original thin-ship theory and Holtrop’s method. The trend of the wave resistance curve is properly estimated even for full ship forms. Of course, further comparisons of numerical results to experimental studies are necessary in order to examine the validity of the present approaches and improve their formulations.

    Visit www.prelimina.com and enjoy the calculator! I'd appreciate your feedback.
    - Josip Basic
     
    Last edited: Oct 10, 2018
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Very interesting though I don't have access to your papers.

    Have you compared your method to Leo Lazauskas' Michlet code?
     
  3. jbasic
    Joined: May 2010
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    jbasic Junior Member

    Dear David, I've modified the post to include the PDF to my paper. I don't think the second one from Noblesse is public. By the way, a new paper about all the details is in the making.
    Yes, Michlet and Maxsurf results were used for the verification of the unmodified thin-ship implementation, which was a starting point.
    By the way, thank you Leo for the advice, fun discussion we had, and for the literature!
     
  4. tspeer
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    tspeer Senior Member

    Sorry, I'm not seeing the pdfs - just links to journal sites.

    Can your method handle circulation around the hull, or the wave drag of lifting surfaces?
     
  5. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Based on his post, the original poster is using Michell's Thin Ship Theory which only works for symmetric hulls. It distributes sources which satisfy the linearized free surface condition over the centerplane of the hull, with the source strength based on the local slope of the hull surface. I have not seen Michell's Thin Ship Theory applied to lifting hull.

    Tom, what do you mean by "circulation around the hull"?
     
  6. tspeer
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    tspeer Senior Member

    I meant a lifting hull.

    I wasn't sure if he was somehow matching the Mitchell's integral with the pressure or potential from the panel code in order to get the wave drag. If that was the case, I was wondering if there was a way of matching the pressures of a hull with side force or the equivalent wave drag of a foil. As I understand it, Mitchell's integral is really a farfield method and doesn't predict the local pressures on the hull. That being the case, it's a little bit like the equivalent body of revolution used in the area rule for aircraft linearized wave drag. A different kind of wave, to be sure, but in both cases it's the farfield that counts.
     

  7. jbasic
    Joined: May 2010
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    Location: Split, Croatia

    jbasic Junior Member

    It is possible to find the hull pressure distribution from the far-field. The tool already calculates the local-flow pressure distribution on the hull. The one thing missing is a quasi-static solver to actually lift and trim the hull by those obtained pressures, i.e. the hull is fixed at the moment. It must be noted that truly planing boats should use "different kind" of Michell's theory (see Tuck & Lazauskas', or Yeung's work). I believe that actual horizontal foils near free surface could employ this so-called "pressure-patch" thing to obtain the wave drag, but I don't know if there is global interest for that.
    Secondly, I was intrigued to try the Michell's theory on a heeled yacht (SYRF Wide-Light design) simply by examining separately two sides of the heeled yacht - and it actually gives adequate results, at least for this design. So I guess it could be possible to predict forces and torques of slightly heeled/yawed hulls approximately with this theory. I'm adding this to my to-do list. Kutta condition is a lie anyway :)
    So in the next weeks you can expect: demihulls; deep transom sterns; lift and trim solver; heeled/yawed hulls; hull form optimisation (with a free-form deformation box).
     
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