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 Josip Basic

    Hi all, sorry to bump a year-old thread, but I'd like to show an update. I've been through some issues, but now I'm continuing to work on this. I've updated the trim and sinkage solver, and fixed the solver on the website prelimina.com
    Currently I'm doing empirical calibration of the solver parameters for many ship types, so hopefully we can get an ultimate preliminary resistance prediction method. Oh, and finishing the Rhino plug-in...
    After seeing many Maxsurf wave-patterns screenshots, I'd like to show you the difference in wave-pressure fields: original theory (top) vs viscous flow enhacements (bottom), in order to see how dangerous is to use Michell's theory on non-thin forms with inviscid flow. Therefore, any kind of introduction of the boundary layer (in our case empirically embedding the displacement thickness) will yield more realistic results. This can be enough for doing a local or global shape optimisation in the preliminary stage, which can later be finished/validated by CFD simulations. We published a paper with some observarions in Ocean Engineering, feel free to drop a comment.
    upload_2020-11-23_22-40-9.png
     
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  2. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Any chance you could add side force to this?
     
  3. jbasic
    Joined: May 2010
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    Location: Split, Croatia

    jbasic Josip Basic

    If you're thinking about obtaining the lateral force, then yes. I tried manually separating two different sides of a heeled hull and got some results of drag, lateral force and the torque. I can make this automatic (adding this to the backlog!). For yawed hulls it's even more simpler, for them I just need to modify surface tangents.
     
  4. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I think you'd need to add some sort of vortex panels along the centerline of the hull and in the wake to superimpose with the Mitchell integral, but the math for that would be beyond me.
     
  5. jbasic
    Joined: May 2010
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    Location: Split, Croatia

    jbasic Josip Basic

    The wake's influence on the wavemaking is incorporated. For Michell's wave-making pressure I am certain that no Kutta or other conditions are needed. You can think of it this way: at the stern (no matter how slender it is), the flow will separate, and behind it, no significant wave system is generated. So no significant harm done to simulate two hull halves separately.

    But the problem is different for local/viscous pressure. I'm currently testing Noblesse's approximation for this, and cannot comment in advance how nonsymmetric sides will be computed. To be continued, I guess :)
     
  6. Anahita Piri
    Joined: Tuesday
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    Location: New Zealand

    Anahita Piri New Member

    Hi there Josip, I first wanted to say thank you for creating such a great online tool - the work put into it is immense and it shows. I was trying to upload some 3D models into Prelimina's resistance tool, however I am unable to successfully upload any 3D models. I attempted to upload a hull and triangular prism to test the 3D view but it seems like only a 2D surface would show. I have attached images of what the .OBJ file is supposed to be, and what Prelimina displays it as instead. I have indeed played with changing the coordinate system of the .OBJ file and it seems to have some impact, but I am yet to meet an orientation that works. The first image displays a hollow triangular prism with sides of approximately 1m long, and 2 m wide and when put into Prelimina only a single triangle is shown as in the second image. The third photo is of a personalized 3D hull when put into Prelimina, which shows two surfaces instead of a solid hull. If you have any idea where I could be going wrong, it would be highly appreciated.

    Kind regards,
    Ana
    Online3DView.JPG PreliminaView.JPG PreliminaViewPrelimina.JPG
     
  7. jbasic
    Joined: May 2010
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    Location: Split, Croatia

    jbasic Josip Basic

    Dear Anahita, thanks for the kind words. I'm using a wide-spread solution (three.js) to load OBJ files, and it's validated using meshes from Rhino, MeshLab, and SolidWorks. Prelimina adopts classic naval arch coord system, X+ stern-to-bow, Z+ up, Y sides. I have no idea besides the advice to double-check your exporter.
     

  8. Anahita Piri
    Joined: Tuesday
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    Location: New Zealand

    Anahita Piri New Member

    Cheers for that amazingly quick reply, as a note for others as well as yourself the issue was resolved by importing the cad file from my online software into MeshLab, and then re-exporting it as an object file from there. This seemed to resolve any issues Prelimina had with the .obj file, and is now functional.
    Many thanks,
    Ana
     
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