Image of chaos behind a stalled sail

Discussion in 'Hydrodynamics and Aerodynamics' started by jlconger, Aug 26, 2014.

  1. tspeer
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    tspeer Senior Member

    It depends on what you want to do. If you want to run a 3D analysis like jconger's, and you want a fine enough grid that the boundary layer is sufficiently resolved for accurate results, and you want to get answers in a reasonable time (hours to a day or two), then you probably need much more than a laptop. You're probably needing a workstation with a GPU accelerator, or a cluster. I'm in the process of purchasing a computer from Microway for that very reason.

    Computers are like guns and cars - at some point, there's no substitute for cubic inches.
     
  2. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    In many ways, a foiling boat is easier to calculate than a floating one. The foils can be calculated separately, using simple tools. We got good agreement with regard to lift, moments, and induced drag for the AC72 foils using a lifting line spreadsheet compared to Navier Stokes (Star CCM+). A panel code like CMARC would do better than a lifting line, and allow you to see where cavitation will occur.

    A Navier Stokes code is necessary if you are dealing with large amounts of separation. The spinnaker is a good example

    For a sailboat, Michlet has some key shortcomings. The main one is it does not deal with sideforce and leeway.

    You're going to need to patch together the results from a number of different tools to put together a VPP. You might get the zero leeway hull wave drag from Michlet, and add in appendage lift and drag, plus hull form drag from a panel code. A panel code plus emirical windage will get you the aerodynamic forces. That will give you a VPP representative of the state of the art in the 12M era.
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    True. IMO, the main limitations are that Michlet is not appropriate for monohull sailboats because they are usually not thin enough, and it does not estimate behaviour in waves.

    Incidentally, a rule of thumb for free-surface calculations is that they are roughly 4 orders of magnitude (3 dimensions + time) more time-consuming than flows without a free surface, e.g. aerodynamic calculations.
     
  4. jlconger
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    jlconger Junior Member

    It is just a hobby for me too. I did CFD for work before I retired, but applying it to sailing is much more fun.

    I've read that CFD played a major roll in the AC72 foil design as they were dealing with cavitation. It would be interesting to know what they ended up changing.
     
  5. Erwan
    Joined: Oct 2005
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    Erwan Senior Member

    Thank you for all these great value comments, I am not in a hurry and will gather info until I feel I can make a reasonnable choice.

    Sorry to have drifted a little bit the thread, so in order to come back a little bit to the core-subjet (modelisation of sail & wing), I would mention that in the "Foiling Week"
    You were scheduled to present an A-Cat wing Mr Speer, but actually you made a great presentation regarding A-Cat foiling.

    I guess someday your A-Cat wing will be presented somewhere

    For the hull I guess an A-cat flying a hull before foiling, should meet Michlet limitations and provide a good idea of hull drag.

    Best regards

    Erwan
     

  6. cenej
    Joined: Jan 2008
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    cenej New Member


    Hi, when I want to access the written pages I received the following message:

    Site not found
    We're sorry, we were unable to locate the site /sailcfd/.

    ... apparently pages are currently not available, will they ever be again?

    Thank you.
     
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