Transom pressure recovery or negative lift?

Discussion in 'Hydrodynamics and Aerodynamics' started by InetRoadkill, Jul 28, 2019.

  1. InetRoadkill
    Joined: Jan 2009
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    Location: San Antonio, Texas

    InetRoadkill Junior Member

    The hull is intended for use as the center hull of a trimaran. So a balance needs to be achieved for light wind performance and more powered conditions. I understand that many tris tend to be overpowered when there's a good breeze, so achieving the absolute best drag performance at speed in not really mandatory.

    But getting back to the original question whether the hull shape is lifting the bow or not, I'm still confused on that point. I understand that viscous drag is going to prevent full pressure recovery. The question is whether the aft hull is producing lift as an inverted airfoil, or is there any static water pressure working against the area (minus any developed lift). There must be some static pressure or the buoyancy would suffer. (Maybe that's what producing the bow lift? Buoyancy center is shifting due to a pressure gradiant?)
     
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    A very common, fundamental mistake is to try to analyze a narrow body using the profile shape. The flow will be three dimensional, not two dimensional.

    Narrow hulls tend to have increased draft as speed increases. Depending on the hull shape the hull may above a certain speed decrease draft, and may eventually plane.
     
  3. InetRoadkill
    Joined: Jan 2009
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    Location: San Antonio, Texas

    InetRoadkill Junior Member

    Yes, a 2D flow isn't going to work here since it's in effect an extremely low aspect wing. Though with most wing profiles, there's a pitch down moment which is why I'm puzzled why a hull would pitch up instead. I'm guessing it has something to do with the fact the hull has an air/water interface creating different boundary conditions. I wish I had a CFD program that could solve viscous flow. It would be fun to play with. But I was hoping there were some general rules established given the rather generic hull shape.
     
  4. Earl Boebert
    Joined: Dec 2005
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    Earl Boebert Senior Member

    My favorite observation about models comes from Alan Turing, in the preface to the last paper he wrote before he committed suicide:

    "... a mathematical model of the growing embryo will be described. This model will be a simplification and an idealisation, and consequently a falsification. It is to be hoped that the features retained for discussion are those of the greatest importance in the present state of knowledge."

    I always took great pains to explain to students the depth of understanding Turing demonstrated in the phrase "It is to be hoped."

    Cheers,

    Earl
     
    CocoonCruisers likes this.

  5. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Yes, it is producing "lift" but just not like you think. Remember that "buoyancy" is nothing more than the summation of the pressures over the submerged surface. As the 3-D body moves through the fluid, it changes the energy (i.e velocity) of the fluid manifested as a pressure increase where the required flow is away from the skin and a pressure decrease where the required flow is towards the skin. These pressure changes form the wake as well as causing a change in the net pressures over the submerged body. Weather you think of it as "lift" or "squat" just depends on which static condition you measure it against.
     
    tlouth7 likes this.
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