Measuring Dynamic Lift

Discussion in 'Hydrodynamics and Aerodynamics' started by mcollins07, Aug 12, 2012.

  1. mcollins07
    Joined: Jan 2006
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    mcollins07 Senior Member

    1) Is the dynamic lift proportional to the size of hollow in the water behind the transom?
    I believe this to be the case, but would be interested in discussions of why or why not.
    2) Has anyone attempted to numerically relate the size of the hollow to dynamic lift?
    Either theoretically or in tank test of models? References would be appreciated.

    My interests are primarily in sailboat hull designs, in the pre-planing or semi-displacement regime. Fn 0.55 – 0.75 If a numerical correlation can be made, a couple of video cameras on the transom of a boat with speed data might be sufficient for comparison between real life hulls.
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Search for papers by Lawrence J. Doctors. He has published several papers
    showing the length and depth of the hollow behind a transom stern. The
    thesis by Robards also has a lot on the measured profiles of transom hollows
    but, unfortunately, it does not consider squat per se.

    Attached is one paper to get you started. I am very impressed by the efforts
    that went into measuring the profiles, but I don't like the theoretical side of
    Lawry's approach. As I have mentioned a few times in previous posts, I
    cannot see how a cavity vented to atmospheric can sustain a pressure, and
    thereby create waves.


    Good luck!
    Leo.
     
    Last edited: Aug 12, 2015
  3. mcollins07
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    mcollins07 Senior Member

    Leo,
    Thanks for the reference and making the point about the cavity being open to the atmosphere. Your guidance is always appreciated.


    I’m hoping to achieve a simpler model than that of resistance, by considering only the vertical component of forces, and looking at dynamic versus static conditions.

    I see squat as only a complicating factor. I want to measure the vertical component of the total force generated on the boat by its velocity. I’d like to neglect consideration of the attitude of the hull also.

    I find that I’m currently searching for formal definitions of terms.
    Is dynamic lift the force represented by the difference in dynamic displacement versus static displacement? Is it represented by the rise of the center of gravity of the total boat? Does the definition of dynamic lift include the vector sum of squat, or are they defined independently? Is squat adequately defined as the downward force (which is created by some combination of a suction or wave formation)?
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    If you are serious about this research then you have a lot of
    collecting and reading to to do.

    There are also several possible pitfalls along the way. For
    example, the paper by Doctors shows the measured profile inside
    the transom hollow, but those results might be inaccurate because
    the probe was subject to self-interference effects. The thesis
    by his student Simon Robards addresses this issue.

    Some research papers have regression equations that relate Froude
    number, transom beam and depth, and the length of the hollow.
    The coefficients of the equations change from paper to paper
    as new results are included, and corrections made to account
    for probe self-interference, or to correct mistakes that snuck
    past peer-reviewers, but not obsessive nerds. ;)

    Some people prefer to present squat as the trim and sinkage.
    Sinkage is sometimes given at the LCB, and sometimes at midships.
    I prefer to show the position of the bow and the stern because
    they can be easily shown on the same graph, and I
    can relate to that format better than "sinkage".

    I understand your frustration with finding definitions.

    Maybe you should consider the sinkage force (tending to increase
    draft) and the trimming moment around midships (or some other
    location). I also like to consider the nominal change in displacement
    because I can relate it to the weight of rowing shells and the crew,
    but others might not like that.

    To complicate matters further, you should also realise that there
    is a change in the elevation of the water around the hull.
    Thus, while it may seem that there should be an increase in the
    wetted area when the hull sinks downwards, if the water level drops
    too, then there will be a much smaller net effect on wetted area.
    This can be a nuisance when looking at towing tank results because
    trim and sinkage will be given relative to the towing carriage and
    not to the water level around the hull.
    It can also affect how you treat the elevation of the water at the
    transom, and how you calculate the loss of hydrostatic pressure on
    the transom as it begins to run dry.

    Finally, if you are still awake, you should be careful of what you
    will include in resistance calculations if you use thin-ship theory.
    Formally, the effects of squat should be small. If they are not then
    you should consider carefully whether you will include the hull wave
    profile in your calculations, and other 2nd order effects.
    Otherwise, you might be cherry-picking some effects and excluding
    others.
     
  5. philSweet
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    philSweet Senior Member

    I think the short answer to the OP's question is no.

    In the one dimensional model, if I have a weight on a spring and push down and release it; it will oscillate with an average height the same as it's quiet equilibrium state. If I have a long row of them and drag a shoe across the row, The springs as a group are supporting the fixed weight of the shoe; and one can use an energy analysis to describe in a general way the entire system based on just the response of the trailing weights behind the shoe. The dynamic lift or sinkage force would be proportional to the square of the weight's vertical velocity at the transom times the mass times the rate of travel. The actual transom depth doesn't figure into it, so volume of the hole doesn't either.

    look at it another way - suppose the hull has a S shape in profile. Horizontal at the waterline, curving deeper, inflection point, then curving back to horizontal at the transom. There will be dynamic pressure near the bow, but it will be matched by a loss of pressure aft of the inflection point provided that the static pressure is sufficient to overcome separation. There will be no net dynamic lift, no vertical velocity at the transom, yet you will have a hollow behind the transom.
     

  6. mcollins07
    Joined: Jan 2006
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    mcollins07 Senior Member

    Phil,
    Thank you for your thoughtful response.
    The one dimensional spring model does introduce a perspective that I had not considered.
    Is not this model illustrating the complications of wave activity? What about the steady state case, or flat water situation?

    In the S-Profile model, pressure is interesting and may be the appropriate route. I was trying to take the view of total energy in such a model. If the work exerted to move water down is reversed after the inflection point, does not the water level return to its original level (less energy loss to resistance). However, the accounting of work introduces résistance, which I was hoping to avoid. An adequate (useful) estimation depends on the relative magnitudes and ability to approximate, I suppose. If a hollow exists, more work was expended to move water down than was returned at the point of the transom.
     
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