Bridgedeck centreboard why don't they work???

Discussion in 'Multihulls' started by valery gaulin, Jan 10, 2017.

  1. pogo
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    pogo ingenious dilletante


    ...........
     
  2. UpOnStands
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    UpOnStands Senior Member

    go argue with Barra. I asked if a medium size catamaran had been built with Dart Keels. Barra posted the above photo of D1395 (but it does have daggerboards). Voila, he thinks its a Dart keel. Dazcat thinks its a Dart keel. Richard Woods says that its not "an integral part of the hull" and so is a dart shape. Hence the question, to him.
     
  3. pogo
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    pogo ingenious dilletante

    I think that with the above posts , and pix, one should be able to judge what kind of keel, what " in between" the Dazcat has.
    ( Don't take Richard's opinion as a dogma, ' cause of the more parabolic hull frames ---in between. Designs often are "soft" , in between.)

    Also the Dazcat's daggerboard in the front section has been explained.


    So what ?
    It's up to you.

    pogo
     
  4. pogo
    Joined: Mar 2010
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    pogo ingenious dilletante

    I don't argue.
    Investigation in aircraft business is my profession ( trouble shooting , MAP ; mise au point) Together with the experience, the knowledge , the experience from 45 years with own boats...
    I try to "present" facts coming from my investigation or experience. and/or I present the results of my analysis. i learn from other members , often they confirm my " ideas" , my conclusions with their superior knowledge -- in much better words.
    Obviously my bad english is a big blocker.
    And, yes, i am an amateur.

    pogo
     
    Last edited: Feb 27, 2017
  5. cmharwood
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    cmharwood Junior Member

    Sorry for the long absence. I've been, among other things, trying to teach some ME students with no naval architecture background what propeller matching looks like. I see that the discussion has moved on, but I'd hate to leave Mr. Speer hanging.

    The section I used was designed to create a leading edge bubble, yes - and it resulted in quite a large one. In some cases, flow reversal occurred along ~60% of the chord length, as indicated by paint streak visualizations. I'll admit that I cannot say with complete certainty what the mechanism is for entrainment into that leading edge bubble (I didn't have access to a time-resolved PIV system to look at the flow in detail), but I can make some educated guesses.

    It's been a long-standing assertion that ventilation must be preceded by low pressure (which is responsible for initial entrainment of air) AND flow separation (which permits the air to remain resident in the flow without being swept downstream). I am fairly sure that even a very short separation bubble (be it a laminar bubble, thin-foil stall, etc) will permit entrained air enough residence time to propagate a larger region of separation. Air moves to occupy that separated region, further expanding the region of separation and moving to fill it in a sort of self-stabilizing behavior.

    If a foil were designed to minimize wetted flow separation, I suspect that bubbles finding their way into the flow would probably just be swept downstream (similar to traveling bubble cavitation).

    A thicker foil section that stalls from the trailing edge will entrain air from the trailing edge forward. Whether that trailing-edge cavity propagates forward and causes full-on ventilation depends upon the modification to the pressure gradient induced by the cavity itself. If open to the atmosphere, the cavity pressure is, by and large, atmospheric - and must be larger than the pressure in the preceding wetted flow. As a result, it'll intensify the adverse pressure gradient and move the point of flow detachment further upstream.

    Sorry for the rambling answer. Brevity isn't my strong suit. I'm glad you asked, and it's a question I hope to address in a small experimental project for my next student. I'd like to use time resolved PIV to track the evolution of a leading edge bubble and trailing edge stall air bubble is allowed to enter the flow. I think it'd answer a lot of questions about the stability of flow and how it's modified by gas entrainment. I have also played with using structural vibration to disrupt ventilation. There's a figure in my thesis (linked previously) that I've attached. Essentially, I used a small shaker motor (part of my modal testing rig) to excite the different natural frequencies of the hydrofoil. Low frequency vibrations (first bending mode) tended to enhance ventilation while higher frequency vibrations (twisting and multi-node bending modes) tended to disrupt the ventilation cavity and re-wet the foil. Why? I have some ideas, but I didn't have the time to investigate them. Another future experiment I think...

    [​IMG]

    For really small models (ca. Breslin and Skalak in 1959, Wetzel in 1957, etc -- sources below), surface tension dominates the sealing effect of the free surface. For models of appreciable size, it's just as you understand it. The free surface is a constant pressure boundary, and thus no chordwise pressure gradient can exist at the free surface. The reduction in chordwise pressure gradients at shallow depths follows from the requirement that the pressure distributions remain continuous. The figure below shows a (simulated) series of pressure coefficient distributions at different depths (immersion/chord=1; AoA=10deg; speed of 2.5 m/s). Similarly the photo shows the paint-streak pattern on a foil at 14 deg AoA (I don't recall the speed off the top of my head), but you can see quite clearly that the separation bubble vanishes at the free surface. The result is a thin layer of high-energy flow that has to be ruptured for air to enter. Swales, Wright, and Rothblum wrote a nice paper on the subject in 1974 (citation at bottom).

    [​IMG]

    [​IMG]

    In part, yes. Ventilation scales with the depth-based Froude number. Higher aspect ratio foils would require higher speeds to hit the Froude numbers I wanted to reach - and the facilities I used were limited to 5 or 6 m/s. Reducing the chord would have made instrumentation more difficult, and would have increased the effects of surface tension.

    Studies with larger aspect ratios would be interesting, but they're not planned at the moment. I'd also posit that a depth of about 1 chord length is where the dominant physics start to kick in. Rothblum and Meyer found that the aspect ratio ceased to affect the fundamental behavior of ventilation for ratios of h/c>1.5 (source below).


    You are absolutely correct that practical designs would have larger spans, but practicality was not the objective of my study. One of the first things I told people during demonstrations of my work was "I would never, ever, ever recommend that anybody use this foil geometry in the real world." The section shape is awful, the span is short, etc. Additionally, we though the results from my study might be applicable to the blades of surface-piercing propellers, which have similarly small aspect ratios.

    Cited:

    1. Breslin, J. P., and R. Skalak (1959), Exploratory study of ventilated flows about yawed surface-piercing struts, Tech. Rep. 2-23-59W, NASA Tech. Mem., Washington, DC, USA.
    2. Wetzel, J. (1957), Experimental studies of air ventilation of vertical, semi-submerged bodies, Tech. Rep. 57, St. Anthony Falls Hydraulic Laboratory, University of Minnesota.
    3. Swales, P., A. Wright, R. McGregor, and R. Rothblum (1974), Mechanism of ventilation inception on surface piercing foils., Journal of Mechanical Engineering Science, 16 (1), 18–24.
    4. Rothblum, R. S., D. A. Mayer, and G. M. Wilburn (1969), Ventilation, cavitation and other characteristics of high speed surface-piercing strut, Tech. Rep. 3023, Naval Ship Research and Development Center.
     

    Attached Files:

  6. tspeer
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    tspeer Senior Member

    It would be really, really, useful to be able to link wetted boundary layer characteristics to the susceptibility of ventilation for practical designs. For example, the formation of a laminar separation bubble on sections with rooftop pressure distributions. Or a correlation between the depth Froude number of the bottom of the cavity for low aspect ratio vs high aspect ratio hydrofoils. I realize that you can't test a huge number of configurations, but if there was a way to correlate ventilation with characteristics that can be computed for actual designs, then the correlation would be useful for configurations that weren't tested. Correlating with boundary layer characteristics is more useful than correlating with geometric parameters, because different configurations can produce similar boundary layer behavior, and a given configuration can have different boundary layer behavior under different conditions.

    Another key parameter is the dihedral angle of the foil. Do vertical foils and inclined foils start to ventilate at the same conditions, and do the cavities extend to the same depth Froude numbers?

    Can inclined foils experience something like a hydraulic jump phenomenon due to the stagnation pressure, and if so, what is the effect on ventilation?
     
  7. UpOnStands
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    UpOnStands Senior Member

    you're doing just great.
    looking at the Dart 18 photo I can see how a lifting body effect could be used to reduce flow separation and thus drag and instability, particularly when the cat is up on one hull. All good points of value to the dingy racer. What I am having a hard time seeing is how the Dart keel would develop active lee resistance when the boat (say 10m) is tacking. Hull is heeled no more than 4 degrees and lee angle is say 4-8 degrees. According to SailCalc I need the hull/keel/foils/whatever to exert a force of about 500 kg. Reducing the flow separation on the hull is nice but where is the force coming from?
    Daggerboards, not a problem, got all the equations to determine size/shape. I can have some confidence in the results.
    LAR, a bit more uncertain, I can use known profiles but the span is short.
    may be use a bottom plate or winglets. More uncertainty.
    But Dart keels?
     
  8. cmharwood
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    cmharwood Junior Member

    I agree completely. A way of modeling ventilation susceptibility as a function of boundary layer characteristics would be extremely useful. However, it's also deceptively difficult. The precursos of ventilation (i.e. flow separation, subatmospheric pressure, and a path-of-ingress for air) are necessary for ventilation, but not sufficient. Ventilation inception, or that first instance of air entrainment, is not deterministic, and it has remained an open question since the 1960's whether there's a good way of scaling it's occurrence. I think (and others I've worked with) think a probabilistic approach would be most appropriate, and I believe that Prof. Sverre Steen at NTNU has done some work in that area.

    This is of my motivations for taking the study down to some even more fundamental levels, however. I think that if I use some canonical geometries (e.g. a flat plate or a plano-convex section), I can focus on the interactions between entrained air and boundary layer separation. I'd like to satisfy the first two conditions for ventilation on a 2D section in wetted flow, but play with the character of the boundary layer and the extent of wetted separation by varying the Reynolds number and AoA. Then by carefully controlling the entrainment of air and using high-speed PIV to record the evolving flow field, I THINK that some meaningful results could be had that relate the probability, time-scale, and extent of ventilation to the preceding pressure distribution and boundary-layer.

    I tried to boil some of my findings down to rule-of-thumb rules (though their generality may be limited) in a recent paper in ASME Applied Mechanics Review. I'll PM you a copy.

    EDIT**

    I neglected to mention: Dihedral messes with everything. There are some results for ventilation on foils with dihedral from Fridsma ca 1963 (too lazy to format the citation). For my part, I found that dihedral causes ventilation to occur earlier, probably in part because dihedral enhances the velocities occurring normal to the free surface. Deviations of even 0.25 deg from vertical moved my ventilation boundaries by several degrees (AoA).

    I did not study the effects of sweep, and I wouldn't want to overstep by commenting on it. I've seen some good discussion of probable sweep effects in the preceding posts.
     
  9. UpOnStands
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    UpOnStands Senior Member

    I look forward to whatever it is the two of you come up with.
     
  10. oldsailor7
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    oldsailor7 Senior Member

    Dart keels are just rectangular keels which have been diagonally cut in half.
    Providing that they have the same area as the rectangular keels, there is no reason why they not behave the same, except the turbulance at the tip would be much less, thus less drag.
    A NEGATIVE would it's extra length for the same area, causing the chance of damage in shallow water.
    A POSITIVE would be it's ability to shed weed.
    The best shape would be the plan view of the Concords wing, since water, for our usage, is incompressable.
     
  11. UpOnStands
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    UpOnStands Senior Member

    you mean rectangular in elevation and symmetrical NACA in plan?
     
  12. brian eiland
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    brian eiland Senior Member

    Dart shaped keel vs Delta shaped keel

    I guess we really should differentiate the shape of those two hull bottoms. As you say the Dart catamaran hull shape is more 'voluminous' than the one seen on the Dazcat. So perhaps that word delta might be a better designation.

    And yes UpOnStands I stand corrected in that Dazcat appears to have combined the two features together on some of their vessels.

     
    Last edited: Feb 25, 2017
  13. brian eiland
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    brian eiland Senior Member

    Well put Ilan
     
  14. oldsailor7
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    oldsailor7 Senior Member

    YES.:cool:
     

  15. brian eiland
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    brian eiland Senior Member

    Interesting posting there Pogo. Probably does more to help describe how the
    rather fat Dart catamaran keel worked.
     
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