About inclined underwater hull form

Discussion in 'Hydrodynamics and Aerodynamics' started by fredschmidt, Feb 18, 2012.

  1. daiquiri
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    daiquiri Engineering and Design

    Ok, but...
    I can see in the mentioned post: http://www.boatdesign.net/forums/sailboats/winged-keel-bulb-keel-40596-3.html#post504290 that you have ommited the keel, but the rudder is still there. How much of the remaining 58 kg of lift are due to the rudder and how much to the hull alone? An analysis of the hull only (no appendages of any kind) is necessary to get a grasp of the amount of lift which a hull alone can produce.

    I agree on the part where you talk about an unfavorable contribution of the keel to the wave drag. How much can a chine give in that condition, and at what induced drag cost, that's a question.

    I am not arguing the effect of the hydrodynamic force-generating chine, it makes sene for me too (if the chine angle is sharp enough). I am questioning the theory of the foil-shaped waterline.

    Which, if true, would prove that the contribution of the hull to the generation of lift (and hence, the induced drag) is quite imperceptible in those cases.

    Cheers!
     
  2. fredschmidt
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    fredschmidt Naval Architect

  3. fredschmidt
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    fredschmidt Naval Architect

    Earl

    I am supposing that the Riptide performance was better in winds with more velocity than lighter winds. Am I correct?
     
  4. Earl Boebert
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    Earl Boebert Senior Member

    Yes, one of the greatest heavy air boats of its era. When hit by a gust, it just leaps out of the water and goes. Eric Sponberg has posted an explanation for this in several places -- it has to do with the narrow stern (which also accounts for the balance of the hull -- it doesn't yaw when it heels.)

    Cheers,

    Earl
     
  5. fredschmidt
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    fredschmidt Naval Architect

    Earl

    Thanks Earl.

    Now we have another explanation.

    The rapid change in board side of low and high pressure may justify also leaps out of the water and the good efficiency in greatest heavy air can be explained with the adequate waterline forms at great angles of heel.
     
  6. Mikko Brummer
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    Mikko Brummer Senior Member

    Actually, the forces on the hull in that post is the hull alone, even if the rudder is still there. From the table in post #84 you can see that the rudder is carrying quite a lot of load in the simulation.

    The rudder is increasing the lift of the hull, by carrying over some of of its lift to hull, so yes, if you would take the rudder away the hull lift would decrease a bit.

    What I'm trying to show in post #43 is that the lift carry-over from the keel fin to hull is an important factor, more important than i would have thought myself. Partially it is because the keel is so shallow and long, so it distributes its lift on a larger portion of the hull. This lift "distributed on the hull" may come with a very small induced drag penalty, due to the Froude effect mentioned earlier. *May* come, I don't know if it does. With a narrow, deep, modern fin there would (probably) be less carry-over to the hull, thus the keel fin would have to take a bigger part of the lift on itself, at the cost of induced drag and losing some of the advantage it has otherwise compared to the low aspect fin.

    Now how does all this relate to the chine and asymmetrical water lines? Maybe not at all, but on the other hand if the chine creates asymmetrical waterlines so that there is more surface touching the water on the windward side of the keel, there is more area for the favorable lift carry over from the keel fin, and the keel root is also protected fron the air interface.

    Lots of questions... looks like I should run the Star simulation as well. I have the canoe body model from sail simulation, but would have to add the keel, skeg & rudder.
     
  7. daiquiri
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    daiquiri Engineering and Design

    A narrow and deep keel is a high aspect-ratio lifting surface, and hence is necessarily a much more efficient (in terms of Lift/Drag ratio) lifting device than a wide, shallow and thick hull body. It is true that if the hull is able to carry a part of the lift necessary to balance the sail forces than the keel will be relieved of the some amount of load. In that case the keel's AoA (leeway) and drag will decrease, but the hull's drag will increase. So I think that either a point will be reached where the division of lift between the two (keel and hull) will keep the same total drag for the same total lift, or there will be a net decrease in the overall hydrodynamic efficiency of the hull+keel at all AoA's, when compared to the "keel lift only" case. Taking into account various lift and interference-associated phenomena (vortex generation at the chine and the boundary layer separation at the keel-hull intersection) I believe that things can only get worse if the hull is allowed to create lift.

    But these are just my opinions, just like most of the stuff written here are, so they have to be verified experimentally before jumping into conclusions.

    I have to disagree with that, if I have understood your words well (that's not granted). The total amount of wetted hull surface is not important in this case, its projection on the vertical plane is. That's because lateral lift is the surface integral of the horizontal pressure components acting on the immersed hull volume. A hull can heel as much as you want, but if the draft (for example) remains constant in the process, the overall projection of the underwater hull on a vertical plane will remain pretty much unchanged, and hence the lift force will not be directly related to the increase of the wetted surface due to heel.

    That would be great indeed. I would love to be able to do it by myself, but I dont have a CFD software with the free-surface modelling capability. By the way, if your software doesn't have it too, then the CFD analysis will be flawed at speeds where waves become important. The analysis will also be in error if only an inviscid method is used, because it will not capture the effects of the boundary layer separation due to keel/hull interference.

    Cheers!
     
  8. Mikko Brummer
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    Mikko Brummer Senior Member

    Agree that there must be an optimum balance between fin and hull lift, probably depeding heavily on speed. I think there will always be some hull lift, because of the carry-over phenomena. Even with a jibing board, angled as much as the regular leeway would be, there will be hull lift. I think this is the reason why the jibibg board fails to improve performance.

    If the chine is reasonably well designed, there will be no vorticity, it will be a "streamline". As a student I did my master exercise in ship theory about a fishing trawler with developable surfaces and chines. A model was built and we did tank tests, put woollen tufts along the bottom and took underwater photos while running in the tank. To our surprise, the tufts all flowed nicely along the chines, nowhere across it, and after all this was just a model designed by students with not much experience. Also, if you look at the keel simulation of the Spring 36, in my hap hasard model it has rather a sharp keel line towards the bow in front of the keel. Yet there is no cross flow or separation across the keel line, except right in front of the keel leading edge, at the large leeway angle of 7 degrees or so. The chine appears to align the flow with itself, to avoid cross flow over it.

    The RANS software used for the Spring does not do free surface, so it will be in error. But it does agree surprisingly well with the VPP which does agree well with real world sailing performance (although not with the Star!). The RANS code is fully viscous.
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Hi Mikko.
    Have you ever tried running your code with simple thin, flat plates?
    I'm very interested to see how various CFD codes do with, for example, the
    lift and induced drag of flat square wings and flat circular disks.
     
  10. fredschmidt
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    fredschmidt Naval Architect

    Mikko


    I really like physics and try to think like a physical and I'm very intrigued by certain explanations.

    The hull is an object that passes through the water with a certain speed, if the shapes of the heeled hull, so the water lines, are symmetrical and it not has an angle of attack it will not produce lift, but if has no angle of attack keel also will not produce lift since it is a wing of symmetrical foils.

    However the whole hull / keel always produces lift and always at the expense of an angle of attack.

    In this case the hull will produce lift although symmetrical.

    And having a tilted symmetric hull is virtually impossible.

    And not being symmetrical and having speed it will always generate lift, to the right side, against the wind, or wrong, in wind direction does not matter.

    There are different pressures acting on the hull surface for the asymmetry. So we have a force caused by the pressure difference.

    Think that hull not produce lift, for me, is to ignore physics.

    If the hull produces lift should be a optimum hull shape associated to the keel, that generate a suitable lift at the lowest possible angle of attack.

    If there are a better hull form why not to be in the form of a asymmetric foil?

    Another point, if hull does not produces lift, how we can explain Hobbie Cat that is a hull with waterlines foil form? The Hobbiet Cat hull not develop lift? A hull without keel.

    What you think about?
     
    Last edited: Mar 21, 2012
  11. daiquiri
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    daiquiri Engineering and Design

    I agree with this, assuming that by "carry-over phenomena" you intend the force acting on the hull due to the pressure field created by the keel. There is a low pressure zone around the keel root at the suction (dorsal) side, and the high pressure zone around the keel root on the pressure (ventral) side of the keel. Since the pressure field cannot have discontinuities at the keel-hull joint, it extends towards the hull in adjacency of the keel root and creates a sideways force - hull lift. In that case, the carry-over exists only because there is a keel. No keel, no induced lift on the hull. The latter situation should be modeled in order to evaluate the lift produced by the hull only (which is of interest here, I think - unless the goals have changed during the 8 pages of discussion ;) ).

    Curiously, the last phrase would contradict your previous observations, because the only mean a chine has to influence the direction of the flow is the production of vorticity. A chine-induced vortex adds a tangential velocity component necessary to modify and align the flow field to the chine. So it is imho more probable that the tufts in your experiments were just too coarse to show the small vortex produced by the crossflow over the chine. The same might be true for the visualization of the flow field around the sailboat in your CFD simulation. You would have to strongly magnify the zone around the chine (provided that the mesh in that zone is sufficiently refined to capture the vortex phenomena) to notice what actually happens.

    Cheers
     
  12. Mikko Brummer
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    Mikko Brummer Senior Member

    Good point, thank you. The mesh in the Spring simulation is certainly too coarse to capture the vortex along the keel line. On the carry-over, yes, that's exactly how I see it too.
     
  13. Mikko Brummer
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    Mikko Brummer Senior Member

    I haven't, no, although I should have. If you have a specific case with dimensions, flow conditions etc. to make it easy for me, I can give it a try.
     
  14. DCockey
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    DCockey Senior Member

    With a chine induced vortex the flow will be aligned with the chine on one side of the chine. Except at it's starting location along the chine, the vortex will be off of the chine and connected to it by a shear layer / vortex sheet. The shear layer will come off the chine tangential to the surface on one side of the chine. The flow on that side of the chine will cross the chine and continue along the shear layer. The flow on the other side of the chine will be aligned with the chine. So the grid refinement needs to continue away from the chine for the height of the shear layer and vortex.
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The attached pdf is a compendium of predictions for thin wings and,
    wherever possible, experimental data.

    I have seen the very attractive plots of the flow around sails on your web
    site. I am thinking that if you can simulate thin complex lifting surfaces like
    sails, then you should be able to demonstrate how well the CFD codes do
    for:
    1. A flat square flat wing (see page 9 ),
    2. a circular planform disk (see page 19), and
    3. a delta wing (see page 23).

    There have been some very interesting studies of frisbees, but that might
    be too difficult to model. A flat circular disk should be much easier.

    I'm not sure if your codes are limited to some maximum Reynolds number,
    but the compendium contains data for low and high Rn.

    If you can model these low aspect ratio wings well, it would
    give greater confidence in your predictions of heeled hulls (to me, at least).

    All the best,
    Leo.
     

    Attached Files:

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