Froude and planing

Discussion in 'Hydrodynamics and Aerodynamics' started by sandhammaren05, Feb 26, 2017.

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  1. gonzo
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    gonzo Senior Member

    Hydrodynamics is part of classical mechanics. If you are not impressed by someone who is called an expert, why do you use claims of expertise (post #16) to force your position? That is a double standard. The only value of an expert opinion is that it usually has more weight. If you are taking that out of the discussion, then the only things left are measurable and repeatable experiments. However, all we have is your opinion about what you saw while laying on your stomach at the stern of a boat.
     
  2. Joakim
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    Joakim Senior Member

    The examples I gave you have nothing to do with cavitation or supercavitating propellers. But since you brought it up, do you think a supercavitating propeller has any thrust in air or deep in water at low rpm and thus no cavitation nor an air bubble? Do you think there will then be backflow and eddies at the trailing edge?
     
  3. sandhammaren05
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    sandhammaren05 Senior Member

    Hydrodynamics is more difficult than classical mechanics. In classical mechanics there is nothing like viscosity. Putting in velocity diffusion with a viscosity coefficient is equivalent to inserting the assumption of the 2nd law of thermodynamics, which cannot be derived from cl. mech. Do you know anything about either subject.? What level?
    Aparently, my obeervations of the flow at the transom of my Glatron are worth more than the simulations that the experts refer to. Don't waste your time trying to lecture me about mechanics and experts, you're way off track (as usual).
     
  4. sandhammaren05
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    sandhammaren05 Senior Member

    There is no backflow on any blade once lift has developed. Supercavitation is not of interest for high speed planing hulls.
     
  5. sandhammaren05
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    sandhammaren05 Senior Member

    The only reason that I haven't posted photos and a video is wind and debris in the water. We had flooding n. of here 2 weeks ago. Again: why don't trailing vortices form and shed continually on a planing hull? Answer that for yourselves and you'll finally understand what I'm talking about.
     
  6. DCockey
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    DCockey Senior Member

    An interesting and informative comment.
     
  7. sandhammaren05
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    sandhammaren05 Senior Member

    But I misspelled Glastron.
     
  8. Joakim
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    Joakim Senior Member

    From sandhammaren05 "paper"
    "Prandtl explained how an airplane wing lifts off. As the plane accelerates down the runway the trailing vortex forms with backflow from bottom to top of the
    trailing edge of the wing (fig. 2) as the opposite circulation develops at the leading edge. There is no net lift here. At a higher speed the trailing vortex suddenly is washed downstream and lift develops on the wing due to circulation:"

    When has Prandtl claimed something like that? Is this misconception the basis of your claims about onset of lift on planing boats. You seem to think there is a specific speed when the flow field around a foil changes from "backflow" to separated at the trailing edge. Then certainly you can show how Prandtl or someone else has calculated this speed. A specific Reynolds number perhaps?

    Do you know how the fig 2 was taken? I mean at which Reynolds number and was that a steady state flow or a transient towards steady state flow.
     
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  9. sandhammaren05
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    sandhammaren05 Senior Member

    What I've stated about wing theory is all very well known in hydrodynamics. I'm happy to take partial credit for the extension to planing hulls. Here are the references. Lanchester in England had the same idea in 1907 (Prandtl in 1908) but didn't publish:

    Prandtl, L., Tragflügeltheorie, Königliche Gesellschaft der Wissenschaften zu Göttingen, 1918.

    Prandtl, L. and Tietjens, O. G., Applied Hydro- and Aeromechanics, McGraw-Hill, N.Y., 1934.

    There is no universal prediction of a speed, there is the prediction and observation of a sharp transition from no lift to lift. Predicting the speed would depend on details of the wing, is non-universal. Steady flow. R in the photo is high, the boundary layer is very thin, ideal fluid theory predicts both the lift and induced drag. See Newman for a complete account.

    Let me add what I have written earlier, and what every student of hydrodynamics knows or should know: if you calculate the force on an object in a steady ideal flow then the net force vanishes unless the circulation about the object is finite. Lift can only be due to circulation, there is no other possibility. D'Alembert first showed that. The relevant flow for calculating forces (other than skin friction, which is less than 10% for a well-designed boat) is ideal fluid theory because the boundary layer is very thin. See Newman. No clearer treatment of the subject exists, to my knowledge. The lift is linear in the circulation, the induced drag (which is due to the tip vortex) is quadratic in the circulation. Prandtl and Tietjens even give a hand-waving 2D calculation of induced drag. Newman and Landau-Lifshitz present the exact inegral.
     
    Last edited: Apr 11, 2018
  10. Barry
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    Barry Senior Member

    Last edited: Apr 11, 2018
  11. baeckmo
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    baeckmo Hydrodynamics

    Interesting referrence Barry, thanks!
     
  12. DCockey
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    DCockey Senior Member

    Lifting surface theory, aka wing theory, has been extended to planing boats for a very long time; to the 1920's or 1930's I believe. However it has also been shown that the extension is the limiting case of very high Froude number when free surface effects can be neglected, and is of limited use in understanding the hydrodynamics of planing boats at typical operating speeds. For hydroplane race boats it may well be applicable.

    This statement is true for an 2D object which is completely surrounded by a fluid. For a 3D object completely surrounded by a fluid the situation is more complicated but the essence remains. However it is not applicable to an object which is partially submerged in the fluid such as a boat.

    How do you define the circulation about an object which is not fully surrounded by a fluid?
     
  13. DCockey
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    DCockey Senior Member

    I've never heard of "a sharp transition from no lift to lift" in either aerodynamics or hydrodynamics as speed increases. Can you provide any references beyond your personal observations leaning over the transom of a boat?

    Can you provide a reference in Newman? Are you referring to Newman's very brief comments on pp 162-163 about the shedding of a starting vortex?
     
  14. gonzo
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    gonzo Senior Member

    The ratio of lift to drag is fairly simple trigonometry. They are the sine and cosine components of the total force. There will be only one angle of attack at which the lift = 0. At all other angles it is either positive or negative. Only submarines look at negative lift as an interesting operational parameter.
     

  15. Joakim
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    Joakim Senior Member

    You have claimed, that there is a certain speed, at which the wing suddenly starts to lift and thus there is no circulation before that speed. That is not at all very well known in hydro nor aerodynamics. Your description of an airplane take off is certainly not something that Prandtl has describes regarding no lift until certain speed.

    I'm not arguing that there is circulation when there is lift for a foil in one fluid. Having backflow at the trailing edge does not mean there is no circulation. There is even plenty of lift and circulation beyond stall when there is a lot more backflow than just at the trailing edge. Even a rectangle traveling at say 45 deg AoA will have lift while having a lot of backflow. Or a normal wing profile will have a decent lift (but a lot of drag) while reversed with obviously a lot of backflow around the very thick and rounded trailing edge.

    Your interpretation of theory is clearly false. You mix separation and dry transom, which are completely different things. And you mix backflow and circulation, which are again completely different things. There can be separation at the edge of the transom and bottom while the transom is wet and there is backflow. The transom can become wet while the speed increases (e.g. my sailboat has dry transom until some speed when it becomes wet and then dry again at even higher speed.).

    The more less imaginary (does not exist in real life) non-circulating flow around an airflow with non-zero AoA does probably have backflow. But not just any backflow. It has to be the one that cancels the circulation to zero. There can be backflow with and without circulation. There can be circulation with and without backflow.
     
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