Froude and planing

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

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  1. sandhammaren05
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    sandhammaren05 Senior Member

    Here's a way to visualize it. Think of a boat with afterplaners, which may be either a thick (wood) or a thin (Al) trailing edge. There is no lift so long there is backflow from the bottom to the top of the afterplaner. Exactly at the point when the top of the afterplaner becomes dry then lift has set in. Before that the lift coefficient is zero. Prandtl's fig. 21 in v. Karman's 'Aerodynamics' is extremely instructive.
     
  2. Joakim
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    Joakim Senior Member

    Please explain how would a foil with a thick trailing edge not have backflow and eddies at the trailing edge? Here is a paper showing measured flow fields with a thick trailing edge: Flow visualization over a thick blunt trailing-edge airfoil with base cavity at low Reynolds numbers using PIV technique https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5610673/

    Look at e.g. fig 5. Does it show backflow? Do you think the foils produces lift?

    Here is the Nasa 1950 report showing normal lift for a thick trailing edge foil: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930082748.pdf

    Think of a water skiing start from the water. Do you feel any lift when the ski is totally submerged? Does the lift go off when the ski is partly submerged? Is there a requirement for the transom of the ski to become dry in order to have lift?

    A planing surface with a transom (aka thick trailing edge) is a special case. It works in the boundary of two phases. Thus the transom does become dry when the pressure at it becomes low enough. There is no such thing for a submerged foil, since there is no other phase to fill the trailing edge. In water you may still have cavitation or ventilation, but that's a different story an not needed for lift to appear.

    I don't have the von Karman's book. Please find the figure from the net, if you want me to see it.
     
  3. Barry
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    Barry Senior Member

    In an earlier note you had said that there is no lift until the transom ventilated.
    Are you suggesting that the total sum of all the vertical components of pressure forces acting on the hull
    on the hull is less than zero from dynamic forces. Ie the force being generated by the acceleration of the water to move the water out of the way of the hull?
    If so then the only force that holds the boat up is bouyancy until ventilation?
     
  4. gonzo
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    gonzo Senior Member

    The Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity. How are you applying that to eddies at the transom?
     
  5. Joakim
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    Joakim Senior Member

    Prandtl did a lot of work on fluid dynamics. E.g he developed the lifting line theory and did a lot of experimental work on foils. But I don't know how that relates to the transom of a boat.
     
  6. markdrela
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    markdrela Senior Member

    Prandtl worked on many things. The Prandtl Number and Prandtl wing theory are completely unrelated.
     
  7. tom28571
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    tom28571 Senior Member

    Well, Prandtl designed the VW bus so there's that. Has as much relevance as Froude and the transom edge.
     
  8. DCockey
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    DCockey Senior Member

    It is fascinating how little of the extensive research on the hydrodynamics of planing (including experiments, theory, analysis and computations) has been referred to in this thread by sandhammaren05.
     
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  9. sandhammaren05
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    sandhammaren05 Senior Member


    Simulations are generally based on limited and/or faulty assumptions. I only trust experiment. I have referred to Newman and Clement, so exactly what reference do you think provides more light in the way of experimental references? Here's one for all of you to chew on. Vortices repeatedly form and are shed from any sharp edge at high enough R. Why, then, doesn't a planing hull form and repeatedly shed backflow at the transom? Answer: because the Kutta condition is satisfied, the circulation density of the vortex sheet vanishes at the transom.

    You will enjoy my next post in a few days. Photos and a video of what happens at the transom as the Froude nr. is increased.
     
  10. sandhammaren05
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    sandhammaren05 Senior Member

    I never used or use the word ventilated. I wrote 'dry'. Buoyancy carries the weight until after the transom is dry. Do you want to worry about the boat riding up on the bow wave before lift sets in? I think that's a detail.
     
  11. sandhammaren05
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    sandhammaren05 Senior Member

    Let's not muddy the water (so to speak) by referring to an example of supercavitation. Planing is not supercavitation. Propellers with thick trailing edges (cleavers) develop lift and run very efficiently. They work all the better if you take care to square the trailing edge with a file or vertical belt sander.
     
  12. Peaky
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    Peaky Junior Member

    I don't think anyone disputes that the transom of a suitable craft runs dry at high speeds. What is disputed is a. That this is the Kutta condition in evidence and b. Planing does not and cannot occur until this condition is satisfied.

    Are you familiar with Dan Savitsky's work?
     
  13. sandhammaren05
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    sandhammaren05 Senior Member

    Look over the transom and you'll see it dry at low speeds, at some speed after
    the bow tries to climb up the bow wave. Just sty looking over the transom as the
    speed is increased.

    Did you see my post of today just above about the Kutta condition?

    Yes.
     
  14. Barry
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    Barry Senior Member

    I wonder when the US sent the first astronaut into space and brought him back how many times the PHD and experts hung their heads over the transom for only experimental feedback? Or to the moon? the first nuclear reaction, or whatever
    Simulations provide the basis for calculations.
    Your comment in a post further on that states that ONLY buoyancy supports the load until ventilation of the transom, actually runs opposite to many EXPERIMENTAL papers written on this subject by experts in the field.

    Your assumptions concentrate on vortices shedding, development whatever and completely ignore the force generated on a plate moving at an angle of attack and the fact that the water that is "shoved" pushed, ( am trying to avoid
    using the term "displaced" to limit confusion) requires a force which acts on the hull, with some horizontal component as well as a vertical component. This very fact of accelerating the water from a zero state to some velocity to get it out of
    the way of the hull, produces a dynamic force from say any speed over zero with a slight bow up attitude to any speed thereafter. It appears that you just chose to ignore this fact.
    How can you ignore F=Ma or Fdot = Mdot x change in velocity
    Every moving object in the known universe works on this proven equation except here, in your guestimate.
    You would suggest then (at speeds lower than transom ventilation) that if we were to drill a hole in or around the stagnation point the only pressure acting on this point would be the pressure developed from buoyancy, ie how far that point is below datum. But even a quick uneducated look on the internet on pressure distributions on hulls at various speeds, DONE EXPERIMENTALLY, (which you want to use as the end all for scientific explanation) with feed back transducers, in a lab, etc, will reveal pressures above buoyancy pressures.

    I don't think that you have convinced many people on your hypothesis and hence the contrarian feedback.
     
    Last edited: Apr 9, 2018

  15. sandhammaren05
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    sandhammaren05 Senior Member

    First, classical mechanics is a lot easier to calculate numerically than hydrodynamics at the air-water interface. Even ORBITS OF chaotic dynamical systems can be calculated for short times by increasing the precision of the calculation as the computer runs. I am not impressed by hearing that someone is called an expert. I have dealt with so-called experts my entire life in theoretical physics. What I starte ignores nothing but irrelevant details.

    Now. Tell me how you can correctly simulate planing with waves and spray created at the interface. What lift coefficient do you obtain for a v-bottom with 12.5 degrees deadrise? Or with any deadrise.
     
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