Usage of aerodynamic symmetrical profiles in water.

Discussion in 'Hydrodynamics and Aerodynamics' started by szlak18, Oct 9, 2014.

  1. szlak18
    Joined: Oct 2014
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    szlak18 New Member

    Dear All
    I have theoretical question about usage of aerodynamic symmetrical profiles in water.
    I use airfoltool tool as there I can find a lot of profile polars. But only up to Re 1000.000.
    I want to use symmetrical profile ie. NACA 0009 for a hull -4 meters which gives me Re about 15000.000
    And now is a question. Is this profile will give me small Cd in water in normal position of this profile (wider end to front) in this RE range?
    I have never seen hull (both in real and experimental) it this shape. All hulls looks like symmetrical profiles but in reverse position – wider end to rear. ). I’m just worry about that, the profile in normal position will make water pressure to high in bow area compared to reverse position when profile will cut water. In movement without drift this may be better but when you swim with any drift (angle of attack higher than 0) bow in reverse position can produce huge rotors and cause big drag.
    I do not have any compromises regarding equipment position and sail configuration on board. Just clear theoretical problem regarding hull shape. I just want to have shape which will give me smallest drag when sailing without tilt with small drift. Where can I find any tool to find one. Or can anyone give me example of such a shape.
    I will appreciate any answer.
     
  2. NavalSArtichoke
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    NavalSArtichoke Senior Member

  3. DCockey
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    DCockey Senior Member

    Airfoil sections are designed and analyzed with two dimensional flow. The flow for design conditions is attached. Resistance is due to skin friction for speeds below transonic.

    The flow around boat hulls is three dimensional. Also boat hulls (other than submarines) generate free surface waves. The flow may be separated. Resistance is due to skin friction, wave generation and possibly separation. The flow around a boat hull is much, much more difficult to analyze than the flow around an airfoil.
     
  4. tspeer
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    tspeer Senior Member

    The flow around an airfoil and the flow around a hull are quite different. The flow can only go around the airfoil and there is no free surface. The flow can go under a hull as well as around it, and there will be waves generated at the free surface. These influences substantially change the drag.

    The bluff bow of an airfoil-shaped hull will produce too much wave drag at the speeds which are probably of interest to you. And if you reversed the shape, the flow separation at the stern would cause a lot of drag.

    The reason boats appear to get away with bluff sterns is because the water is not coming in from the sides there, but is really flowing up from the bottom. If you look at the angle between the buttlines and the surface at the stern, you'll see the angle is quite shallow.

    That's all any of us want! :)

    I suggest you look into using Michet to analyze the drag of your hull. It is the only free code I know of that can calculate the wave drag of slender hulls.

    However, Michlet cannot calculate the effect of drift (leeway) on the hull or the keel. You will have to estimate those effects separately. You may be able to neglect the effect of leeway on the hull if it has well rounded sections. The effect of leeway on the keel can be estimated using wing theory.

    Read Principles of Yacht Design by Larsson & Eliasson. They go into this with enough detail to get you started.
     
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  5. Fanie
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    Fanie Fanie

    Could it be that that shape improves pitch poling ?
     
  6. szlak18
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    szlak18 New Member

    Thanks for valuable inputs.
    My main interest is RC airplanes and this is why I made small error in reasoning. In aerodynamics we have common environment – air. Here we have border between two environment and shape move on border of these.
    Now I get it.
    So can anyone tell me what’s are contributions of the individual friction factors. Which one is the bigger and which is a smallest one.

    Friction of water to the surface –wet area – finishing of surface

    Front and back wave production –shape of hull

    Rotors – shape of hull

    Any other I forgot…

    Maybe this will be better to build submerged hull as this is common environment but it depends which friction factor is biggest.
    Thank you for free tool Michlet. I will go through this.
     
  7. DCockey
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    DCockey Senior Member

    Wave making is one difference in the flow past an airfoil and a boat hull. Another major difference is that the flow around an airfoil can only go around either side of the airfoil while the flow past a hull can go around under the hull as well as around either side. This is due to the fundamentally different shapes of hulls and wings, not the presence of the free surface between water and air. Consider a submarine or missile with a shape created by revolving an airfoil profile. The pressure and velocity profile along the submarine/missile will differ significantly from the pressure and velocity profile along a wing with same airfoil profile.
     
  8. DCockey
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    DCockey Senior Member

    szlak18, are what you call "rotors" the same as is commonly called trailing vortices in aerodynamics? Usually drag from trailing vortices from boat hulls is relatively insignificant. It can be significant from sailboat foils (keels, centerboards, etc)

    The relative significance of wave making drag vs skin skin friction drag vs "everything else" depends on the shape of the hull and the speed. At very slow speeds friction drag is much larger than wave making drag. At higher speeds both wave making drag and skin friction drag are important. At very high speeds skin friction drag may be predominant.
     
  9. szlak18
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    szlak18 New Member

    For me rotors mean when wing has stall. Than you have detachment of air stream from profile – generally turbulent flow.
    Wing has high aspect ratio but you still have air flow from top to bottom on wing tip which gives you trailing vortices. Hull is something like small aspect ratio wing. If someone wants to use aerodynamics profiles then it should be supersonic profiles. Than it should make sense. Does it make any sense?
    I realize that the Re numbers are much different in water so low speed aerodynamics profiles are good only in length of max 0,2 meter – this gives you RE about 1000000 so only keel and/or rudder.
    I have some analysis of international class CANOE. It says that we have laminar flow only on first 20% of its length. Than you it changes to transitional till 80% of length to change to stall with rotors. Theoretically change to turbulent flow is on Re 5*10^6. So transitional flow can theoretically cover all length of CANOE hull 5,2 m. But for sure you will have – turbulent flow on last 20% of length. So maybe this would be good to use modified CANOE profile. Modification will be to make stern symmetrical to bow on vertical axle.
    This would allow to “calm down” water partials using water viscosity and at least to avoid turbulent rotors and keep transitional flow. I do not need planning surface.
    By the way. Has anyone be able to provide me measuring of CANOE hull to allow me to create hull model?
     
  10. cmckesson
    Joined: Jun 2008
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    cmckesson Naval Architect

    Rotors = Separation

    It is clear from the canoe example, that Polish "rotors" means English "Separation."
     

  11. kapnD
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    kapnD Senior Member

    Hee hee
     
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