How do we reduce skin friction ??

Discussion in 'Powerboats' started by tunnels, Oct 16, 2011.

  1. Jimboat
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    Jimboat Senior Member

    it has to do with control of boundary layer trip. nothing to do with air drag. all hydrodynamic issue.
     
  2. Panos_na
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    Panos_na Junior Member

    Check some photos from the Airhull 25 RIB, which is high rated from the Greek magazines for its performance and innovations! http://www.airhull.com

    [​IMG]

    [​IMG]
     
  3. ConnClark
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    ConnClark Duck Ring user

    Here is something that might show some promise on reducing skin friction. The theory is being applied for aerodynamics but could be used for hydrodynamics as well. It basically helps control the turbulent boundary layer by stealing energy from the microscopic turbulence and converting it to heat.

    http://www.sinhatech.com/AIAA-2006-3030-245.pdf
     
  4. tunnels

    tunnels Previous Member

    round and round and got no where !! Any one able to down load ?? :eek:
     
  5. Petros
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    Petros Senior Member

    Yes, I downloaded and read it. Interesting, it looks like they used a laminar flow airfoil section, in both a wind tunnel and on a section of an actual light aircraft. They put a flexible skin on it supported at intervals that allowed the skin to oscillate or flutter at a certain frequency. This reduced the boundary layer thickness and reduce drag and improved lift, with a total L/D improvement of about double (100 percent improvement!). This is not a theoretical improvement, but an actual measure improvement on a flying aircraft.

    this is really BIG, it is completely passive and does not require any power, blowing or suction (like attempts to reduce the boundary layer in the past). I suspect it sets up a series of tiny surface vortexes that has the effect of reducing the amount of air that is trapped against the effective foil surface. This is an awesome discovery. The only real problem I see is the membrane would get fatigued and eventually wear out and need to be replaced regularly.

    Ha! and everyone assumed that the flutter that is often observed in the old tube and rag wings and fuselages was not good for drag, someone finally tested it. Tube and rag aircraft rules!

    That certainly can be used on a boat hull, you would build it skin-on-frame. There is a certain relationship between viscosity and the frequencies of the oscillating skin that is optimum, not sure what that would be for water. May be related to fluid density, Rn, or viscosity, and speed.

    This is really a brilliant discovery. Now lets see if it can be made useful.
     
  6. tunnels

    tunnels Previous Member

    Can the same system be adapted to be used on boats ?? :confused:
     
  7. MikeJohns
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    MikeJohns Senior Member

    No, sails operate at much lower Rn's and for hulls and foils the kinematic viscosity of the water is too high. Soaring birds have also evolved a high L/D efficiency probably a similar effect in action.

    Fish keep flow attached but it gets much harder once you get RN's over the tripping point. Smaller marine mammals like dolphins can keep the flow attached and laminar. For the rest it's more about keeping turbulent flow attached rather than encouraging a big laminar separation 'bubble'.

    I doubt the old canvas covered aircraft really fall into the same regime either, I suspect it's only suitable for a low range of specific conditions and the fluttering diaphrams will be quite small.
     
  8. Petros
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    Petros Senior Member

    I think it could be used on boat hulls, but that depends on what are the critical factors in terms of Rn or viscosity or what ever the controlling factor. On the aircraft wing the critical factor on the spacing of the flexible film chambers was the air speed, Rn was well over 10^8. It seems for some hull sizes and speeds it is a possibility, but again long term durability may become too costly except for something like a racing boat.
     
  9. ConnClark
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    ConnClark Duck Ring user

    This technology is applicable to a boat hull its just needs time and money to adapt and develop it. Right now there is more of a market for it above the waves.
     
  10. MikeJohns
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    MikeJohns Senior Member

    I'd be interested in how you think it could be applied effectively to boats, considering it's principally about extending laminar flow on Lifting Foils (wings) to improve the Lift to Drag ratio.

    You need to consider what any theoretical gain of L/D actually means in context of the total drag equation. There are completely different drag regimes between aircraft and boats.
     
  11. ConnClark
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    I say it will because any thing that delays transition from laminar to turbulent boundary layers will show real world benefits to unplanned or unsimulated disturbances.

    http://www.waset.org/journals/waset/v31/v31-103.pdf

    "Experiments with such coatings of a towing model performed at velocities 10 – 20 m/s in a natural basin [6] showed the drag reduction about 20%. "

    Upon further investigation it is being applied toward ships

    http://www.smooth-ships.itu.edu.tr/papers/Paper6k.pdf
     
  12. Petros
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    Petros Senior Member

    on the test section there was form and skin drag reductions of up to 40 percent, that alone would be a reason to do it. there were up to 12 percent increase in lift at any given AOA.

    all planing hulls generate lift, and therefore will have induced drag. As well as having skin friction and form drag. So even if there is no lift generated by a displacement hull, there is still significant benefit, and on most hulls there is both lift and drag present.

    It could possibly be applied to lifting foils, rudders and dagger boards, and certainly to smaller hulls like sailboards and perhaps dingys, depending on the rules. How about propeller surfaces? Perhaps not durable enough. It would be interesting to know what size and speeds in water this gives an advantage. I suspect anywhere you have turbulent flows (past transition) there will be benefit.
     
  13. MikeJohns
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    MikeJohns Senior Member

    Yes I've seen those papers before and it's an older concept than that, it was all about Dolphins at one stage even 20 or 30 years ago various films furs and skins were being trialled but nothing ever came of it in the real world.

    That's why I said you need to put some figures to "real world benefits" and ship hulls would be a classic case, look at where you get the transition in a surface displacement ships hull in a seaway ...... .

    You are much more interested in what the flow does after the transition, hanging onto a laminar flow for a fractional longer (relative to body length) would show no real world improvement that I can see.
     
  14. ConnClark
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    ConnClark Duck Ring user

    MikeJones,

    According to one paper they have increase the transition distance by a factor of 5.2 and say that by varying the material properties at different locations much more is achievable. I think the first place you will see it will be improving laminar flow over propellers and rudders where it will get you the most bang for the buck.

    Conn
     

  15. ConnClark
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    ConnClark Duck Ring user

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