Turbulence vs turbulence

Discussion in 'Boat Design' started by messabout, Mar 19, 2010.

  1. messabout
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    messabout Senior Member

    Intrigueing article at Discovery.com/tech/waterpipe_flow. The brief article describes methods to reduce or eliminate turbulence in pipes by introducing deliberately generated turbulence downstream. The research seems to indicate that the concept could be applied to ships. It is said to work remarkably well in pipe systems.

    This reminds us of the "turbulator" devices that are sometimes seen on sailplanes. Turbulators or trippers appear somewhere near the wing leading edge and are said to promote attachment and delay separation. When done correctly the result is quite impressive. I have some personal experience with this gimmick on slow flying aircraft. I have never heard of this being applied to boats, but there might be some merit for turbulators on dagger boards and similar foils. Any one done experimental work of that sort?
     
  2. gonzo
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    gonzo Senior Member

    Yes, it works on masts. You can either use a small straight edge or create roughness in the leading edge. It is also use in engine intakes for high performance
     
  3. daiquiri
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    daiquiri Engineering and Design

    Turbulators are nothing new and are being regularly used whenever it is necessary to promote the flow (actually, the boundary layer) from laminar to turbulent.
    The most common reason for doing that is to avoid laminar separations or laminar bubbles and to reduce the pressure drag in blunt (or bluff) bodies. Golf balls are a classical example, a sail mast is another.

    It works because laminar flows, though having smaller friction drag when compared to turbulent flows, are less resistant to separation. Once a laminar flow separate, the pressure drag component becomes predominant over the friction and the overall drag (pressure +friction) becomes higher than if the flow was plain turbulent but attached to the body. It happens often for blunt (non-streamlined) bodies in which case becomes convenient to encourage a turbulent transition in the flow.

    It is also used for scale-model testing, in order to improve the similarity of the flows between the model and the real-size ship, and also whenever there is a need to eliminate fluctuating aero(hydo)dynamic forces due to laminar flow instabilities/separations.

    That said, I was not able to open that page at Discovery.com, the link appears broken. I'm pretty curious to read that story about placing turbulators downwind in order to reduce the drag upwinds, can you perhaps give another link or print the page in pdf (and post it here)? ;)
     
  4. TollyWally
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    TollyWally Senior Member

    Is this like the sharkskin idea that the rough surface ends up making it "slicker"?
     
  5. daiquiri
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    daiquiri Engineering and Design

    Shark skin is a very interesting and complex combination of microscopical variable-geometry turbulators and cavities/channels controlled thanks to feedback sensors (nerves). together they act on the viscous sub-layer (which is the part of the boundary layer nearest to the surface), creating a system of micro-vortices which influence the slippage of the flow along the body. Something that we have just started to comprehend and mimic, it goes far beyond simple turbulators described above. ;)
     
  6. messabout
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    messabout Senior Member

    Daquiri; I tried to raise the article on Discovery. No result. When I used the search function for "water pipes" the search returned hookah. Wrong kind of water pipe for this discussion. The original article appeared in one of the headline presentations when you first open explorer. The principals in the research were said to be some people at Harvard University in collaboration with some German science types. The article said that there may even be some potential for this method in vascular flow situations. The health care part is probably where the money is.

    Gonzo; deliberate roughness in engine manifolds is not for the purpose of increasing flow rate. The reason is to try to encourage homogenous mixtures of air and fuel. This is a sometimes helpful ploy with carbureted engines or those that use throttle body injection. In both those cases the fuel component is introduced well upstream of the valves. It is common to have heavier fuel separate from the much lighter air. Separation occurs most dramatically where the mixture turns a corner. (most common manifolds have bends and crooks) There the fuel is literally centrifuged out of the air mass. Thus it is worth trying to re mix the two components such that wet flow is diminished. When an engine breathes wet fuel, combustion is slower and less complete. Roughness is only used downstream of the manifold direction changes. Most modern automotive engines, these days, are using port injection systems where the fuel is injected very close to the inlet valve. In that case seperation is not a problem.

    On older style engines, we sometimes use a tiny protrusion on the under side of the inlet valve that may behave just like the turbulators or trippers. That is effective for improving flow past the valve seats at certain fuel/air velocities only. The tripper lips, as they are called, work at a restricted range of throttle openings so they are not used on full race engines.
     
  7. gonzo
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    gonzo Senior Member

    There is a difference between the Vortec type heads and the roughness effect on the inside of a curve in the intake. The roughness creates tiny vortexes that are smaller than the big one that would be created with a smooth wall.
     

  8. FAST FRED
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    FAST FRED Senior Member

    www.airtab.com/

    Here is how the concept is used on commercial trucks.

    FF
     
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