Moving surface boundary layer control on a hydrofoil

Discussion in 'Hydrodynamics and Aerodynamics' started by mij, Nov 11, 2013.

  1. mij
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    mij Junior Member

  2. daiquiri
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    daiquiri Engineering and Design

    In absence of more details about the intended goals, a general reply could be - it could, but why doing it? :)

    The boundary layer control with moving surfaces is generally intended for delaying (in some cases even eliminating) the stall. It becomes functional at high angles of attack, when conventional airfoils lose the aerodynamic efficiency. Lift generation with moving surfaces (Magnus-effect rotors, mostly) been tried many times throughout the history of the aeronautics, but no solution has lasted the test of the real-life situations and economics.

    At low angles of attack an airfoil with a moving-surface high-lift device is less efficient than a well-designed conventional foil, and hence brings just unnecessary mechanical complications, weight, higher costs, structural issues and maintenance problems. Besides these, at low angles of attack a moving-surface high-lift device also causes a collateral effect of increasing the pressure coefficient around the leading edge, exactly what you don't want to have on a hydrofoil (cavitation).

    At high (near-stall) angles of attack - well, you won't get there if you have designed your hydrofoil system properly.

    These are, in brief, general considerations regarding your idea. However, if it is trying to address some specific problem which I have ignored here, we can discuss it. :)

    Cheers
     
  3. mij
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    mij Junior Member

    Thanks Daiquari. What I was wondering was if it would be possible to have a fixed foil but still be able to control the amount of lift by adjusting the rate of rotation with a moving surface device. From your explanation I can see that I have misunderstood the effect of leading edge devices. Can I ask a follow up
    question? What about trailing edge moving surfaces? I have read that these can increase lift at low angles of attack.
     
  4. tspeer
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    tspeer Senior Member

    What you are looking for is circulation control. In principle, you could have a rotating trailing edge that would do this. However, it would have to work by using the boundary layer to entrain the outer flow and bring it around the trailing edge. There would necessarily be a separation point on a blunt trailing edge, and the moving wall effects could be used to control where that separation occurred.

    However, I think it would be a comparatively weak form of circulation control, compared to trailing edge blowing or just a conventional trailing edge flap.

    It could be useful for a proa, where you'd have a rotating cylinder on both edges to allow the section to be equally effective in both directions.
     
  5. mij
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    mij Junior Member

    Thanks for the explanation. A foiling proa sounds like an interesting challenge, but I guess that would be 4 rotating cylinders (per foil assuming a T-foil configuration)?
     
    Last edited: Nov 11, 2013
  6. mij
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    mij Junior Member

    Could this approach also be applied to a wing sail? A front rotating cylinder would make the wing less susceptible to stalling, and the rear would provide the lift? I should note that I'm thinking more of applications for large vessels or small cruisers.
     
    Last edited: Nov 12, 2013
  7. tspeer
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    tspeer Senior Member

    Search for papers by Lars Ericsson on moving wall effects. Examples include:

    Lars E. Ericsson, Martin E. Beyers, "Universality of the Moving-Wall Effect," Journal of Aircraft, 2000, Vol.37: 508-513, 10.2514/2.2627.

    Lars E. Ericsson, "Moving wall effect in relation to other dynamic stall flow mechanisms," Journal of Aircraft, 1994, Vol.31: 1303-1309, 10.2514/3.46651.

    Rotating cylinders have also been used for boundary layer control at critical points on wings, such as flap hinge lines.

    Although I suggested rotating cylinders on both leading and trailing edges, I'm skeptical that approach would be better for hydrofoils than a conventional leading edge and sharp trailing edge.
     
  8. rwatson
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    rwatson Senior Member

    Attached Files:

    Last edited: Nov 12, 2013

  9. mij
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    mij Junior Member

    Thanks for the refs, unfortunately not a journal I have easy access to, but I will track them down.

    To satisfy my curiosity I might try it out (on a small scale). I've been thinking about a simple foil comparison test, but I will start a new thread on this so as not to confuse the issues.
     
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