flow straightener

Discussion in 'Boat Design' started by clmanges, Aug 3, 2008.

  1. clmanges
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    clmanges Senior Member

    It's going to take me a while to figure out what (if any) new propulsion system to put on my rowboat, so in the meantime I'm thinking of doing an experiment with a passive horizontal foil to straighten out the turbulence caused by the boat's transom. The turbulence is mostly chaotic, but with each stroke of the oars, I see a very clear Karman street vortex (indicating drag) form in its wake. The harder I pull, the harder the boat sucks me backward. :(

    I found this paper

    http://www.springerlink.com/content/9nedcrl8qe3hgff2/

    and it occurred to me that my transom might be thought of as an upside-down form of a backward-facing step. :idea:

    What I'm thinking is to keep the foil just above the bottom of the boat, so it doesn't get fouled, make it of neutral bouyancy, and mount it on a pivot, so it can adjust its own pitch. I think it would oscillate on its own, but I'm not sure.

    The paper doesn't deal with pitching foils; only pure plunging motion, so I'm not certain about this. Also, the test model in the paper was quite small, so I don't know what effect scale factors would have. Do note in Section 4.1 of the paper that a stationary foil was found to induce drag, while the same foil when plunging produced thrust.

    What think ye?

    Knocking the rust off my brain,
    Curtis
     

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  2. rwatson
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    rwatson Senior Member

    I dont think the plate will help if your transon is square.

    That 'pitching foil' concept was for light aircraft, and I dont think that would 'hold water' for a boat that isnt completely immersed in its 'flow medium'
     
  3. clmanges
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    clmanges Senior Member

    Two questions:
    1) Is neutral bouyancy really important if I pivot the foil some way back from the LE; say at the quarter-chord point (for a NACA 001x foil)?

    2) If so, I need to compute volume of the foil. Is there a table somewhere of cross-sectional area of NACA profiles, expressed as a relation to chord length or some other parameter? I haven't found one yet. I suppose that I can work up a rough approximation if I have to.

    Curtis
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    As you increase speed, the transom becomes drier, and there is an attendant loss of hydrostatic pressure. That is a type of drag in itself.

    The Karman street vortices have a vertical axis. I'm not sure that a horizontal ly-placed foil will do much to reduce those (vertical) vortices.

    Have fun!
    Leo.
     
  5. clmanges
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    clmanges Senior Member

    All this reading I'm doing must be paying off . . . a couple of weeks ago, I wouldn't have understood this comment for a day or so -- this time I got it in less than a minute. At first, I thought it highly unlikely that I could make this bathtub go fast enough under oars to make a difference in the depth at the transom, but you may be right. However, there is also a little pitching of the boat caused by shifting my weight with each stroke. How to know which motion is doing what? I don't know, but I do know that, when I stop rowing, I no longer see the vortex.
    That's another thing I hadn't thought of, but would the remaining chaotic turbulence be an indicator of horizontal vortices?

    Believe it or not, I am having fun. Thanks.
    Curtis
     
  6. tspeer
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    tspeer Senior Member

    Vortices can only meet a free surface at a right angle. Could the vortices he sees be the legs of a U-shaped vortex that wraps around his transom? In that case, the horizontal foil may be the most effective.

    The flapping could be problematic to get right. If the flow is oscillatory, then the foil motion needs to be phased properly or it could actually make the drag worse. I would incorporate a spring in the foil supports that would spring-load the foil to center. A selection of springs could then be used to vary the natural frequency of the foil motion to better tune it to the flow.

    But I wonder if it's even necessary for the foil to move at all. If the flow is oscillating as it comes off the transom, the Katzmeyer effect will produce some thrust from a fixed foil.
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Yes, they could to be due to the legs of U-shaped vortices from the sides of the hull. (So where do these vortices detach? At the transom edge, or further forward on the hull?) They could also be due to unsteadiness of the flow.

    The flow behind the transom depends on the transom depth-based Froude number, and there are at least four different regimes that have been identified.

    IMO, a transom stern is not a particularly good hydrodynamic choice at the low Froude numbers achievable by human rowers. And the variation in speed during the rowing stroke makes it all the more difficult to get clean flow off the transom. I wonder if converting to a canoe stern would be better than adding more complexity in the form of a fixed of flapping foils.

    Cheers,
    Leo.
     
  8. clmanges
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    clmanges Senior Member

    The vortex shown does not wrap around the transom; it's about half as wide, and appears about four or five feet behind the boat.

    If that's true, then what is the nature of the other turbulence I'm seeing?

    Katzmayr is one of the things I haven't researched yet; I'm basing the idea of a moving foil on the quote from my first post:
    I know that the situation with my boat has different conditions than those of the experiment in the paper, though.

    It would be pretty easy for me to arrange the foil to be either fixed or free-moving. Adding a spring would be only a slight complication, I think.

    What about adding a little strip of flexible material to the TE of the foil, to act as a sort of automatic camber?

    Thanks for the input.
    Curtis
     

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  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It would help to know the transom depth-based Froude numbers.

    What are the speed variations during the rowing stroke?

    How deep is the transom at rest?

    Leo.
     
  10. clmanges
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    clmanges Senior Member

    I've actually considered something like this, but haven't yet figured out how to do it on my plastic hull. Any ideas?
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Just another quick observation... When you pull on the oars, your upper body and arms move towards the bow, which means the boat will decelerate.

    With sliding seats, the maximum speed of a rowing shell is attained when the rowers are sliding towards the stern, and slowest when they are accelerating towards the bow.

    Leo.
     
  12. clmanges
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    clmanges Senior Member

    Leo, I can't help you with speed variations . . . I don't really know how fast I'm going in general (except that it's not nearly as fast as the kayaks and canoes) -- let alone on a second-by-second basis. It's a real slug.

    Transom depth at rest is about four inches, I think. This is the same boat I referred to in my other thread:

    http://plasticboats.com/
     

  13. tom28571
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    tom28571 Senior Member

    From a momentum transfer point of view, that is true but the net transfer has to be zero, not counting friction.

    The second part implies that the max speed is when the boat is coasting and slowest when thrust is being applied. Wish I had a car like that. Not likely.
     
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