sail aerodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by Guest, Mar 21, 2002.

  1. Doug Halsey
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    Doug Halsey Senior Member

    Pressure is not the curvature of the flow. Nor is it the boundary layer.

    If the flow does not follow the shape of the sail, that can often be deduced from the pressure distribution.
     
  2. Skyak
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    Skyak Senior Member

    By conservation of momentum the force on the foil must equal the change in momentum of the fluid which is a function of curvature and velocity which is also a function of curvature.

    Flow separation can certainly be detected in the pressure distribution. What I am saying is that you need the sail curvature to know what needs to be done about it or the pressure sensors are no better than a tuft of yarn.
     
  3. Doug Halsey
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    Doug Halsey Senior Member

    Of course, there will be curvature somewhere in the flow, but you do not need curvature at a particular point on a surface in order for its local pressure to be varying. A foil doesn't need any curvature anywhere on it to generate a force.


    If your display indicates that the head of the sail is stalled, you would know that you need to add twist, regardless of what the current shape is. In this example, a tuft of yarn might tell you the same thing, but it might not be as visible & you can probably get better resolution with the sensors to detect smaller problems.

    The sensors could also help you optimize more subtle things not involving flow separation, that telltales aren't sensitive to - like variations in the spanwise load distribution.

    Especially in real-time applications, knowing the actual shape may be less important than knowing how you should change it.
     
  4. Skyak
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    Skyak Senior Member

    I don't know what to do about any pressure distribution without knowing what the shape of the foil is -and I would say that you don't either. All of these actions are based on an assumption of foil shape and it's reaction to input. I am not saying you don't know your sail or what to do, I am saying that assumption brings error.

    With shape and boundary layer measurement, a program can completely optimize and control the foil.
     
  5. Doug Halsey
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    Doug Halsey Senior Member

    I'm not saying that it wouldn't be desirable to know the shape, just that there are situations where the pressure data alone can be very helpful.

    But it seems like you're saying that if the pressures (or the tufts) tell you that the head of the sail is stalled, then you wouldn't think to ease the vang unless you knew the shape?
     
  6. BOATMIK
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    BOATMIK Deeply flawed human being

    Problem with small boats, of course, is the lack of man/personpower.

    It's a lot of fun to play around with smoke trails or readouts, but if you can't feel the maximum power through the seat of your pants without actually luffing the boat or luffing the sail then there are far bigger steps to be made. That kick in the pants when a decent boat, set up properly hits the right AoA is unmistakeable.

    The big step of course is to move this subtle area of sensing by collecting auditory, visual, balance into action using muscle memory rather than requiring conscious movement - which can all be done in the one apparatus - oneself.

    Pretty amazing really.

    One thought is, that with the pressure strips, the range of reliable calibration of the standard device might not be relevant to boats sailing in much lower apparent wind velocities. The AC boats don't have to because 1/ they are fast and 2/ they cancel racing when it is below a certain windspeed which is quite high relative to what most of us spend quite a bit of time in.

    The other reason is their use. Sailplane and aircraft tests use them because they can find areas of separation and reconfigure flaps, foil sections, gaps between elements and the like - much like the AC guys need to know.

    Good idea about fitting them to a batten pocket!!

    Interesting discussion point though.

    Cheers
    MIK
     
  7. Skyak
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    Skyak Senior Member

    What I said is that using MEMs to measure on the AC wing is far more useful because the wing surface is well known and adjustments are precise -alpha, camber, and twist.

    Applying the the same technology to soft sails is less useful because you do not know the shape precisely and cannot adjust it precisely. I offered my own scheme that would capture sail shape and boundary layer measurement that I consider much more useful.

    In your example of the head stall -you loosen the vang, but maybe it was already loose -the main sheet is too tight and the traveler is too far leeward. Or maybe a batten caught on the backstay...or didn't pop through after a tack... Soft sails have too many overlapping controls, too many degrees of freedom, and too many complexities to be controlled without knowing shape.

    You could use pressure data? GOOD NEWS! It's easy and priced reasonably! I can help you if you like. I found a good MEM pressure sensor ~$4 to less than $2 in volume. ST also makes a huge line of cheap micros and bluetooth connection should work fine to get the data to a tablet or phone.
     
  8. Skyak
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    Skyak Senior Member

    MIK,
    even a cheap sensor has plenty of precision 0.2mBAR relative, 2.6 absolute, so there is no problem with wind speed.

    Flogging conditions of sails are so destructive nothing can survive. This is another reason these MEM strips make so much more sense applied to a wing.

    Stretching sails is an art. There are masters of that art that like it that way. I love all the visceral inputs and outputs of sailing, but if I had my way I would make the sail with the same precise control the AC boats have -alpha, camber and twist...
     
  9. Erwan
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    Erwan Senior Member

    For the boundary layer issue, I think Mark Drela & Tom Speer have already provided the topic with interesting comments in "Sail Aerodynamics"
    The boundary layer is likely to be turbulent all the way, transition starting at around 5% of the leading edge.
    Identifying area with, and measuring the drag price of separation or the lift give up due to separation could be more useful ??

    Happy Sunday
     
  10. CT249
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    CT249 Senior Member

    Don't tufts already provide an accurate indication of separation?
     
  11. Skyak
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    Skyak Senior Member

    We don't have an issue with boundary layer. If anything we have a disagreement on what there is to know about it -likely by placing different values on experience and theory. I am arguing for using emerging technology to make the link between soft sails and the theory that today's personal computers are capable of modeling.

    Mark D. and Tom S. have done a great job teaching fluid dynamics here and elsewhere on the web. It is fair to say that what I have learned from them is what inspires me to argue with sailors of much greater experience.

    About a page has passed and it seems that I am promoting and others are criticizing a system I have not even detailed yet. My thought is I should do a bit more investigating on sensor accuracies and start a new thread with the proposal for closed loop development of sails and trim.
     
  12. Skyak
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    Skyak Senior Member

    Yes, but separation is just one condition, it can initiate in places you don't have a tuft, and tufts actually contribute to boundary layer growth.

    Measuring the boundary layer tells you so much more. It tells you about drag, mass defect, and is the best check of theory vs reality.
     
  13. daiquiri
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    daiquiri Engineering and Design

    And how would the sailor on board use the boundary-layer data thus obtained?
     
  14. powerabout
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    powerabout Senior Member

    he would collect it and give to so the company that makes the CFD software for sails and in the long run we get better modelling software.
    New sensors are certainly a great gift to add real data and hence further all this theory
    Look at what happened to "modelling of foils" somebody made one that software cant explain so I guess we have a new model now?

    the order of the world always has been;
    1. smart guy invents it
    2. university makes a course on it
    3. software vendors follow up ( quite often software designed by a uni)
    4. future graduates slowly improve it
     

  15. Mikko Brummer
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    Mikko Brummer Senior Member

    So how would you measure the boundary layer on board a sailboat?
     
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