sail aerodynamics

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

  1. philSweet
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    philSweet Senior Member

    I'd like to go back and revisit the video posted by Mikko in post 481. Specifically, I'd like to look at those trailing edge vorticies again. I had started to post this question a couple times before, but there was always something else being worked on.

    There were two explainations offered - Mikko talked about convection of separation for the windward side, and TSpeer mentioned a starting vortex. I'd like to look at the problem using classical dynamic response - specifically, Theodorsen's lag function.

    Mikko - can you tell us what the Cx,Cy,Cz values are in the corner of the video?

    Also, can you show us the pitch rate, yaw rate, and roll rate functions that were used so we can tie a state vector to the video images?

    My suspicion is that there is a dynamic advantage to the sloop rig, specifically, an advantage to using a particular sized jib (in absolute terms) due to its ability to avoid the worst-case zone of Theodorsen's model. By being able to spit out small vorticies in a timely manner, the lift is greater than if a monorig of the same planform had been used. The monorig would have to shed the vorticies at a different phase lag and would require different helm response to try to manage the lift.

    I'd like to isolate the roll motion from the pitch motion and look at that first, if possible. I think it is likely that roll dynamics, which are hard to see, are primarily responsible for triggering those vorticies.
     
  2. brian eiland
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    brian eiland Senior Member

    Rotating Spar on a J-90

    Diaquiri,..I've searched thru my external hard drive and not found that article or news clipping.

    But I did find this report that likely if we could find the subsequent reporting to this one on their rotating mast experiments on their J-90 would prove interesting
    Blackwing: Rotating Spar on a J/90
    Interesting that a LEADING EDGE device should prove very effective ;)
     
  3. brian eiland
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    brian eiland Senior Member

    J-90 specs

    http://www.jboats.com/j90-performance
    ...almost mastheaded, and sizable genoa ;)
     
  4. DCockey
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    DCockey Senior Member

    Not surprising to me. Leading edge shape can have a substantial impact on whether the flow stays attached, or how quickly it reattaches if it separates. That in turn can affect drag and maximum lift.
     
  5. daiquiri
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    daiquiri Engineering and Design

    There is a good reason why both trailing and leading edge high-lift devices are used on airplane wings. The attached drawing schematizes the effects of the two on the lift and drag curves of the wing. Essentially, both L.E. and T.E. devices modify the camber of the airfoil, but they act at the opposite sides of the foil chord thus causing different effects.

    The drawing should be compared to the explanation given by Prof. Drela in the post #576 (link: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-39.html#post745109), as they are complementary to each other.

    The trailing-edge device increases the local angle of incidence of the foil in the aft part of the chord. Hence, for a given AoA the CL increases through the increase of the CL,0 (lift coeff. at zero AoA), and it is highlighted by the Thin Airfoil Theory (T.A.T.) through the lift/incidence influence function f_a(x), explained in Prof. Drela's attached pdf file.

    The same lift/incidence influence function f_a(x) does not indicate a possibility of increase of the CL through the modification of the leading-edge camber. That's because the T.A.T. is an inviscid theory which can evaluate the CL,0 but cannot estimate the stall angle of the airfoil. And the L.E. device does exactly that - it increases the lift by extending the lift curve towards higher stall AoA, while maintaining essentially the same CL,0 and lift curve slope (the latter one being equal to 2*Pi for all airfoils in T.A.T.).

    Hence, both L.E. and T.E. devices do increase lift and change the polar curve of the airfoil. But they do it through different aerodynamic mechanisms, only one of which is captured in detail by the T.A.T.

    Cheers
     

    Attached Files:

  6. Mikko Brummer
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    Mikko Brummer Senior Member

    I looked at the sims again, hiding the sails, and definitely the vortex is not only due to convection of the windward side. There is a vortex detached from the windward side/luff, travelling to the leech, but the vortex from the leech is generated a little later during the the bow pitching up, detaching from the leech when the bow starts to pitch down again. There is plenty of lateral movement, too, at the height of the upper jib leech. So it could be the roll dynamics like you suggest.

    When halving the motions, the vortex from the leech disappears. Likewise with the origin of the motions, the Star, but the Star had double the motion.

    Those Cx,Cy,Cz values are related to the corresponding axial forces (drive, sink, heel) but cannot be used for anything, partly because the ref velocity is true wind, not apparent, but also because the waves are there as a physical element affecting the coeffs. I cannot post the motion functions here but I can send them in a private communication.

    It could be that the period of the motions is too short for a boat this big. I could run the sim without the roll and see what happens (when I get the time). From a practical experience, I would say that good driving would be one that tries to minimize roll (constant heel) (?). Also, from olympic coaches I hear that the best sailors appear to be able to keep up a constant, steady speed, no accelerations/decelerations, and they claim that a trained eye actually can see this from the outside, coach rib.
     
  7. CT249
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    CT249 Senior Member

    ....and rated much, much slower than the comparable Thompson 30, which has a low foretriangle and a smaller genoa.

    However, PHRF rated Blackwing considerably slower than the standard J/90 - 60 seconds for Blackwing compared to the 51 recommended by the designer for the standard J/90. A recent ad for Blackwing states that "She has since been converted to a sloop rig to the original rig plan supplied by Rod Johnstone."

    So if J/90s teach us anything about rig geometry, it would be that una-rigged wingmasts are slow in yachts, that big foretriangles are slow in yachts, and that a fixed mast with a medium foretriangle is fast in yachts. Of course, it's a lot more complicated than that, but it does seem to be hard to see the Blackwing experiment as anything more than another entry in the long list of failed wing masts.

    With great respect to Ben Hall, a great sailor, maybe it wasn't conservatism that stopped other people trying wingmast una rigs in sportsboats, but the fact that they knew it wouldn't work.
     
  8. tspeer
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    tspeer Senior Member

    I think it might be helpful to put some numbers to this discussion. Here is a wingmast design based on the Clark Y airfoil, that is 10% of the total chord. The smooth lee side condition is 35 deg of mast rotation. I've shown rotations of 0, 10, 20, 30, 35, 40 and 50 degrees.

    [​IMG]

    Here are the calculated polars for these sections:
    [​IMG]

    For each angle of attack or lift coefficient, there is a mast rotation that produces the minimum profile drag. It's only in the neighborhood of 6 - 7 degrees angle of attack that the smooth lee side profile (blue curve) has the best performance. Less mast rotation is needed at lower lift, and at the highest lift the mast needs to be over-rotated from the smooth lee side condition.

    I've attached plots of the pressure distribution and boundary layer displacement thickness for each mast rotation, taken at the angle of attack that produces the best lift/drag ratio for that rotation. This isn't exactly the same as the envelope of minimum drags, but it's close.

    What's interesting is the location of the stagnation point is very similar for all of these cases. It's near the leading edge, a little to the windward side. The extent of the separation bubbles varies quite a bit across the range, so going by telltales on the sail to judge the size of the separation bubbles is not going to be a good guide to trimming the mast. But something that indicates the stagnation point location will be a consistent guide to the best mast rotation.
     

    Attached Files:

  9. CT249
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    CT249 Senior Member

    Fascinating - thank you very much!

    The dramatic change in upwash interests me, because I assume that it would cause significant effects on a jib if one was added. Would it be true to say that an over-rotated shape could be more beneficial on a sloop than on a una rig, because the upwash would improve the jib's angle of attack? Or would the jib affect the upwash and the entire flow so much that lessons cannot be drawn from these illustrations of a una rig?

    The information you've been giving us is the first theory I've seen that explains why de-rotation can be fast in a breeze, which is something that many wing-mast classes know in practice.
     
  10. Doug Halsey
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    Doug Halsey Senior Member

    I've always thought of wingmasts in terms of reducing drag, but seeing the mast/sail geometries side-by-side for all the different rotation angles really points out what a tremendous tool they can be for controlling the camber.

    No doubt, that's obvious to many of you, but these figures really gave me a better appreciation of it.
     
  11. tspeer
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    tspeer Senior Member

    The amount of upwash is due to the amount of lift being produced by the section. Here is the same section at a constant 10 deg angle of attack, with different mast rotations. A better comparison would have been to have kept the same position of the sail and rotated the mast, but I think these still show how you can trim the sail and then rotate the mast to suit.

    The leech of a jib is far enough away from the mast that the main effect is to change the apparent wind angle to the mainsail. The details of the flow around the mast and main are essentially the same, and you'd trim the mast the same way.

    When you're dealing with a sloop, people put a lot of attention on the 2D section interaction between the main and jib. But it's easy to forget about the induced drag and 3D effects. Assuming the flow is mostly attached, the induced drag is much more important than the profile drag, and getting the best compromise between the height of the center of effort (controlled by twist) and the lift is very important, too. These are controlled by the combined spanwise lift distribution of the main and jib.
     

    Attached Files:

  12. BobBill
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    BobBill Senior Member

    Clueless me reads, ten or so times and loves it...and has, in the back of head the idea that very very soon we gonna see practical begin to dominate foilage...meaning air and water foils...with the former getting simpler. So simple, relative to practicality, the soft wing will end up leading edge, if the combo sail doesn't take over...

    But the above is rhetorical, do not want to detract from or alter direction of thread.
     
  13. Mikko Brummer
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    Mikko Brummer Senior Member

    Attached, the spanwise distribution of lift & drag of the complete boat, including hull & superstructures (the crew on the rail as well, although not shown).

    There a kink to zero lift at approximately the heeled deck height - not sure where this comes from, there's a considerable positive pressure on the fore deck inside the jib, from lift carry over. Maybe that has a component against the lift of the hull side, as well as vortices along the cabin sides, and inside the cockpit.

    A positive kink due to the edge vortex under the boom, and a buldge in the drag distribution above the jib - this probably more due to flow separation in the mainsail head than a tip vortex there.
     

    Attached Files:

  14. Mikko Brummer
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    Mikko Brummer Senior Member

    I drew this sketch to put into perspective what Prof. Drela states in post 576. Showing it to some sailors the comment was "If that's true, my mainsail trimmer is going to claim a very substantial raise". The red areas control 50% of lift, and together with the blue 82%.

    For the sloop configuration, the narrow red strip in the mainsail leech controls most of the lift, while the jib actually carries most of it.
     

    Attached Files:


  15. brian eiland
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    brian eiland Senior Member

    I don't understand what you are saying,...particularly about the jib 'carrying most of it'??
     
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