sail aerodynamics

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

  1. brian eiland
    Joined: Jun 2002
    Posts: 4,920
    Likes: 172, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    BOOK reference, Understanding Aerodynamics....

    Tom, much thanks for that reference.

    I was looking thru the 'preview' offered on-line, and particularly at pages 329-334 (but missing page 330).
    Multielement Airfoils & the Slot Effect
    I would really like to have a hard copy that I could read over a number of times to try and absorb more thoroughly what was being said (age limited learning curve :)). And I would really like to see page 330.
    What I did notice at the very beginning of his presentation is that he made a definitive distinction between the slot widths (gaps) used in aeronautical applications (much smaller relative to overall chord than.....). That next page was missing from the review :( .

    I suspect he was going to draw some distinctions between aircraft explanations and sailing explanations regarding the slot effect.


    I'm afraid in the overall scheme of things that book has just too much info for me to consume. particularly in relation to amount of sailing aero included,....and the rather hefty price.

    But thanks again for that reference





    For my dense mind, would you mine just re-explaining this "suppression of the leading-edge suction peak" ?
     
  2. brian eiland
    Joined: Jun 2002
    Posts: 4,920
    Likes: 172, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    ....from that book review again, Understanding Aerodynamics....

    Page 332 introduces the subject of 'dumping velocities/effects', which I recall Tom you have posted before. Ah, here is an older posting of yours I pulled up from another forum.

    http://steamradio.com/pipermail/multihulls/2001-September/007046.html

    http://www.boatdesign.net/forums/sailboats/code-zero-sails-aerodynamic-questions-53291.html#post737374
     
  3. philSweet
    Joined: May 2008
    Posts: 2,281
    Likes: 161, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Doug, does your hull's hydro drag model include induced drag from side force? And the logical follow-on - Are the sideforces and their associated hydro draq being recomputed when you change the sail drag?

    I'd like to see the same chart run on incremental sail lift. It also has a crossover, but a different one.
     
  4. Doug Halsey
    Joined: Feb 2007
    Posts: 426
    Likes: 113, Points: 53, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    Yes, the hydrodynamic drag due to sideforce is included. Unlike the basic curve of hull drag versus speed, however, I didn't have any experimental data, so I just used standard wing theory, with an input parameter that lumps together the effects of the presumed foil span & efficiency factor. Since these calculations were focused on downwind sailing, most cases involved small sideforces & the drag due to sideforce wasn't very important. I repeated the calculations with the parameter set to zero, and also with it set to 10 times the value used for the chart, and they still told the same story about the effect of the aero drag on the downwind performance.

    Yes, the code balances all the aero & hydro force components for each point on all the curves it computes.

    I'm not quite sure what you mean. Are we still talking about downwind sailing? Or cases where increasing the aero lift would run up against heeling moment constraints? Or cases where the hydro drag due to sideforce would become large? Studying the possible beneficial effects of adding aero drag involved a very limited set of possibilities; studying aero lift seems a lot more open-ended.
     
  5. markdrela
    Joined: Jun 2004
    Posts: 306
    Likes: 28, Points: 28, Legacy Rep: 324
    Location: MIT Aero & Astro

    markdrela Senior Member


    Pages 1,2 of the PDF show the surface Cp(x) distributions and pressure force vectors with a jib and mainsail far apart.
    Pages 3,4 of the PDF show the same things when the mainsail leading edge is brought close to the jib trailing edge, with all the angles and freestream held fixed.

    Note that the original strongly negative pressures (and hence high velocities) at the mainsail leading edge have been greatly suppressed. But the opposite happens at the jib leading edge.

    Note that the CL is virtually unaffected by this rather drastic change in the overall geometry. So Smith misstated his "Circulation effect". This effect doesn't increase the lift, but only redistributes it, from the mainsail onto the jib.

    The second configuration will definitely have a greater maximum lift, but the overall incidence must be increased to obtain it.

    If the mainsail element has an ugly unfaired mast, then the second configuration will have much less drag, because the mast is now in a low-velocity region. And there's now a short favorable pressure gradient immediately behind the mast, so the separation behind the mast can quickly reattach, so it better negotiate the adverse pressure gradient over the rest of the mainsail.
     

    Attached Files:

  6. Doug Halsey
    Joined: Feb 2007
    Posts: 426
    Likes: 113, Points: 53, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    I assume you're talking about AMO's Wright Brothers' Lecture, which is long & fairly difficult to follow at some points. But, after reading & rereading portions of it, I don't think what he said would contradict anything you just wrote.

    The attached file excerpts his summary of the 5 primary effects of gaps. In the section on the "Circulation effect," he only mentions the effect that the downstream element has on the forward element, not the effect on the overall circulation. In the later section elaborating on this, he even mentions that "...at 10 deg. angle of attack, according to inviscid-flow calculations, the single-element airfoil carries slightly more load than the three-element system."
     

    Attached Files:

  7. Mikko Brummer
    Joined: May 2006
    Posts: 514
    Likes: 50, Points: 28, Legacy Rep: 258
    Location: Finland

    Mikko Brummer Senior Member

    Uli,

    Even if the jib isn't sealed in its back part, there's the gunwhale & the deck & the cabin forming the "foil". The curve is created integrating slices in the x-direction. But you are probably on the right track, I should take a closer look at the slices at zero lift height. Thanks.

    Another interesting curve would be dividing the lift curve with the local "chord" width, for a cl-distribution - the lower part of the jib below the boom must be heavily loaded, which is why it stalls first when you bear away.
     
  8. philSweet
    Joined: May 2008
    Posts: 2,281
    Likes: 161, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    But I think this is being negated by high pressure on the heeled deck. This would also account for the dip in drag at the same height.
     
  9. CT249
    Joined: May 2003
    Posts: 1,228
    Likes: 67, Points: 48, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    Very interesting, and like you I'm glad to find that the figures bear out gut feeling and experience.

    As a matter of interest, do you have curves for hulls that are less widely differentiated in drag, and do you know what hulls are represented by available drag figures?
     
  10. Doug Halsey
    Joined: Feb 2007
    Posts: 426
    Likes: 113, Points: 53, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    I don't have a big collection of drag data - just the ones shown in Marchaj's Sailing Theory and Practice . (International Canoe, "Gimcrack", New York 32, 6 Meter, 5.5 Meter, "Narrow Finn", Tornado catamaran & A-Scow)

    My use of this VPP has been pretty limited. It's a Visual Basic implementation of a Fortran code I wrote in 1980 to give some students a research tool for class assignments. (I taught a "Theory Of Sailing" course at a local university.) That code sat unused for 20 years, until 2000 when I started working on my trimaran (Broomstick). Then the code just served as a stepping-stone in developing more elaborate codes specifically intended for hydrofoils. But these codes either have simplified drag formulas built in, or else read data files generated by a separate vortex-lattice code. In either case, there's no experimental drag data used.
     
  11. Mikko Brummer
    Joined: May 2006
    Posts: 514
    Likes: 50, Points: 28, Legacy Rep: 258
    Location: Finland

    Mikko Brummer Senior Member

    Great book indeed... one of the lessons I learned from it was that you can only draw so much conclusions from 2D aerodynamics analysis into 3D. Especially when the 3D case is as complex as a sailbot with several sails, masts, spreaders etc. For instance, zero skin friction is not a valid criteria for flow separation in 3D. Also, stagnation point (line really) is much more complicated in 3D.
     
  12. Mikko Brummer
    Joined: May 2006
    Posts: 514
    Likes: 50, Points: 28, Legacy Rep: 258
    Location: Finland

    Mikko Brummer Senior Member

    Great video by the author

     
  13. Remmlinger
    Joined: Jan 2011
    Posts: 241
    Likes: 20, Points: 18, Legacy Rep: 172
    Location: Germany

    Remmlinger engineer

    Thanks Mikko for this interesting link!
    Doug McLean's remarks about the misconceptions in explaining lift are a helpful clarification. After that, one is waiting for his "right" explanation, but that is somewhat disappointing. He himself calls it his "weak link". I think you can not explain lift without explaining the Kutta-Joukowski condition. Doug McLean doesn't even mention it. I still prefer Ludwig Prandtl's explanation of lift that he gave in his textbook in 1931.
    A side note: The Kutta-condition is only applicable in viscous flow. Without viscosity the rear stagnation point would be on the upper surface of the airfoil and there would be no circulation and hence no lift. So McLean's statement at 21:12 minutes in the video, that viscosity is not needed to create lift, is wrong!
    Uli
     
  14. BobBill
    Joined: Oct 2009
    Posts: 728
    Likes: 12, Points: 18, Legacy Rep: 157
    Location: Minnesotan wakes up daily, in SE MN, a good start,

    BobBill Senior Member

    Mikko, I too thank you for the vid. I am not a math person, nor aeronautical or marine architect type person. I do love physics and science but as sailor, just a swab who likes to build, pet and sail boats in the water. I watched the vid and most of it made sense to me, even some of the esoteric stuff, and raised some theory ?s.

    That said, I am stuck and thrive under my first instruction generally, when it comes to sail and boat trim, namely, "If it looks good, it is good, if not, change it!"

    But, every bit helps and I do like the academic side. Ya never stop learnin'! Merci encore!
     

  15. markdrela
    Joined: Jun 2004
    Posts: 306
    Likes: 28, Points: 28, Legacy Rep: 324
    Location: MIT Aero & Astro

    markdrela Senior Member

    That's not a good way to put it.
    The explicit Kutta condition is not needed in viscous flow (e.g. in a NS code).
    The explicit Kutta condition IS needed in inviscid flow to mimic an important lift-controlling viscous effect which was ignored -- namely vorticity shedding.

    I think what McLean meant was that the inviscid equations of motion are sufficient to very accurately describe the overall flowfield and hence the lift, which is certainly true. Their only missing piece is setting of the overall circulation. In a real flow this is set by viscous vorticity shedding. In the inviscid model this is set by imposing an explicit Kutta condition.

    Saying "you need viscosity to have lift" is like saying "you need a spark plug to make a car go". Well, sort of. The compressed air/fuel mix must be ignited, but exactly HOW it's ignited is a relatively trivial point.

    The explicit Kutta condition is an ad-hoc replacement to the spark plug, but functions just as well.
     
Loading...
Similar Threads
  1. Mikko Brummer
    Replies:
    52
    Views:
    2,034
  2. Grunf
    Replies:
    12
    Views:
    991
  3. wesley Sherman
    Replies:
    10
    Views:
    658
  4. Federico Ferretti
    Replies:
    3
    Views:
    1,039
  5. schakel
    Replies:
    4
    Views:
    1,016
  6. Yull
    Replies:
    10
    Views:
    1,266
  7. Manfred.pech
    Replies:
    7
    Views:
    1,536
  8. andysailor
    Replies:
    4
    Views:
    1,139
  9. CocoonCruisers
    Replies:
    75
    Views:
    6,690
  10. mustafaumu sarac
    Replies:
    25
    Views:
    1,506
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.