1. HappyMonday

    HappyMonday Moderator Staff Member

    Twitter user KnoelWester has found an interesting trail over Bristol, and is interested in whether I've ever seen anything similar to the 'donuts' in this formation, and an explanation of what causes this.

    Anybody know the technical? I'm sure I've seen it mentioned here.



    http://twitpic.com/c82c0h

    [​IMG]
    Last edited by a moderator: Nov 20, 2013
  2. captfitch

    captfitch Senior Member

    I haven't pinned it down but when I see a trail like that I think turbulence. I say that because it's fairly rare to see a rough trail like that and I assume it's because no one typically stays in the rough air for very long.
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  3. Jazzy

    Jazzy Senior Member

    Vortices entrain and drag each other together under conditions of power or propinquity. They regenerate a NEW vortex, generally at right angles to their previous axes. This is described as a "sinusoidal mutual inductance instability".

    This enlightens:

    [video=youtube_share;XJk8ijAUCiI]http://youtu.be/XJk8ijAUCiI[/video]

    More can be found under "Crow Instability - More about vortices", "Trails seen from Space" at http://jazzroc.wordpress.com

    In this case the vortices are aircraft wake vortices, and ordinary aircraft will make "doughnuts" (out of the cylindrical tip wake vortices) in turbulent conditions or high-angle-of-attack conditions where greater energy is being put into the wake vortices. A heavily-loaded aircraft traveling slower than normal will do it, or a laden bomber with low aspect ratio wings traveling at high speed will do the same. The vortices behave this way whether they are visible or not. At high altitudes the engine trails are swept into the wake vortex and render them visible.

    The wake vortices of flight 175 were plainly visible, written in flames and smoke, briefly attached to WTC 2 after the impact, stuck to the tower face, and linked to each other with subsidiary rings and braids.

    The centerline of a vortex is a line of least pressure, and so will attach itself to a surface if it can, whether it's an aircraft wake vortex attached to a tower, or a tornado attached to the ground.

    Jay knew this stuff before I did.

    If you find the Japanese smoke ring experiment (GIANT VORTEX BOX 20M TEST) in YT's suggestions and play it, I hope you laugh as much as I did. :)
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  4. Mick West

    Mick West Administrator Staff Member

    It's not turbulence. What you are seeing is not even really a regular contrail. It's what I've been calling a "hybrid contrail", formed as a combination of both exhaust contrail and aerodynamic effects.

    It forms when the conditions are just right so that the lower pressure in the center of the wake vortices creates favorable ice formation/accretion conditions for a bit longer than the surrounding air, and hence larger and longer lasting ice crystals which sink a bit faster than the rest of the contrail.

    I took this photo at Mammoth Lakes a few days ago:

    [​IMG]

    The thick part is the regular exhaust contrail. The wiggly lines are the hybrid part, which has somewhat separated from the regular contrail by this point.

    The wiggle and clumping is from the crow instability, which really just affects the center of the wake vortex more than the trail in general - where you'll get the less dramatic "pendules"

    http://en.wikipedia.org/wiki/Crow_instability

    The separation depends on the view angle, and the relative persistence of the trails. Often you'll just get the hybrid contrails left behind in that kind of "chromosome" pattern.

    Also discussed here. with nice photos:
    https://www.metabunk.org/threads/929-Different-evolution-of-symmetric-and-asymmetric-contrails
    Last edited: Nov 20, 2013
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  5. Unregistered

    Unregistered Guest

    Interesting stuff. Can two wing vorticies merge into one? I've only ever seen wing vorticies in pairs in which I can imagine pairs of donuts. I would not say the donuts are at right angles to the original vortex either. Given me some thing to think about though, thanks.

    @KnoelWester
  6. Mick West

    Mick West Administrator Staff Member

    No, they can't merge into one because they are rotating in opposite directions, and are roughly equal in magnitude.

    There's some discussion of it in this 1972 book

    https://plus.google.com/photos/107393796095434664991/albums/5363662113705530081?banner=pwa
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  7. Mick West

    Mick West Administrator Staff Member

    The references to Aurora date back to a rumored experimental aircraft from the 1980s and 1990s, and the phrase "donuts on a rope" used to describe the contrails.

    http://en.wikipedia.org/wiki/Aurora_(aircraft)



    This is the 1992 Photo
    http://deepbluehorizon.blogspot.com/2010/11/its-baccccck.html
    [​IMG]

    Since then any kind of "pulsed" looking contrail has been given the same suspicion. But they are most likely all just regular contrail in different air conditions. More stable air would results in more distinctive "donuts"

    [​IMG]

    The donuts would seem more symmetrical if the trail is directly overhead. They are also more apparent with a single engine jet plane. The 11.2.3 example above was from a single engine fighter jet. The relative infrequence of such contrails (depending on where you live) makes seeing one more notable.

    All planes have two wake vortices, regardless of the number of engines, as the vortices come from the wings, not the engines.
    Last edited: Nov 20, 2013
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  8. Ross Marsden

    Ross Marsden Senior Member

    Correct, the original photo is of the spectacular Crow Instability.

    The photo above with the caption 'Pre 1972 "Donuts on a Rope" (from "Clouds of the World")' looks more like pendules than thye donuts in the photo above that captioned '(C) Steve Douglass'.

    Incidentally, the donuts are not a consequence of the Crow Instability.
    This paper The Effects of Aircraft Wake Dynamics on Contrail Development shows how both the pendules and donuts in contrails are predicted by the fluid dynamics of the situation.
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  9. Unregistered

    Unregistered Guest

    I won't pretend to understand that wake dynamics paper, but I get the picture. Thanks guys.
  10. Mick West

    Mick West Administrator Staff Member

  11. Jazzy

    Jazzy Senior Member

    I don't want to appear to be picky, but it can indeed be turbulence.

    What is needed to power any wake vortex to instability is increased angle of attack. A change in the real angle of attack is what results from turbulence. Even if it happens for only for a second at cruise speed, that is eight hundred feet of forward motion, which is easily enough space in which to generate a single "donut".
  12. Jazzy

    Jazzy Senior Member

    Superb.

    And there's only one of us with egg all over his face. :)
  13. Mick West

    Mick West Administrator Staff Member

    I see what you mean, but you can also get pretty much the same result from level flight through calm air. The crow instability is an exponential amplification of asymmetries between a vortex pair, so it does not take much to kick it off.

    You don't see it much because hybrid contrails are relatively infrequent, not because flying though rough air is relatively infrequent.

    And I think there's a lack of clarity here as to what is a "donut". There's the rings that sometimes form in a crow-instability hybrid contrail situation, and then there's the puffs that sometimes form along a linear contrail. Two radically different things.
  14. Jazzy

    Jazzy Senior Member

    In spite of my recent drubbing, I can see immediately here you have something wrong. Namely: "The vortex aerodynamic contrails indicate the regions that would form hybrid contrails at high altitude."

    The vortices you indicate emanate from deployed flaps during landing. At cruise speed and altitude these would be inboard, and generate nothing.
  15. Mick West

    Mick West Administrator Staff Member

    Bugger. I remember vaguely thinking that, then forgetting about it.
  16. Jazzy

    Jazzy Senior Member

    You may be right there. I formed the impression that a "puff" was merely a failed "donut". I need time to mull it over... :)
  17. Mick West

    Mick West Administrator Staff Member

    Question, does full flap deployment change the initiation point of the wake vortex on a wing from the end?

    [​IMG]

    Meaning - is what we see above still the plane's wake vortices? Or a different smaller vortex caused by the abrupt geometry change in the wing at that point? Is there still another vortex starting at the wingtip (I'm thinking there must be).
    Last edited: Nov 20, 2013
  18. Mick West

    Mick West Administrator Staff Member

  19. Mick West

    Mick West Administrator Staff Member

    Here's a handy addition:
    [​IMG]
    Last edited: Nov 20, 2013
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  20. Ross Marsden

    Ross Marsden Senior Member

    Referring to Post #18 "This is an interesting perspective:"

    There is a remark (under the linked image):
    The visible vortices from the outside flap edge are no puzzle, but that only one of them seems to have a rotating cloud around it is very interesting. Is there a whisp of something entering the scene from the far left? What is that?
  21. Mick West

    Mick West Administrator Staff Member

    I don't think so, I think it's a bit of an optical illusion

    [​IMG]
    Last edited: Nov 20, 2013
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  22. captfitch

    captfitch Senior Member

    When the flaps are out you get a whole new set of vortices that can be super strong and compact because there is absolutely no mitigation. Remember, wake vortices are essentially evidence of the massive lost and misdirected energy at the tips of the wings as the high pressure air wants to try to get back up to its low pressure friend above it. The wings are often designed to mitigate the vortices. Flaps, however, are used at low speed and the efficiency of flight isn't as important then, so who cares if you get massive vortices. I have seen a couple pics where there are vortices off all four tips.
  23. MikeC

    MikeC Senior Member

    Not only is there no mitigation of Flap Vortices, but the pressure differential that causes them is much higher than wingtip vortices - hence they often cause contrails when wingtip vortices do not.
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  24. captfitch

    captfitch Senior Member

    Also, not to get too deep, but the tips are at a slightly lower angle of attack by design to enable the wing to stall "correctly". So they may not be creating such huge pressure differences even in clean configuration. When you dirty up the wing with flaps the sections have radically different AOAs and the portion at the flap has a ton of more energy to force out.
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  25. captfitch

    captfitch Senior Member

    Beat me too it. Exactly right mike.
  26. Jazzy

    Jazzy Senior Member

    That's a head-banger. My feeling is that a wake vortex is generated by the whole wing (on one side), but there's an equally-large additional vortex when the flap comes down, and each affect each other downstream. The tip vortex won't show much, except for a spiral fluctuation in the flap vortex centerline, which you can sometimes see. If the tip vortex does show, it will also be slightly spiral by interaction with the flap vortex.

    The vortex centers are reduced pressure, and colder than their surroundings. A visual demonstration of Boyle's Law, or maybe it's Charles' Law, er, P*V/T = k

    Captfitch rules...
  27. captfitch

    captfitch Senior Member

    I'm also reminded of the rule of thumb that states the worst wake turbulence is behind a heavy, clean, slow aircraft. The aircraft in these pics are dirty and slow but also light so I would imagine the primary vortex created by the entire wing span is both diminished (due to the new escape path the air took around the outside of the flaps) and possibly altered by the existence of the adjacent vortices at the flap. So jazzy, I would guess your partially correct except the vortex sizes are probably quite a bit different, owing to the different AOAs of the causing surface. Mathematically I could suppose that because the aircraft is still moving the same mass of air regardless of configuration, the resultant energy of the two vortices per wing when dirty would be the same as one vortex when clean. Give or take I'm sure.
  28. Mick West

    Mick West Administrator Staff Member

    I think that would be "worst" in the sense of "longest lasting relative to size". i.e. the vortex is so stable that it can flip a Cessna several miles behind. "Heavy" makes it big and powerful. "Clean" makes it stable, as flaps or gear would introduce major instabilities which would make the vortex dissipate much sooner. "Slow" seems counterintuitive, but the faster the plane the more likely the wake turbulence will be chaotic and not smooth.

    Think of an oar moving through the water and the vortex it creates. If the oar moves too fast it's going to churn up the water. If the shape of the oar is broken up, the the vortex is not going to be messed up.

    See here about 3:30 in:

    ‚Äč
  29. Mick West

    Mick West Administrator Staff Member

    Last edited: Nov 20, 2013
  30. captfitch

    captfitch Senior Member

    Wow, that's impressive and I wouldn't have guessed it would have looked like that. I wonder if that's coincidental?
  31. Mick West

    Mick West Administrator Staff Member

    I don't think so. I think it's like this:

    [​IMG]

    But the perspective foreshortening from the other side makes it look unlike that.
    Last edited: Nov 20, 2013
  32. Mick West

    Mick West Administrator Staff Member

  33. Jazzy

    Jazzy Senior Member

    The flap vortex is indeed the main vortex. No coincidence. Wonderful picture.
    Last edited by a moderator: Nov 20, 2013
  34. Mick West

    Mick West Administrator Staff Member

    Why do you say that?

    There's a very narrow vortex coming from the flap, but it seems to me this just gets entrained in the much larger wake vortex, and end up in the center. The wake vortex originates at the tips of the wing, but that's not really what causes it - that's just the edge of the field.

    In the below image, the flap vortices are just barely visible.
    [​IMG]

    In this image it looks more like it's wrapped around the outside:

    [​IMG]

    http://www.dailymail.co.uk/sciencet...ur-trails-sonic-booms-created-planes-sky.html
    Last edited: Nov 20, 2013
  35. Jazzy

    Jazzy Senior Member

    I said it was a head-banger. It seems to me that the flap vortex runs dead center of the main vortex, and therefore is the main vortex. It appears maybe to be another form of illusion. I give up... LOL
  36. Mick West

    Mick West Administrator Staff Member

    The creation of the wake vortices is described as the "rolling up of a vortex sheet"

    [​IMG]

    [​IMG]

    So really it's the entire wing surface creating the vortex. The flap vortex seems like a smaller version of this.
    Last edited: Nov 20, 2013
  37. Cairenn

    Cairenn Senior Member

    And folks wonder why we still don't understand what all happens in a tornado well.
  38. MikeC

    MikeC Senior Member

    There's some good basic info about vortices in the New Zealand CAA's booklet on wake turbulence - it's a fairly quick 700k download.

    It is mostly about landing/takeoff situations, but has some nice diagrams about the nature of vortices - including from helicopters :)
  39. Mick West

    Mick West Administrator Staff Member

    Three dimensional vector field advection in an invisible medium is pretty much impossible for the poor human mind to grasp. The visualizations are always a kind of 2D approximation, as you have to cut away most of the volume to make the interesting bit (generally some kind of isosurface) visible, but then you can't really see the whole picture.
  40. Cairenn

    Cairenn Senior Member

    The first heavily filmed tornado was on April 2, 1957 in Dallas. We have a lot footage, but not a lot of instrument readings. I know that OK City tornado in 1999 was the first one that they managed to bracket with doppler.

    I think 'rocket science' is easy compared to weather
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