1. Jan-61

    Jan-61 Member

    Nice pics around here!
    About the 'crow instability' (what has it to do with crows?): by this we recognize 4-engine-jets. I noticed that when a B747/A380/A340 leaves a persistent ctrl, these 'hybrid' crow-instability-curls convert towards virga-shapes. I guess the virga-trail above the Berlin high-tension-pylons came from a 4-engine jet. Any other ideas?
    BTW, I read the Hybrid-ctrls-log on contrail science.


    Jan.
     
  2. Mick West

    Mick West Administrator Staff Member

    The crow instability (named after S. C. Crow) is just what happens to the wake vortices, and it not really a function of contrails. It's just the same (but invisible) even if no contrails form. You see the squiggles when the contrail gets entrained in the wake vortices, and that bit persists longer (the hybrid contrail). So you get them both with 2 an 4 engine jets.

    I think they might be more common with 4 engines though, as the outer engine contrail tends to feed more directly into the vortex.

    This video shows crow instability in the hybrid contrails of a two engine jet:

     
    • Like Like x 2
  3. A380

    A380 Member

    Yes, there are many misconceptions.

    Exhaust gases from the engines are not in rotation. Rotations behind planes result particularly from turbulences on the wings. Decisive are mass, form and angle of attack. If there is a heavy as Boeing 777 with two engines there are also shapes of Crow Instability and even with aircraft such as B737 and A320, they are seen.
     
  4. Jan-61

    Jan-61 Member

    Though I use to perceive these hybrids strikingly coming from 4-engine aircrafts, while then other aircrafts only generate exhaust contrails (maybe with them, this 'Crow instability' remains invisible, I am not that far introduced in the mere scientific background of contrails); although here in Europe. Indeed, in seldom cases I saw it also from 2-engine jets, be it usually in a more modest way (I can't remember sawing this so manifest as in the vid).

    I assume that the zipper-shaped contrails (virga?) have to do with the hybrid contrails.


    Jan.
     
  5. Mick West

    Mick West Administrator Staff Member

    Can you post an example of a "zipper shaped" contrail?
     
  6. A380

    A380 Member

    I think he means the wake vortices in the lower area of the contrail:

    [​IMG]

    They sink, they are blown or they dissolve. The upper section has not fallen into the wake vortices and remains relatively straight.

    Wake vortices can be heard at 00:28:

     
    Last edited by a moderator: Nov 20, 2013
  7. Trailspotter

    Trailspotter Senior Member

    • Like Like x 4
  8. Jan-61

    Jan-61 Member

    Right, these I meant, thnx :)



    Jan.
     
    Last edited by a moderator: Nov 20, 2013
    • Like Like x 1
  9. Jay Reynolds

    Jay Reynolds Senior Member

  10. CbIncus

    CbIncus Member

    Crow instability development over Chisinau, Moldova on April 30th, 2013.

    IMG_7398 (Копировать).JPG IMG_7399 (Копировать).JPG IMG_7400 (Копировать).JPG IMG_7401 (Копировать).JPG
     
    • Like Like x 1
  11. Jay Reynolds

    Jay Reynolds Senior Member

    Ten years ago I introduced a description of the Crow Instability in aircraft wakes to the chemtrails crowd. They weren't too happy when their 'big discovery' was explained.
    It took about a month before they got resigned to the facts and went off on another tear, this time claiming ship tracks were made by airplanes.

    Part 1

    Part 2
     
    • Like Like x 1
  12. JRBids

    JRBids Senior Member


    5-11code copy.

    Oh, the "code"? ;)
     
    Last edited by a moderator: Nov 20, 2013
    • Like Like x 1
  13. Ross Marsden

    Ross Marsden Senior Member

    • Like Like x 1
  14. Jay Reynolds

    Jay Reynolds Senior Member

    They called them hemmorhoids. Fairly descriptive, in an odd sort of way!
     
    • Like Like x 2
  15. skephu

    skephu Senior Member

    Can anyone give a more detailed explanation of how contrail pendules form? I tried to search the literature but didn't really find anything enlightening.
     
  16. Trailspotter

    Trailspotter Senior Member

    It is my understanding that contrail pendules also result from the Crow instability. At high relative humidity (RHi about or greater 100%) the exhaust contrails would spread fast and envelope the wake vortices. Having formed inside this envelope, the hybrid contrails would break up in loops and hoops, bulging out as pendules. In the first photo of #6 above, there is a denser loop shining through the pendulus on the right.

    See Large Eddy Simulation of Contrails at:
    http://flowgallery.stanford.edu/research.html
     
    Last edited: Mar 23, 2015
    • Agree Agree x 1
  17. Mick West

    Mick West Administrator Staff Member

    • Informative Informative x 5
  18. skephu

    skephu Senior Member

    Thanks! Now this explanation is quite heavy and technical. I'm trying to simplify it so that laypersons can understand it. But I don't understand it completely myself. So let's try to put it into simpler terms:
    1. As the airplane flies, it pushes down the air, which generates two huge counter-rotating vortices.
    2. The cores of these vortices spin very fast, and they will not stay straight; instead they start to assume a corkscrew-like shape.
    3. Where these two corkscrews (which are mirror images of each other) are closest together, they push down the air between them strongly. This is where the pendules will appear.
    Is this roughly correct?
     
  19. Trailspotter

    Trailspotter Senior Member

    1. Correct.
    2. Roughly correct. The shape of vortice core is actually not a helix (corkscrew-like), but an irregular wave.
    3. Hmm. Where the counter-rotating vortices come close, they break and reconnect forming series of loops. These loops then produce pendules.

    Watch again the video in post #2. It shows the evolution of the pair of counter-rotating vortices revealed by the entrained part of exhaust contrails. The remaining exhaust contrails dissipated before the vortices had broken up into loops. Now imagine that the exhaust contrail persisted longer than the vortices. In this case, the condensed vapour from the loops will not dissipate but remain as pendules under the straight part of the contrail.
     
    • Informative Informative x 1
  20. skephu

    skephu Senior Member

    I'm looking at Figure 11.2.iii in post #17. That one shows that there is strong downward motion where the two vortex cores are closest to one another, and that's where the blobs (pendules) form. In this phase there is not yet breaking/reconnecting/etc.
     
  21. Trailspotter

    Trailspotter Senior Member

    This 40+ year old theory has been revised. Look up the link^ I've posted above:

    [​IMG]
     
    Last edited by a moderator: Sep 10, 2017
  22. skephu

    skephu Senior Member

    OK so we should forget about Figure 11.2.iii in Scorer's book? So the pendules form in a later stage, after the vortices have broken up into individual loops?

    I found this interesting image that shows how the elongated horizontal loops eventually turn into vertically oriented loops. Are these vertical loops that correspond to the pendules?

    [​IMG]
     
    • Informative Informative x 2
  23. Trailspotter

    Trailspotter Senior Member

    Yes, the reconnected parts of the loops move apart and out of plane that may explain the pendules' shapes.

    Here is a similar series of my photos from November 22, 2012. They show different steps of evolution of the same contrail from an overhead flying plane, but do not necessarily picture the same section of it. The actual contrail images (3-5) are quite similar to the simulated images above.
    Crow_pendules.
    Individual large-size images are attached.
     

    Attached Files:

    Last edited: Mar 24, 2015
    • Like Like x 1
  24. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    How exactly does "vortex-entraining" work?
     
  25. Trailspotter

    Trailspotter Senior Member

    There must have been a Metabunk discussion on the wake vortices and hybrid contrails, but I can't search for relevant posts right now. Here is an image that I've picked from this thread:
    https://www.metabunk.org/debunked-i...aerosol-spraying-aerodynamic-contrails.t7856/
    [​IMG]
    It also can be seen as a visualisation of the wake vortices' formation by aerodynamic contrail. The entraining happens at some distance behind the aircraft, where the sheet edges rolls upon themselves with the formation of two tubes.
     
  26. Mick West

    Mick West Administrator Staff Member

    Same way tornado entrainment works. The vortex is lower pressure, so water can both condense and persist longer in it, but it can also capture(entrain) water particles from the outside air - i.e. from the exhaust contrails and/or the full wing aerodynamic contrails.

    https://en.wikipedia.org/wiki/Entrainment_(meteorology)
     
  27. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    Sounds plausible. On the other hand all aeroplanes have vortices but not that type of narrow core.
     
  28. Mick West

    Mick West Administrator Staff Member

    Huh? Surely at cruise altitude all wake vortices are basically the same?
    [​IMG]

    Moving this to a more appropiate thread
     
  29. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    That was basically my point. They are "basically the same", but they don't all result in narrow optically dense contrails of the type shown. Why?

    Some are obviously bigger or more irrotational than others, but I suspect latent heat release is a big factor.
     
    Last edited: Aug 2, 2017
  30. Trailspotter

    Trailspotter Senior Member

    Well, the wake vortices are basically the same, but their interactions with the engine exhaust very likely depend on the aircraft type and design.
     
  31. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    I am not convinced that this meteorological reference to entrainment is applicable to a continuous vortex that is unbounded, by the ground, for instance. Are you suggesting water vapour moves radially across the vortex?
     
  32. Mick West

    Mick West Administrator Staff Member

    Because that type of "hybrid" contrail depends on the atmospheric conditions. It's not really that the water particles are being sucked into the core, that always happens, it's that they persist longer in there.

    A) In some contrails the entire thing persists, so you can't see the vortex cores, even though they are still there, and are still entraining.

    B) In some contrails the entire thing dissipates, so again you can't see the vortex core.

    C) In some contrails the dissipation rate is such that the lower pressure in the vortex core maintains a ice-supersaturated state for longer, allowing the particles to grow and/or persist longer.
     
    • Agree Agree x 1
  33. Trailspotter

    Trailspotter Senior Member

    No, I do not think so. It got wrapped around and then probably tightened up.
     
  34. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    I'm not sure random YouTube videos of contrails are within the guidelines, but this is a good video of an aerodynamic contrail, and wake vortices wrapping up the engine exhaust:

    Source: https://www.youtube.com/watch?v=f22A3eML6Fo
     
    Last edited: Aug 14, 2017