Crow instability, hybrid contrails, and pendules.

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.
 

Mick West

Administrator
Staff 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.
.

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:

 

A380

Senior 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.

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.
 

Jan-61

Member
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:


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.
 

A380

Senior Member.
Can you post an example of a "zipper shaped" contrail?

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



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:

 
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Jan-61

Member
Can you post an example of a "zipper shaped" contrail?

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



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

Right, these I meant, thnx :)



Jan.
 
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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
 

JRBids

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



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


5-11code copy.jpg

Oh, the "code"? ;)
 
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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.
 

Trailspotter

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.
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
These contours of ice particle area density show contrail development over twenty minutes after the aircraft has passed as seen by an observer on the ground. The periodic puffy structures are formed by the Crow instability and persist long after the vortices themselves have dissipated.
Content from External Source
 
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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?
 

Trailspotter

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?
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.
 

skephu

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.
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.
 

Trailspotter

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


This 3D rendering of aircraft wake development shows jet exhaust concentration as a translucent isosurface, with isosurfaces of positive and negative vorticity in red and blue respectively. The images are generated, from the top, 60, 90, and 120 seconds after the aircraft has passed. The initially parallel wing-wake vortices are perturbed by ambient turbulence and form periodic vortex loops that hasten the dissipation of the vortex system (the Crow instability).
Content from External Source
 
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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?

 

Trailspotter

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?

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.png
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Trailspotter

Senior Member.
How exactly does "vortex-entraining" work?
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/

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.
 

Mick West

Administrator
Staff member
How exactly does "vortex-entraining" work?

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)
Entrainment is a phenomenon of the atmosphere which occurs when a turbulent flow captures a non-turbulent flow. It is typically used to refer to the capture of a wind flow of high moisture content, or in the case of tropical cyclones, the capture of drier air.
Content from External Source
 
Huh? Surely at cruise altitude all wake vortices are basically the same?


Moving this to a more appropiate thread
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.
 
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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)
Entrainment is a phenomenon of the atmosphere which occurs when a turbulent flow captures a non-turbulent flow. It is typically used to refer to the capture of a wind flow of high moisture content, or in the case of tropical cyclones, the capture of drier air.
Content from External Source

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?
 

Mick West

Administrator
Staff 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?
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.
 
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