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[Phys-L] physics of Kelvin-Helmholtz clouds



Hi Folks --

I suppose you've all seen the viral photo of Kelvin-Helmholtz clouds
aka fluctus clouds in Virginia earlier this week.
https://weather.com/news/weather/news/2019-06-20-kelvin-helmholtz-wave-clouds-virginia
https://cloudatlas.wmo.int/clouds-supplementary-features-fluctus.html

If you see such clouds, take pictures immediately, because they
are not going to last long.


The essential physics is not particularly complicated. Here's a
video that is very simple and "mostly" correct:
https://www.youtube.com/watch?v=qgamfo86FQo [1]

Also some experiments (just data, not much explanation):
https://www.youtube.com/watch?v=mf_143gkKSQ [2]

You can find more by googling.

Some corrections:

-- Part of what video [1] says about the Bernoulli principle
is wrong.
In particular, the timing and alignment of the arrows in the
video is incorrect, as is the associated narration. In fact,
air flowing above and below the wing does not arrive at the
trailing edge at the same time. The air that goes over the
top arrives earlier, even though it traveled a longer path.
This is due to circulation. Here's the correct timing:
https://www.av8n.com/irro/lecture_e.html

-- Given a known correct velocity field, applying Bernoulli's
principle is straightforward. The tricky bit is calculating
the velocity field. The calculation for the outside of a
wing is vastly more complicated than for the inside of a
venturi. If the video had used a venturi analogy rather than
a wing analogy, several of the details would have been more
correct.

-- Friction does not simply drag the peaks downwind; it spins
up circulation. Vortices. So the top of the wave gets wrapped
into a fiddle-head shape.

There is of course a ton of vorticity in the boundary layer
between the two fluids, even before the K-H waves form.

In general, non-experts have a terrible tendency to underappreciate
the importance of vorticity in fluids.
http://www.feynmanlectures.caltech.edu/II_40.html
http://www.feynmanlectures.caltech.edu/II_41.html