## Sunday, May 21, 2006

### Spinning Round (or Square or Hexagonal...)

From Slashdot, here's this unreal article in Nature.
Tomas Bohr and colleagues made plexiglass buckets, 13 and 20 centimetres across, with metal bottoms that could be rotated at high speed by a motor. They filled the bucket with water and spun the bottom to whip up the liquid into a whirlpool that rose up the sides of the container.

This set-up is very similar to the rotating bucket that Isaac Newton used in the seventeenth century to investigate centrifugal forces.

The researchers found that once the plate was spinning so fast that the water span out to the sides, creating a hole of air in the middle, the dry patch wasn't circular as might be expected. Instead it evolved, as the bucket's spin sped up, from an ellipse to a three-sided star, to a square, a pentagon, and, at the highest speeds investigated, a hexagon.
I mean, symmetry breaking is one thing, but where do pentagons come from? Why do things "quantize" in this way, when there's nothing obvoiusly "quantal"about the problem?

Floor said...

If you look closely to the shape of the paterns you can see they are not real pentagons and triangles. You can see it easy in the "triangle", the lines are not straight.
I think it is some kind of standing wave in a circle.
The amount of wave periods seem to depend on the rotaion speed.
I cant say this is THE solution or explain it with mathematics. It's just a guess.

Floor said...

My idea is suggested multiple times there.

Peter said...

Clearly I hadn't read the comments either! It makes sense, I guess, but then why wasn't this mentioned in the Nature article?

Anonymous said...

Is that a goldfish in the middle?

Qubit said...

It looks like a parabolic star.A collection of many waves which are in straight lines.

Gary Ray said...

I would bet that these come from the vibrations of the apparatus that spins the buckets. If the bearings were changed to another type I bet the shapes would change. All machines that spin in bearings have vibrations and I think it is the apparatus design that is causing the shapes.

I have not read the article so I may be putting my foot in my mouth. I am an old GE Field Engineer (retired from that job) and have a good story about vibrations and turbine bearings.

Another old Field Engineer used to take a standard Styrofoam cup; fill it up with coffee and place it on a shaft bearing of a large steam turbine bearing while doing a vibration test. He said he could tell the vibration (in mils) from counting the rings that would from on the top of the coffee.

Anyway the story goes to the point that I think it is the machine that is causing these shapes from the vibrations.

Nice blog, I read political blogs ( left left left if you must know) and I think I’ll start reading science and engineering blogs. Keeps my blood pressure down.

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Anonymous said...

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Anonymous said...

http://ams.allenpress.com/archive/1520-0477/63/11/pdf/i1520-0477-63-11-1294.pdf
shows polygonal hurricane eye walls.
Perhaps polygonal hurricane eyes and saturn's hexagonal north pole are caused by smaller, unseen, rapidly moving atmospheric high pressure spots or tornados moving linearly along the polygon wall then turning sharply and clearing out clouds in its path.

didonai said...

The north pole of Saturn exhibits this same pattern on an enormous scale. One might wonder about the sides of the 'bucket' on that planet in which weather patterns are 'spun' at high force. It could be a huge impact crater with steep sides. Since the planet is covered with a dense atmosphere, the physical side of the planet is not visible.

didonai said...

The north pole of Saturn exhibits this same pattern on an enormous scale. One might wonder about the sides of the 'bucket' on that planet in which weather patterns are 'spun' at high force. It could be a huge impact crater with steep sides. Since the planet is covered with a dense atmosphere, the physical side of the planet is not visible.

Anonymous said...

In the case of frigid gigantic Saturn, the north pole's extreme cold may create a super high pressure zone unlike anything man is familiar with on earth, of course, with a temperature much colder and a pressure much higher than the surrounding atmosphere resulting in the hexagonal north pole zone relatively free of clouds.

Anonymous said...

http://www.space.com/scienceastronomy/061109_monster_storm.html
shows that Saturn's south pole hurricane eye is also hexagonal, but smaller. As I mentioned, I think frigid gigantic Saturn's polar hurricane eyes might be caused by super cold temperatures resulting in super high pressures much much colder and higher pressure than the surrounding atmosphere. Saturn's atmosphere might be a constant battle between the super cold temperatures of space fighting frictional/high pressure heat from the super high rotational speeds of such a gigantic planet.