Recipe for Making Symetrical Holes in Water

scottZed writes "Danish researchers found a simple way to make curiously shaped air holes in a bucket of water. Simply rig the bucket to have a spinning plate at the bottom, and depending on the speed, you can get an ellipse, three-sided star, square, pentagon, or hexagon. The effect may help explain such shapes seen in atmospheric disturbances on Earth and other planets. One practical use: really trippy washing machines."

Interesting Effect

[Anonymous Coward]

It looks like the end result of system resonance set up between the harmonics and the properties of water. It would be cool to artificially vary the viscosity of the water with polymers, or add salts to increase specific gravity to note the affect on the pattern properties. OK, some of you are thinking, this guy is a nut but it just proves how never ending the learning process is as it relates to even the simplest things observed in nature. I like it.

Re:Just a resonance?

[pheede]

Yes, he's the grandson of Niels Bohr. His two cousins, Henrik and Jakob Bohr, are also professors at the Department of Physics at the Technical University of Denmark.

Re:Wow

[Decaff]

But seriously, what if in the ocean the waterflow is spinning very hard itself under certain conditions, wouldn't that be a possible explanation for the disappearances in the Bermuda Triangle?

You don't need an explanation for the disappearances in the Bermuda Triangle, at least no special explanation. The disappearances there occur at no greater frequency per unit of shipping or flight than anywhere else in the world.

Re:Just a resonance?

[ZombieWomble]

Well, if you have waves in the bucket, and the circumference of the hole is a multiple of that wavelength, then it's very natural that this phenomenon should happen.

In that situation, there would be perfect axial symmetry as these wavelengths would be identical in all directions, giving a fixed circular standing wave pattern once the flow stabilised (given a symmetric bucket, obviously). However, in this case, we have a breakdown in axial symmetry, and instead have slowly rotating geometric shapes instead.

mmmm tasty

[Bob_Geldof]

mmm ... tasty eigenmodes abounding. Congratulations. We study this kind of crap all the time in the Applied Mathematics program at Univ. of Washington. Can't imagine it's so different for other programs. That being said, I like the nice pictures. I've seen some interesting pictures where a thin layer of fluid is trapped between two cyliders. The inside cylinder is rotated and you can see through the outer one. At certain Reynolds http://en.wikipedia.org/wiki/Reynolds_number [wikipedia.org] numbers you get different forms, such as a stream going helicaly around the cylinders, much like a barbers pole. I think this picture might be in Garrett's "Atmospheric Boundary Layers" book (no, I will not help you find it. That would be cheating and I don't get any of the royalties, so I don't care.). --

Easy to explain

[EmbeddedJanitor]

What we're seeing are shapes caused by friction. As a moving fluid (liquid or gas) moves against something that is not moving, or is moving at a different speed, the friction causes waves just like those waves that surfer guys ride or the waves in the atmosphere caused by a wind blowing over a mountain.

So why the different shapes? As the bucket speeds up, three things happen. There's a different speed differential between the bucket and the water, the water depth decreases and the extra g forces increase (effectively increasing local gravity). This changes the wavelength of the wave. So, since the bucket has a finite circumference and is circular, standing waves will form that go back to their own starting point which will make shapes of integer numbers of sides. (non integer numbers of sides will not form a standing wave).

Reminiscent of the work of Hans Jenny

[DIGITAiLor]

This reminds me of the work of the Swiss doctor Hans Jenny in the 60s. Dr. Jenny sent audible simple sine waves through various media and photographed the patterns that would emerge.

The results were often strikingly beautiful and symmetrical. His two books on the subject, full of high-quality imagery, were recently reprinted as one volume. He called his study of wave properties "cymatics."

The photographs illustrate the multi-sensory aspect of all phenomena. Frequency and wavelength show their existence in many forms and media, all representing the same phenomena. You can string a violin bow over sand on glass and see some incredible webs of patterns emerge in the sand. It's amazing to think that both aural and visual feedback from the same source can be produced so simply. And, importantly to myself at least, have both be aesthetically pleasing.

http://www.cymaticsource.com/ [cymaticsource.com] has the reprints of the books. I think they also relate it to a lot of more sketchy spiritual stuff that the good Dr. never mentioned AFAIK.

In this case in TFA, the researchers have seen the amazingly symmetrical and simple visual representation of the interaction between fluid, vessel, and frequency (rotation). It does make sense that such a simple phenomenon (rotating fluid) would have a simple, fundamental visual pattern. I bet it looks a lot more interesting than it sounds though.