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."

Re:Just a resonance?

[Anonymous Coward]

Thomas Bohr is the grandson of Niels Bohr.

- AC

Re:TFSummary says "Three-sided star..."

[Anonymous Coward]

Well, there is a difference between a three sided star and a triangle. Where a triangle has got three angle pairs (hence the name I suppose) the three sided star has got six angle pairs. Three near the center and three at the points. I would draw you some ascii art but that probably won't look right.

Re:Sloppy reporting.

[msparshatt]

I've never considered /. "the best in science journalism," and think you should think hard about life, the universe and everything if you do.

The GP was referring to Nature magazine, which does describe itself as "the best in science journalism"

Re:TFSummary says "Three-sided star..."

[tOaOMiB]

Isn't a three-sided star the shape of a mercedes-benz logo [mercedes-benz.com.vn]? I guess they really meant three-pointed star...

Re:Instability?

[NeutronCowboy]

You're confusing randomness with chaos theory. Randomness is essentially us saying "We might know the principles at work, but it's too complicated for us to make an accurate prediction on what is exactly going to happen." Brownian motion is one such example. We have a good idea on the physics behind it, but the huge number of interactions that take place mean that we can only predict the behavior of the entire system, not of single particles in it. Furthermore, single particles do not show a propensity to do anything in particular. You won't find random particles moving in circles, for example.

Chaos theory deals with systems where we can calculate effects on single objects in the system, and where these objects exhibit non-random patterns. You mentioned fractals already (although strictly speaking, that's defined as a complex system rather than a chaotic one), and population growth patterns are another.

Photos and video

[Falkkin]

Some better photos can be found here, along with a video. Unfortunately the video seems to show the vortex from its side rather than the top. Pretty cool though!

http://dcwww.camp.dtu.dk/~tbohr/RotatingPolygon/ [camp.dtu.dk]

Re:Sloppy reporting.

[m0nstr42]

Seems that they realize that this is but baby steps, and there needs to be much more work done.

Amen. I'm getting sick of people reading a /. summary of a summary of someone's legitimate results and deciding then and there that the original research (whose message is now 2x re-interpreted by the successive authors) is crap. These people do this for a living; many hold tenure positions at prestigious research institutions that are reserved for the brightest in their fields. Most of their really significant results appear in peer-reviewed publications. They're probably slightly more qualified to decide what is significant in their fields than you are.

Popular media tends to mangle the crap out of stories in an effort to make it accessible to a wide variety of people. This is necessary for the sharing of information and the generation of public interest in scientific progress. If you're semi-intelligent and a particular story catches your eye, you should know enough to read between the lines a little bit. If you want to make any claims regarding validity, you need to find the original publications and make a slightly better assessment than a half-page web story can provide you with.

Re:Sloppy reporting.

[iamlucky13]

To expand on the parent, the effect is called "similitude." The Reynolds number is a dimensionless number that involves the velocity of flow, the size of any defining flow feature (like pipe diameter), and the viscosity of the fluid. These are the primary factors that effect how a fluid flow will act on a larger scale.

Unfortunately, this sort of thing doesn't work very well on a small portion of a system. Instead, computational fluid dynamics involves breaking the flow up into discrete elements, figuring out what each element should be doing (typically according to the equations used on larger or simpler systems), then figuring out how that effects the element next to it. Then you do the whole thing over again with new initial conditions defined by how all the elements effected each other. Then you do it once more. Then you keep doing it over and over until the difference between subsequent iterations gets small enough to make you happy (assuming you didn't screw up and it diverges). The ability to do this with a computer definitely opened new gateways for engineering with fluids, but it's still only an approximation, and there are some effects they have trouble figuring out. I don't think anyone can really appreciate the difficulty of some of the common problems like long-term or highly accurate weather or climate predictions until they've tried to solve a finite element problem involving just 4 elements (especially if you have complicating factors like heat transfer). Then you look up at the sky and multiply the difficulty by several billion or so.

A couple of my friends in school worked summer research projects with one of our physics professors looking at a related effect known as Stewartson layers (basically, the shear rate of a fluid isn't actually linear across a flow in which velocity changes with position, like we usually model it as...sometimes the flow forms in "sheets"). I don't know all the details, but like the effect in the article, this one isn't well understood.

Re:Just a resonance?

[Anonymous Coward]

And he is the son of Aage Bohr (Nobel prize in physics).

Re:Sloppy reporting.

[Old Wolf]

It seems obvious to me that they meant "span" (the past tense of "spin"):

  http://www.usingenglish.com/reference/irregular-ve rbs/spin.html [usingenglish.com]