Via press release: (Santa Barbara, Calif.) –– Scientists can use cylinders as small as teapots to study the mechanisms involved in powerful hurricanes and other swirling natural phenomena.
The earth’s atmosphere and its molten outer core have one thing in common: Both contain powerful, swirling vortices. While in the atmosphere these vortices include cyclones and hurricanes, in the outer core they are essential for the formation of the earth’s magnetic field. These phenomena in earth’s interior and its atmosphere are both governed by the same natural mechanisms, according to experimental physicists at UC Santa Barbara working with a computation team in the Netherlands.
Using laboratory cylinders from 4 to 40 inches high, the team studied these underlying physical processes. The results are published in the journal Physical Review Letters.
“To study the atmosphere would be too complicated for our purposes,” said Guenter Ahlers, senior author and professor of physics at UCSB. “Physicists like to take one ingredient of a complicated situation and study it in a quantitative way under ideal conditions.” The research team, including first author Stephan Weiss, a postdoctoral fellow at UCSB, filled the laboratory cylinders with water, and heated the water from below and cooled it from above.
Due to that temperature difference, the warm fluid at the bottom plate rose, while the cold fluid at the top sank –– a phenomenon known as convection. In addition, the whole cylinder was rotated around its own axis; this had a strong influence on how the water flowed inside the cylinder. Rotation, such as the earth’s rotation, is a key factor in the development of vortices. The temperature difference between the top and the bottom of the cylinder is another causal factor since it drives the flow in the first place. Finally, the relation of the diameter of the cylinder to the height is also significant.
Ahlers and his team discovered a new unexpected phenomenon that was not known before for turbulent flows like this. When spinning the container slowly enough, no vortices occurred at first. But, at a certain critical rotation speed, the flow structure changed. Vortices then occurred inside the flow and the warm fluid was transported faster from the bottom to the top than at lower rotation rates. “It is remarkable that this point exists,” Ahlers said. “You must rotate at a certain speed to get to this critical point.”
The rotation rate at which the first vortices appeared depended on the relation between the diameter and the height of the cylinder. For wide cylinders that are not very high, this transition appeared at relatively low rotation rates, while for narrow but high cylinders, the cylinder had to rotate relatively fast in order to produce vortices. Further, it was found that vortices do not exist very close to the sidewall of the cylinder. Instead they always stayed a certain distance away from it. That characteristic distance is called the “healing length.”
“You can’t go from nothing to something quickly,” said Ahlers. “The change must occur over a characteristic length. We found that when you slow down to a smaller rotation rate, the healing length increases.”
The authors showed that their experimental findings are in keeping with a theoretical model similar to the one first developed by Vitaly Lazarevich Ginzburg and Lev Landau in the theory of superconductivity. That same model is also applicable to other areas of physics such as pattern formation and critical phenomena. The model explains that the very existence of the transition from the state without vortices to the one with them is due to the presence of the sidewalls of the container. For a sample so wide (relative to its height) that the walls become unimportant, the vortices would start to form even for very slow rotation. The model makes it possible to describe the experimental discoveries, reported in the article, in precise mathematical language.
The other UCSB author is postdoctoral fellow Jin-Qiang Zhong. Additional authors are Richard J. A. M. Stevens and Detlef Lohse from the University of Twente and Herman J. H. Clercx from Eindhoven University of Science and Technology, both in the Netherlands.
Willis Eschenbach says:
November 30, 2010 at 3:53 pm
I agree, the Ginzburg-Landau equations are fundamental to the field of nonlinear dynamics and phase space landscapes. To re-iterate my earlier posting:
This is not the Coriolis effect, that is linear, this is nonlinear. A classic Hopf bifurcation laminar-turbulent transition. With increasing rotation speed the system is gradually driven away from equilibrium. At a certain disequilibrium point, nonlinear pattern formation breaks out. The increase in warm fluid transport at the critical transition point could possibly tie in with (and give support to) Willis’ Constructal Law (systems converge to maximise flow of something) despite the hard time his theory was given on a recent thread here. The “boundary forcing” referred to is well known in for instance Rayleigh-Benard pattern formation convection in liquid helium.
oMan says:
November 30, 2010 at 3:40 pm
Phlogiston: agree, this experiment is getting at something interesting about how the system is moving energy from one place (hot base) to another (cool top). When the system isn’t rotating, I would bet there are convection cells in the fluid with vertical components but no (or only minor, random) horizontal components. When it rotates, there are shear forces working at the edge of the cylinder (between zero horizontal velocity at the wall, and non-zero velocity in the adjacent fluid). Laminar flow up to some critical velocity and distance from wall; then…? Apparently then there are these vortices, which maybe act to “re-order” the turbulence? And maybe they act as part of the vertical convection cells, somehow increasing the heat flow? I gots no physics here, just some physical intuitions. Help!
I’m not an expert at the maths either, I just try to get my head around chaotic and non-linear phenomena qualitatively. Pure turbulence is chaotic and devoid of emergent pattern formation – so i dont think that vortices are part of actual chaos in the strict sense – correct me if I’m wrong someone. The phenomena of spontaneous nonlinear pattern formation, or “emergent pattern” occurs at the boundary as a system transitions from linear – e.g. laminar flow – to chaotic. A major feature of this pattern formation is the attractor or “strange attractor” – “strange” because it appears for no apparent reason. I think that vortices are attractors – thus attempts to mechanically describe why they occur are futile. (Attractors are a “subset of the system phase space to which the evolving system converges”. The “phase space” is the multi-dimensional set of all parameters which describe the system.) Although emergent pattern is at the transition to chaos only and thus might be unstable, this instability is opposed by the phenomenon of Lyapunov stability by which attractors, once formed, have more stability and persistence than might be expected – thus the vortices from an aircraft wing persist several miles behind the plane. Chaos and turbulence by contrast destroy emergent pattern, so the term “chaotic attractor” is incorrect and a contradiction.
If vortices are trying to “re-order” the turbulence as you suggest (good description), this could be the operation of Lyapunov stability in reasserting the attractor.
As I mentioned in my reply to Willis (and original comment) the mention of increased flow at the nonlinear transition seems to agree with the prediction of “Constructal Law” that a system converges on a state that maximises flow – provided that the transitional pattern formation regime is favoured – as Lyapunov stability states that it should be.
At least poster has stated that the Coriolis effect is not applicable in the lab. However, there are similar effects, on a small scale, relating to conservation of angular momentum that were once demonstrated by astronauts. Maybe videos exist on YouTube.
The maths for all of this are the Navier – Stokes equations and they do not have analytical solutions so computer modelling and re – iteration is necessary which is the world of the climate modellers – only they have even more variables.
It is difficult determining if this experiment has any relevance. Their observations are already known.
I found many people here reducing these findings to a ridicule, it is the lack of research into the very basic simple things that has failed science. Fluid dynamics , vortices and chaotic flow are not well understood. Temperature and its dynamics in the mix are also some what of a mystery. That some one of intelligence is doing basic research should be applauded.
jack morrow says:
November 30, 2010 at 9:39 am
Wow, what deep,intriguing experiments. Takes real “scientists” to come up with yet another hair brain model. Where do all these people come from? I’m sorry, but I seem to have lost faith in the scientific community as a whole.
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You have not been paying attention. Newton, Faraday, Galileo all did this kind of superficially simple experiment and had the insight to make something of it. Many of their contemporaries went poo-poo in much the same way that you do. They were wrong.
The experimental setup described in the article is rather old. The result is new observations in the critical transition region from laminar to tubulent flow, if I am skimming the article properly. That’s important.
All of the poo-poo- ers did not read the article properly or understand what was being described. For example it had nothing whatsoever to do with the coriolis effect.
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LazyTeenager says:
December 1, 2010 at 5:11 am
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I think it is a real bad sign when the first thing I do in the morning is to agree with a LazyTeenager comment.
But you are absolutely correct. The most important experiments are the simple ones, and here we have an incredibly simple mechanism that displays, predictably and repeatably, a very complicated behavior that is found in many places.
I’m disappointed in the snotty comments about some simple experiments that advance understanding of complex natural phenomena.
I expected better of the folks here.
To those snarking at the snarkers:
If there is anything of true import going on here, or any phenomena that have not been seen before, I’m completely missing it. True, the last paragraph makes some vague comments about this research relating to Ginzburg and Landau’s work in superconductivity, but then shooting paperclips with an elastic band can be related to rocket launches — doesn’t make it important.
If it weren’t for the quotes from the principle physicists gushing over the “unexpected phenomenon” of a vortex forming when the cylinder is spun fast enough, or that vortices keep away from the walls of the container, I would be content to simply ridicule this as a very bad press release — and Lord knows there are plenty of those. But the quotes from the scientists themselves seem remarkably damning.
I’m very happy that those with in-depth knowledge about fluid dynamics can peer into this morass of triviality to find significance, but to the general reader this is nothing more than a couple of physicists playing with toys that can be bought from any science shop.
Of course Newton and Galileo and a host of others have done small, simple experiments. But with the vast amount of research already done in the field of fluid dynamics, ranging from pure research to children’s toys, I have to assume that someone at some point has already noticed the “unexpected phenomenon” that “[w]hen spinning the container slowly enough, no vortices occurred at first. But, at a certain critical rotation speed, the flow structure changed. Vortices then occurred inside the flow”. And yes, “the warm fluid [is then] transported faster from the bottom to the top than at lower rotation rates.”
For those of us without specialised knowledge, it is indeed not at all “remarkable that this point exists,” nor that “[y]ou must rotate at a certain speed to get to this critical point.”
Hence — snarks.
Enough with the jeers you mob…. Observation is science…… “The authors showed that their experimental findings are in keeping with a theoretical model similar to the one first developed by Vitaly Lazarevich Ginzburg and Lev Landau in the theory of superconductivity. “……..
This is what they are doing, then expressing the findings in mathematical terms…… They are doin’ science guys. So quit th’ sneerin’, ya hobos.
CodeTech says:
November 30, 2010 at 2:50 pm
Tell you what… for those 2 people in the entire world who think my post was mocking the study of vortices, check the first two points… they both say “(a good thing)”.
It is truly unfortunate that with the state of Science lately, the remaining 12 points of my post would be unsurprising, if they were to occur.
In fact, the study of vortices is serious business, and has innumerable real-world benefits (including those winglets I mentioned, they actually reduce fuel use or increase aircraft range, depending on your perspective).
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damn serious business. For three years I worked for a company that studied forebody vortices on high performance aircraft ( F16 etc). never did much work with VGs most of the work was focused on trying to use the vortex as a control, trippy stuff.
By chance, do you mean “yobbos”?
And yes — as I said, there’s a vague reference to Ginzburg and Landau. Perhaps they too discovered that when cylinders are spun vortices appear (but only if spun fast enough). I’ve read countless press releases in which some claim is made (always at the end) that this relates to some important previous work, but upon closer investigation the link is about as important as that between shooting paperclips and rocket science.
The point remains that given the information provided, this looks like nothing more than an over-hyped play date. Is there more to it? Maybe. But not from where an average reader is sitting.
See Figures 6 & 7:
White, W.B.; Chen, S.-C.; & Peterson, R.G. (1998). The Antarctic Circumpolar Wave: A Beta Effect in Ocean-Atmosphere Coupling over the Southern Ocean. Journal of Physical Oceanography 28, 2345-2361.
http://sam.ucsd.edu/sio219/white.pdf