Tag: turbulence

Inside the Canopy
If you’ve ever gone into the woods on a windy day, you know that conditions there are drastically different than in the open. To blowing wind, trees of different sizes act like enormous roughness that disturbs the flow. Inside the canopy, flows can become incredibly complicated and many of the common techniques used by researchers no longer hold.
You can get a sense for this complexity with the second image above, which visualizes data from a wind tunnel experiment. The gray blocks represent roughness elements – the trees of this wind-tunnel-scale forest – and the large, blue arrow shows the direction of the flow. The thin colored lines show the paths taken by particles in the flow. The lines’ colors indicate what height the trajectory began at.
Notice how the blue and purple lines are relatively straight and oriented in the direction of the flow. This indicates that the flow here is relatively steady and uncomplicated. At the lower heights, though, especially in the green and yellow regions, the pathlines are far more twisted and complex. The flow here is turbulent, and the particles’ trajectories don’t necessarily correlate at all to the winds higher up. (Image credit: T. Japyassu and R. Shnapp et al.; research credit: R. Shnapp et al.; submitted by Ron S.)
September 5, 2019
Phytoplankton Swirls
A winter bloom of phytoplankton appears as green and teal swirls in this false-color satellite image of the Gulf of Aden. Although phytoplankton can be an important food source for fish and other marine animals, in recent years we’ve observed more frequent toxic blooms. Currently, physical sampling of the phytoplankton is necessary to determine what type they are, but scientists are working to use multi-spectral imaging to identify different species remotely. As harmful as they can be, blooms like these help visualize the flow and mixing in different coastal regions. Here, for example, we can see distinctive turbulent eddies in the Gulf that are tens of kilometers across. (Image credit: N. Kuring/NASA; via NASA Earth Observatory)
July 22, 2019
Order in Chaos
Although turbulent flow is chaotic, it’s not completely disordered. In fact, order can emerge from turbulence, though exactly how this happens has been a long-enduring mystery. Take the animations above. They show the flow that develops between two plates moving in opposite direction that are separated by a small gap. (The formal name for this is planar Couette flow.) The visualization is taken in a plane at a fixed height between the plates.
Initially (top), the flow shows narrow bands of turbulence, shown in green, separated by calmer, laminar zones in black. As time passes, these areas of laminar and turbulent flow self-organize, eventually forming diagonal stripes that are much longer than the gap between plates (bottom), the natural length-scale we would expect to see in the flow. Researchers have wondered for years why these distinctive stripes form. What sets their spacing, and why are they along diagonals?
To answer those questions, researchers explored the full Navier-Stokes equations, searching for equilibrium solutions that resemble the striped patterns seen in experiments and simulation. And for the first time, they’ve found a mathematical solution that matches. What the work shows is that the pattern emerges naturally from the equations; in fact, given the characteristics of the solution, the researchers found that many disturbances should lead to this result, which explains why the pattern appears so frequently. (Image and research credit: F. Reetz et al., source; via phys.org; submitted by Kam-Yung Soh)
June 11, 2019
Astrophysical Turbulence
Subsonic turbulence – like the random and chaotic motions of air and water in our everyday lives – is something we have only a limited understanding of. Our knowledge of supersonic turbulence, where shock waves and compressibility rule, is even more tenuous. In part this is because, although we can observe snapshots of supersonic turbulence in astronomical settings like the Orion Nebula shown above, we cannot watch it evolve. On these scales, features simply don’t change appreciably on human timescales.
This has limited scientists to mostly numerical and theoretical studies of supersonic turbulence, but that is starting to change. Researchers are now building experimental set-ups that collide laser-driven plasma jets to generate boundary-free turbulence at Mach 6. Thus far, the observations are consistent with what’s been seen in nature: at low speeds, the turbulence is consistent with Kolmogorov’s theories, with energy cascading from large scales to smaller ones predictably. But as the Mach number increases, the nature of the turbulence shifts, moving toward the large density fluctuations seen in nebulae and other astrophysical realms. (Image credit: F. Battistella; research credit: T. White et al.; see also Nature Astronomy; submitted by Kam-Yung Soh)
April 25, 2019
Amber Waves
When I was a teenager, I liked riding my bike along the river boardwalk near my house. There were fields there, like those in the image above and video below, with tall grass that would bend and sway in the wind. The long stalks undulated almost like a fluid, and they were mesmerizing. This video gives you a higher vantage point, where you can see the larger patterns of motion. What you’re seeing, I think, are some of the large-scale turbulent variations in the wind. Rather than being uniform and laminar, the wind contains pockets of turbulent gusts, which the sway of the long grass reveals to the naked eye. In terms of physical mechanism, I suspect it’s similar to how wind imprints its patterns on water. (Video and image credit: N. Moore)
February 6, 2019
Understanding Jupiter
The swirling clouds of Jupiter hide a complicated and mysterious interior. For decades, scientists have worked to puzzle out the inner dynamics of Jupiter’s atmosphere and what could be going on inside it to generate the flows we see visibly. Near Jupiter’s equator, we see strong jets that flow either east or west, depending on their latitude; this creates the stunning cloud bands we’re used to seeing on the planet. Toward the poles, though, things look more like what we see above – swirling but unbanded.
Through theory, experiments, and simulations, scientists have tried to work out exactly what ingredients are necessary to make Jupiter look this way, but it’s pretty tough to recreate the conditions simply because Jupiter is so extreme. You need a lot of rotation, a lot of turbulence, and a way to stretch that turbulence if you want to imitate Jupiter. There’s been progress recently, though, and it suggests that the jets we see on Jupiter are far more than skin-deep. Instead, they likely stretch deep into the Jovian atmosphere at the equator and ride somewhat shallower toward the poles. (Image credit: NASA JPL; research credit: S. Cabanes et al.)
December 24, 2018
Growing Droplets
The moisture in clouds eventually condenses into droplets that grow into raindrops and fall. Some steps in this process are well understood, but others are not. In particular, scientists have struggled with the problem of how droplets grow from about 30 microns to 80 microns, where they’re big enough to start falling and merging.
Laboratory experiments and numerical simulations (below) have shown that turbulence can help drive small water drops together. When droplets are tiny and light, they simply follow the air flow. But when they’re a little heavier, turbulent eddies (seen in orange below) act like miniature centrifuges, flinging larger water droplets (shown in cyan below) out into clusters, where they’re more likely to collide with one another.
Although this effect has been seen in experiments and simulation, it’s been difficult to capture in clouds themselves. But a new set of test flights (above) confirms that this mechanism is present in the wild as well! (Image credit: UCAR/NCAR Earth Observing Laboratory, P. Ireland et al., source; research credits: M. Larsen et al., P. Ireland et al.; via APS Physics; submitted by Kam-Yung Soh)
November 20, 2018
Dinosaurs, Propellers, and Hiding Objects
The latest FYFD/JFM video is out, and it’s all about the interactions between structures and flows! We learn about plesiosaur-inspired underwater robots, how turbulence affects air-water interfaces, and how adding a tail can help hide an object in a flow. If you missed one of the previous episodes in this series, you can find them all here. (Image and video credit: T. Crawford and N. Sharp)
November 14, 2018
An Introduction to Turbulence
With some help from Physics Girl and her friends, Grant Sanderson at 3Blue1Brown has a nice video introduction to turbulence, complete with neat homemade laser-sheet illuminations of turbulent flows. Grant explains some of the basics of what turbulence is (and isn’t) and gives viewers a look at the equations that govern flow – as befits a mathematics channel!
There’s also an introduction to Kolmogorov’s theorem, which, to date, has been one of the most successful theoretical approaches to understanding turbulence. It describes how energy is passed from large eddies in the flow to smaller ones, and it’s been tested extensively in the nearly 80 years since its first appearance. Just how well the theory holds, and what situations it breaks down in, are still topics of active research and debate. (Video and image credit: G. Sanderson/3Blue1Brown; submitted by Maria-Isabel C.)
November 12, 2018
What Makes Turbulence So Hard
Turbulence – that pestersome, unpredictable, and chaotic state of flow – has been a thorn in the sides of mathematicians, physicists, and engineers for centuries. It is certainly one of – if not the – oldest unsolved problem in physics. Over at Ars Technica, Lee Phillips has a nice overview of the situation, including what makes the problem so difficult:
The Navier-Stokes equation is difficult to solve because it is nonlinear. This word is thrown around quite a bit, but here it means something specific. You can build up a complicated solution to a linear equation by adding up many simple solutions. An example you may be aware of is sound: the equation for sound waves is linear, so you can build up a complex sound by adding together many simple sounds of different frequencies (“harmonics”). Elementary quantum mechanics is also linear; the Schrödinger equation allows you to add together solutions to find a new solution.
But fluid dynamics doesn’t work this way: the nonlinearity of the Navier-Stokes equation means that you can’t build solutions by adding together simpler solutions. This is part of the reason that Heisenberg’s mathematical genius, which served him so well in helping to invent quantum mechanics, was put to such a severe test when it came to turbulence.
Phillips goes on to describe some of the many methods researchers use to unravel the mysteries of turbulence computationally, experimentally, and theoretically. This is a great introduction for those curious to get a sense of how turbulence, stability theory, and computational fluid dynamics all fit together. (Image credits: L. Da Vinci; NASA; see also: Ars Technica; submitted by Kam Yung-Soh)
November 2, 2018