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The Blob—a localized blob of turbulence created in a tank at the University of Chicago—is helping scientists better understand the laws of turbulent motion. Above, the motions that make up the Blob visualized through trajectories of tracer particles colored by speed.
Image Credit: Takumi Matsuzawa
Scientific Frontline: Extended "At a Glance" Summary: Isolated Turbulence and "The Blob" Tank
The Core Concept: "The Blob" is a pioneering experimental setup in which a perfect, stationary ball of turbulence is generated at the center of a water tank by firing synchronized water jets. This configuration isolates the chaotic swirling of fluids from boundary interactions, allowing scientists to study turbulence in its purest, undisturbed form.
Key Distinction/Mechanism: Unlike traditional experiments that use mechanical instruments like paddles or grids—where the stirring mechanism and container walls inevitably interfere with the fluid's natural motion—this method suspends the turbulence entirely in the center of the tank. This free-floating mechanism allows researchers to observe how turbulent eddies organize, expand in a sharp front, and decay without external physical disruption.
Major Frameworks/Components:
- Sharp Front Spreading: The experiment provides the first visual evidence in water that turbulent eddies organize to spread in a sharp front, a mechanism previously only observed in superfluid helium in the 1990s.
- Two-Stage Energy Decay: The data reveals that an isolated ball of turbulence loses energy in two distinct stages, driven by the size and growth patterns of the initial eddies before they hit the container walls.
- Extended Theoretical Models: The discoveries directly challenge and extend classical models for the evolution of freely decaying turbulence, originally developed by physicists A.N. Kolmogorov and G.I. Barenblatt.
Branch of Science: Experimental Physics, Fluid Dynamics, and Theoretical Physics.
Future Application: A fundamental understanding of turbulent behavior is crucial for optimizing the aerodynamic design of airplanes and wind turbines, advancing the engineering of fusion reactors, and modeling complex systems like weather patterns.
Why It Matters: Turbulence is a ubiquitous but paradoxically complex physical phenomenon that governs everything from human blood flow to ocean currents and hurricane formations. By successfully isolating turbulence from environmental interference, scientists can finally map its fundamental rules, definitively ruling out flawed decay theories and establishing a clean baseline for future physics research.
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| A visualization of the chaotic motions of swirling fluids, known as turbulence. This image, which was recently awarded first prize in the annual UChicago Science as Art contest, captures the trajectories of particles in water as turbulence winds down in a specially designed tank, with the color representing the speed of the particles. Image Credit: Takumi Matsuzawa |
In a tank on the bottom floor of a University of Chicago research laboratory, scientists summon “The Blob” into existence by firing water jets to create an artfully choreographed series of rings.
First created three years ago in the laboratory of UChicago Prof. William Irvine in collaboration with graduate student Takumi Matsuzawa, The Blob is one of the only ways that researchers can study the strange properties of turbulence—the chaotic swirling of fluids such as air and water—in its purest form: stationary in a lab and isolated from boundaries.
Turbulence is a bit of a paradox. It governs everything from the movements of ocean currents and hurricane clouds to the swirling of cream in your coffee and blood in your veins. But as widespread as it is, turbulence has been fiendishly difficult for scientists to understand, compared with most other everyday physics phenomena.
In a new study, the Irvine lab reported its first findings from the strange blob. They include several insights into the behavior of turbulence as it spreads and dissipates—including that it lingers far longer than visible to the human eye.
“It’s a totally fundamental question—what does turbulence do when you let it loose?—and yet we had no way to study it in such a clean setting before,” said Irvine, a professor of physics and member of the James Franck and Enrico Fermi Institutes.
The study was co-authored by Prof. Nigel Goldenfeld, a theoretical physicist at the University of California, San Diego and Minhui Zhu, then a graduate student at the University of Illinois at Urbana-Champaign and now at Argonne National Laboratory, and was published in the Proceedings of the National Academy of Sciences.
Turbulence has been fiendishly difficult for scientists to understand, compared with most other everyday physics phenomena.
How turbulence spreads
Understanding the rules of turbulence is crucial for designing planes and turbines, and for building fusion reactors, among other uses. But simulating turbulence and testing it in experiments has been difficult.
Researchers always want to study the purest, simplest form of a phenomenon in order to understand the basics and extract the fundamental rules of its behavior. The trouble is that by creating turbulence to study it, you are always interfering with the system in some way. If you stick a paddle into a tank of water to stir it up, both paddle and tank walls unavoidably interfere with how the motion plays out.
That was, until The Blob.
Irvine and Matsuzawa created an experimental setup in which vortex rings were fired into the center of a tank from all corners. This created a perfect, stationary ball of turbulence on its own at the center of the tank.
“It’s a very unique experiment to ask the question, because in no other situation do you have turbulence separated from the walls—with properties not controlled by the box where it exists, but by how you made it,” said Irvine.
In this study, Matsuzawa set up a camera to take high-speed images to track the movements of The Blob as it played out. It first expanded, filled the chamber, and then gradually decayed back to rest.
A visualization of the chaotic motions of swirling fluids, known as turbulence. This image, which was recently awarded first prize in the annual UChicago Science as Art contest, captures the trajectories of particles in water as turbulence winds down in a specially designed tank, with the color representing the speed of the particles. Image courtesy of Takumi Matsuzawa
One new observation concerned how turbulence spreads. Unlike, say, tea molecules spreading diffusely out from a teabag in a teacup, turbulent eddies organize themselves to spread in a sharp front. However, this effect had only been experimentally observed in superfluid helium in the 1990s as part of a collaboration by Goldenfeld with the late famed experimentalist Russell Donnelly.
Irvine and Matsuzawa’s experiment captured evidence of the same effect in water for the first time. Using modern flow visualization techniques, they were able to perform more nuanced measurements than previously possible to confirm this mechanism of turbulence spreading.
How turbulence decays
The team also discovered something unexpected about how turbulence died out.
They observed two distinct stages in how the energy decayed in The Blob: early on, the energy dropped in one characteristic way, but later it followed a different pattern of decrease.
To dig deeper, Irvine and Matsuzawa created turbulence with a different method—by placing a plastic mesh, or grid, into the water tank and shaking it. When they did, they saw that the energy decayed in just a single pattern.
“What this shows is that you can have two different laws of decay of turbulence in the same box,” Matsuzawa explained.
The difference comes down to the structure of the eddies at the start. In The Blob, the largest eddy starts out about as large as the blob itself and keeps growing until it reaches the size of the container. By contrast, turbulence generated by the grid already contains eddies as large as the container from the start.
These findings extend a model for the evolution of freely decaying turbulence, first developed by A.N. Kolmogorov and G.I. Barenblatt, the scientists said. However, it had to be extended to take into account surprising new findings about the technical details by which turbulence evolved once created.
“What this shows is that you can have two different laws of decay of turbulence in the same box.”Takumi Matsuzawa, PhD’23
“Even a single isolated blob of turbulence is a complex system. It’s amazing that a minimal theoretical picture can still capture the essential behaviors observed in the experiment,” said Zhu. “The analysis of the experimental data performed by the team was able to rule out previously proposed theories for the decay of turbulence, thus providing new puzzles for theorists.”
“This work is an example of how fundamental aspects of one of the most complex physical phenomena can be explored scientifically through innovative experiments and imaginative theory, via a deep collaboration between theorists and experiments,” Goldenfeld commented. “Crucial to the success of this collaboration—spanning seven years from initial conception to completion—was the willingness of talented scientists to improvise and persevere on a very challenging problem!”
The scientists plan to continue to work with The Blob to explore the properties of turbulence, in particular to explore further the unexpected features of turbulent spreading and decay that they uncovered for the first time.
Note: Irvine and Goldenfeld dedicated the paper to their fond memory of Russell Donnelly, who carried out seminal work on turbulence as a professor at UChicago and later at the University of Oregon.
Additional information: The study used the resources of the National Science Foundation Materials Research Science and Engineering Center and the UChicago Research Computing Center
Funding: U.S. Army Research Office, Simons Foundation, Brown Foundation, Schmidt Science Fellowship.
Published in journal: Proceedings of the National Academy of Sciences
Title: Nonlinear diffusion and decay of a blob of turbulence spreading into a quiescent fluid
Authors: Takumi Matsuzawa, Minhui Zhu, Nigel Goldenfeld, and William T. M. Irvine
Source/Credit: University of Chicago | Louise Lerner
Reference Number: phy041326_01