An isolated blob unveils secrets of turbulence

by | Jul 3, 2023

Turbulence is hard to control, but a turbulent blob created using vortex rings provides fundamental insights into this elusive state of matter.
Abstract swirling image

Turbulence, a highly irregular fluid motion characterized by chaotic changes in the magnitude and direction of the flow velocity, occurs in a wide array of physical scenarios spanning different scales — from the swirl of air currents encircling the wings of an aircraft to the movement of interstellar gas in galaxies.

Describing fluid motion analytically is very difficult because any fluctuations that arise in the flow interact with each other, changing their motion and making the overall flow very unpredictable. It has made developing theoretical methods for calculating the properties of turbulent flow very difficult, hindering our ability to study these physical processes effectively. Even the most powerful supercomputers can’t accurately simulate the behavior of a turbulent fluid flow.

This has left laboratory experiments as the main approach to studying turbulence, but even here problem arise. In experiments, turbulence often occurs when a fluid interacts with different structures, such as walls, pipes, grids, or spinning plates. This makes it difficult to control the energy, momentum, and rotation created by these boundaries on the turbulent flow. Scientists have therefore recognized the significance of generating turbulence away from the physical boundaries of the experimental apparatus in order to study its properties more accurately.

“Turbulence can be found everywhere. Stirring coffee with a spoon is a good example,” said Takumi Matsuzawa, a graduate student at the University of Chicago, and William Irwin, his supervisor and a professor at the same university. “Nevertheless, manipulating this ephemeral phase of matter is not as easy as the other conventional phases of matter, such as solids and liquids.”

“In many cases, the material boundaries, like the spoon in the previous example, obscure what the turbulence has been fed,” added Irwin. “This led us to question whether it is possible to create an isolated blob of turbulence and hold it in place.”

A turbulent blob provides answers

To achieve this, Matsuzawa, Irwin, and their colleagues proposed a new method of creating an isolated turbulent “blob” using fluid vortex rings, whose collision results in the formation of a region of chaotic fluid motion.
“Our proposed approach entails building turbulence by placing eddies one at a time, like Legos,” said Irvine in a press release. “Nobody truly knows what an eddy is, but a vortex ring, aka a smoke ring, is a good candidate, as it is a robust fluid structure and can travel far from the material boundaries. Moreover, its properties can be fully measured so we know what we feed into turbulence.”

However, there was a problem in the previous studies: when the researchers collided the rings formed by the impulsive drawing of water through orifices in water tanks, the turbulence arose for only a very short time — almost immediately after the collision occurred, the original rings recombined into smaller rings, which moved outward and away from the collision region, resulting in the disappearance of the turbulence.

In a recent study published in Nature Physics, the team adopted a new approach. Instead of firing eight rings from the corners of a cubic water tank, they sent out multiple sets of eight rings at repeating intervals, allowing the outgoing rings to interact with the incoming rings. This approach paid off, whereupon reaching a frequency of several ring sets per second, the rings did not have time to leave the collision region and, together with the incoming batch of rings, formed a stable blob of turbulent fluid.

“Our approach provides unique design principles to localize, position, and control turbulence,” Irvine said. “The properties of the blob are set by those of vortex rings; the size is set by the ring radius; the inner turbulent intensity is set by the energy carried by the rings. If we combine helical loops, we could also inject the other conserved quantities such as angular [momentum] and helicity, whose roles in turbulence are not well-known.”

Future studies

Despite the fact that this achievement is by itself groundbreaking, the scientists did not stop there, and say they are continuing to delve deeper into the study of turbulence using their new method.

“We are currently investigating how turbulence freely evolves in a quiescent environment,” Irvine continued. “This is an important question regarding how turbulent fluctuations spread and die out. We are also interested in studying how turbulence ‘forgets’ what has been fed.

“It is believed that turbulence is universal in the small scales even if the vortical structures in the input are different. Our system would be ideal to study this memory in turbulence by tuning the input by combining various vortex loops.”

In future the team say they plan to use their method to investigate the most fundamental questions about the nature of turbulent behavior and the interaction of turbulent flow.

“Some of the questions that could be explored following our study include: what happens at the interface of turbulent and non-turbulent flows?” Matsuzawa concluded. “How are conserved quantities such as energy and [momentum] transported across the interface? Are there different types of turbulence depending on the combinations of conserved quantities?”

Reference: Takumi Matsuzawa et al., Creation of an isolated turbulent blob fed by vortex rings, Nature Physics (2023), DOI: 10.1038/s41567-023-02052-0

Feature image credit: TT on Pixabay

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