New insights into how fluids transform from order to disorder

Unraveling the turbulence

A 3D reconstruction of the collision dynamics of two vortices. Credit: Ryan McKeown / Harvard SEAS

Turbulence is everywhere – it makes our planes vibrate and creates tiny whirlpools in our bathtubs – but it is one of the least understood phenomena in classical physics.


Turbulence occurs when a flow of ordered fluid breaks into small vortices, which interact with each other and break into even smaller vortices, which interact with each other and so on, becoming the chaotic vortex of disorder that makes rafting in fun white waters.

But the mechanisms of that descent into chaos have baffled scientists for centuries.

When they don’t understand something, physicists have a practical solution: destroy it together. Do you want to understand the fundamental building blocks of the universe? Destroy the particles together. Do you want to reveal the mechanics behind the turbulence? Destroy the eddies together.

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) may have identified a fundamental mechanism by which turbulence develops by breaking the vortex rings against each other, recording the results with ultra-high cameras resolution, and reconstruct the collision dynamics using a 3D visualization program. Together with the analysis of the numerical simulations performed by the collaborators of the University of Houston and the ENS in Lyon, the researchers acquired an unprecedented vision on how fluidic systems transform from order to disorder.

The research is described in Science advances.

Unraveling the turbulence

Vortex cannons fire in a 75-gallon aquarium to produce vortices. Each vortex was dyed in a different color, so the researchers were able to observe how they interact. Credit: Harvard SEAS

“Our ability to predict the weather, understand why a Boeing 747 also flies with turbulent currents in its wake and determine global flows in the ocean depends on how well we model turbulence,” said Shmuel Rubinstein, associate professor of physics applied to SEAS and corresponding author of the article. “However, our understanding of turbulence still lacks a mechanistic description that explains how energy flows on smaller and smaller scales until it is eventually dissipated. This research opens the door to that kind of understanding.”

“Trying to make sense of what’s going on in an extremely complex system like turbulence is always a challenge,” said Rodolfo Ostilla-Mónico, assistant professor of mechanical engineering at the University of Houston and correspondent author of the article. “At each scale of length, the vortices strive and compress each other to generate a chaotic image. With this work, we can begin to isolate and watch simple couple interactions and how they lead to rich dynamics when there are enough of them. “

Physicists have been using vortex colliders to understand turbulence since the 1990s, but previous experiments have been unable to slow down and reconstruct the mechanics of the collision as it descends into chaos. To do this, the researchers synchronized a powerful scanning laser sheet with a high-speed camera, capable of taking hundreds of thousands of images per second, to quickly scan the entire collision in real time.

Unraveling the turbulence

When the vortices collide, the edges form antisymmetric waves. The crests of these waves develop into finger-like filaments, which grow perpendicularly between the colliding nuclei. Credit: Harvard SEAS

They used vortex cannons in a 75-gallon aquarium to produce vortices. Each vortex was dyed in a different color, so the researchers were able to observe how they interact when they collide violently. It takes less than a second for the rings to disappear in a puff of dye after the collision, but a lot of physics happens at that time.

First, the rings extend outward as they crash into each other and the edges form antisymmetric waves. The crests of these waves develop into finger-like filaments, which grow perpendicularly between the colliding nuclei.

These filaments counter-rotate with their neighbors, creating a new series of miniature vortices that interact with each other for milliseconds. Those vortices also form filaments, which in turn form vortices. The research team observed three generations of this cascade cycle, each equal to before, only smaller: a Russian nesting mess doll.

“This large to small scale similar behavior emerges very quickly and in an orderly fashion before everything turns to turbulence,” said Ryan McKeown, a SEAS graduate student and first author of the paper. “This cascading effect is truly exciting because it could indicate a universal mechanism for the functioning of these interactions, regardless of scale.”

In addition to the experiments, the research team also developed numerical simulations to understand the dynamics of the failure and quantify the evolution of the energy spectrum of the waterfall. Turbulence has a very specific and well-defined energy spectrum. While this system is considerably simpler than the turbulence that shakes a plane, the researchers found that the energy spectrum at the breakdown of the advanced stages follows the same telltale scaling of the fully developed turbulence.

“This is an excellent indication that although it is a different system, it is creating the same turbulence conditions for a short time. It is a starting point,” said McKeown.


Experts investigate how order emerges from chaos


More information:
Ryan McKeown et al., Turbulence generation through an iterative cascade of elliptical instability, Science advances (2020). DOI: 10.1126 / sciadv.aaz2717

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Harvard John A. Paulson School of Engineering and Applied Sciences

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Unraveling the turbulence: new insights on how fluids transform from order to disorder (2020, February 29)
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