Going with the flow: New insights into mysterious fluid motions

January 24, 2020

Water issuing from an ordinary faucet tells a complex tale of its journey through a pipe. At high velocities, the faucet's gushing stream is turbulent: chaotic, disorderly -- like the crash of ocean waves.

Compared to orderly laminar flows, like the faucet's steady stream at low velocities, scientists know little about turbulence. Even less is known about how laminar flows become turbulent. A mix of orderly and disorderly flows, transitional flows occur when fluids move at intermediate velocities.

Now, Dr. Rory Cerbus, Dr. Chien-chia Liu, Dr. Gustavo Gioia, and Dr. Pinaki Chakraborty, researchers in the Fluid Mechanics Unit and the Continuum Physics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), have drawn from a decades-old conceptual theory of turbulence to develop a new approach for studying transitional flows. The scientists' findings, published in Science Advances, may help furnish a more comprehensive, conceptual understanding of transitional and turbulent flows, with practical applications in engineering.

"Turbulence is often touted as the last unsolved problem in classical physics - it has a certain mystique about it," said Cerbus. "And yet, under idealized conditions, we have a conceptual theory that helps explain turbulent flows. In our research, we're striving to understand if this conceptual theory might also shed light on transitional flows."

Finding order in disorder

Scientists have long been captivated by turbulent flows. In the fifteenth century, Leonardo da Vinci illustrated turbulent flows as collections of swirling eddies, or circular currents, of varying sizes.

Centuries later in 1941, mathematician Andrey Kolmogorov developed a conceptual theory that revealed order underlying the energetics of seemingly disordered eddies.

As depicted in DaVinci's sketch, a stream plunging into a pool of water initially forms a large, swirling eddy, which quickly becomes unstable and breaks apart into progressively smaller eddies. Energy is transferred from the large to ever-smaller eddies, until the smallest eddies dissipate the energy via the water's viscosity.

Capturing this imagery in the language of mathematics, Kolmogorov's theory predicts the energy spectrum, a function which describes how the kinetic energy - the energy from motion - is apportioned across eddies of different sizes.

Importantly, the theory says that the energetics of the small eddies is universal, meaning that although turbulent flows may look different, the smallest eddies in all turbulent flows have the same energy spectrum.

"That such simple concepts can elegantly elucidate a seemingly intractable problem, I find it truly extraordinary," said Chakraborty.

But there is a catch. Kolmogorov's theory is widely thought to apply only to a small set of idealized flows, and not the flows of everyday life, including the transitional flows.

To study these transitional flows, Cerbus and his collaborators conducted experiments on water flowing through a 20-meter-long, 2.5-centimeter-diameter glass cylindrical pipe. The researchers added small, hollow particles with approximately the same density as water, allowing them to visualize the flow. They used a technique called laser doppler velocimetry to measure the velocities of the eddies in the transitional pipe flows. With these measured velocities, they computed the energy spectrum.

Surprisingly, the researchers found that, despite seeming distinct from turbulent flows, the energy spectrum corresponding to the small eddies in the transitional flows conformed to the universal energy spectrum from Kolmogorov's theory.

Beyond furnishing a new conceptual understanding of transitional flows, this finding has applications in engineering. Over the past two decades, Gioia and Chakraborty's research has shown that energy spectra can help predict friction between the flow and the pipe - a major concern for engineers. The more friction in a pipe, the more difficult it is to pump and transport fluids like oil.

"Our study combines esoteric mathematical ideas with factors that engineers care about," said Chakraborty. "And, we've found that Kolmogorov's theories have wider applicability that anyone thought. This is an exciting new insight into turbulence as well as into the transition to turbulence."
-end-


Okinawa Institute of Science and Technology (OIST) Graduate University

Related Turbulence Articles from Brightsurf:

Turbulence affects aerosols and cloud formation
Turbulent air in the atmosphere affects how cloud droplets form.

Atmospheric turbulence affects new particle formation: Common finding on three continents
New particle formation (NPF) over three countries is investigated using aerosol physicochemical quantities and turbulence information.

Laser technology: The Turbulence and the Comb
While the light of an ordinary laser only has one single, well-defined wavelength, a so-called ''frequency comb'' consists of different light frequencies, which are precisely arranged at regular distances, much like the teeth of a comb.

Return of the Blob: Surprise link found to edge turbulence in fusion plasma
Correlation discovered between magnetic turbulence in fusion plasmas and troublesome blobs at the plasma edge.

Researchers unveil the universal properties of active turbulence
Turbulent flows are chaotic yet feature universal statistical properties.Over the recent years, seemingly turbulent flows have been discovered in active fluids such as bacterial suspensions, epithelial cell monolayers, and mixtures of biopolymers and molecular motors.

Unraveling turbulence
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) may have identified a fundamental mechanism by which turbulence develops by smashing vortex rings head-on into each other, recording the results with ultra-high-resolution cameras, and reconstructing the collision dynamics using a 3D visualization program.

Researchers develop first mathematical proof for key law of turbulence in fluid mechanics
Turbulence is one of the least understood phenomena of the physical world.

A new parallel strategy for tackling turbulence on Summit
A Georgia Tech team developed an algorithm for simulating turbulence on Summit, the world's most powerful and smartest supercomputer.

Turbulence creates ice in clouds
Vertical air motions increase ice formation in mixed-phase clouds. This correlation was predicted theoretically for a long time, but could now be observed for the first time in nature.

Turbulence meets a shock
Interaction of shocks and turbulence investigated with a focus on high intensity turbulence levels.

Read More: Turbulence News and Turbulence Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.