Author: admin

A sneeze. Ocean currents. Smoke. What do these have in common? They’re instances of turbulence: unpredictable, chaotic, uneven fluid flows of fluctuating velocity and pressure. Though ubiquitous in nature, these flows remain somewhat of a mystery, theoretically and computationally.

“Most flows that we encounter in nature are turbulent — it does not matter whether it is the flow outside the airplane that makes us fasten our seatbelts, or the flow in a small stream,” said UC Santa Barbara mathematics professor Björn Birnir. “Turbulence is difficult to understand because the mathematical models that describe it are nonlinear, stochastic and the solutions are unstable. This made it necessary to develop new theories to truly understand the nature of turbulence.”

Fortunately, Birnir and Luiza Angheluta of the University of Oslo are getting us closer to being able to characterize turbulence, with an approach that captures some of the myriad complex phenomena that occur over the evolution of a turbulent flow. Their research is published in the journal Physical Review Research .

‘The most important unsolved problem’

Described in 1964 by famed physicist Richard Feynmann as “the most important unsolved problem of classical physics,” turbulence has accumulated its fair share of laws and theories, as researchers over two centuries contributed valuable insights and approaches to the study of this highly complex phenomenon. However, because of its nonlinearity, general unpredictability and also its multi-scale nature, generating the math that holds true for everything from the tiniest fluctuation to the entire flow with all its interacting vortices and eddies has been one of the primary challenges of the field.

This is particularly true of the turbulent flow called Lagrangian turbulence, where an observer follows the flow (as in an airplane). It starts by an initial ballistic flow (all particles stuck together and flowing in the same direction), goes through large Lagrangian vortices and later Eulerian turbulence (a homogeneous flow with smaller but more complex vortices).

“There has been a lot of speculation,” said Birnir, who directs the Center for Complex and Nonlinear Science at UCSB. “The ballistic region has a certain scaling. The Lagrangian region has another scaling, and

A simulation of Lagrangian turbulence, with a ballistic region on the right, superdiffusing into a Lagrangian region, followed by an Eulerian region and finally, according to the researchers, a region of “free eddies.”

then it looked like it was going into this region where there was Eulerian scaling.” Each scaling regime contains the math that best describes the forces and unique phenomena in only that particular evolution of the turbulent flow. “So you are basically seeing three types of scalings but there was no theory behind it and there was in fact no proof of it.” Instead of becoming clearer, the study of turbulence had become more confusing, he added.

While the ballistic and Eulerian flows have fairly well-established scaling laws, the region between them was relatively less understood.

“Different scaling regions, in time, is one of the main characteristics of Lagrangian turbulence,” Birnir explained. Another unique characteristic is the Lagrangian framework’s approach, which is to follow the turbulence from the point of view of a particle — a “tracer” — within the flow, as opposed to from a stationary point outside the flow, as is the case with Eulerian turbulence, where the flow is more homogenous.

Birnir and Angheluta investigated the statistical properties of a fully turbulent Lagrangian velocity field using a modeling framework called stochastic closure theory, which captures randomness as part of the system. They also used a set of relations called the Green-Kubo-Obukhov relations to characterize the effects of various forces on and conditions of the flow such as diffusion and viscosity, as well as the chaotic dynamics of the entire system.

The result is a mathematical model that demonstrates the presence of a Lagrangian scaling regime in the “passover region” between ballistic flows and Eulerian turbulence, while also connecting the three scaling regimes as the turbulent flow evolves from its initial conditions through the ballistic region, as it superdiffuses into the chaotic, multi-scale fluctuations and flows of the Lagrangian region and transitions to the more homogenous Eulerian region. Additionally, the researchers identify a fourth region “free eddies” — free-floating, rapidly swirling vortices that are disconnected from the earlier turbulence. Their results, according to an introduction of their work in Physical Review Research, “show excellent agreement with Direct Navier-Stokes simulations.”

This enhanced statistical understanding of Lagrangian turbulence will be useful for tackling real-world puzzles of turbulence, such as tracking ocean currents, and predicting weather patterns and how pollutants and airborne pathogens spread.

“This gives us a little more foundation for calculating things like the spread of COVID and other aerosols,” said Birnir, who plans to write a biomedical paper to provide information on how to use this model to better calculate the infectiousness of diseases carried by airborne pathogens.

Plastic polymers are ubiquitous in our lives, and while their resilience makes them ideal for a variety of uses, finding means to effectively dispose of them represents a major challenge. Recently, a variety of plastivore insects have been discovered that possess the remarkable ability to consume and rapidly degrade petro plastics. Focusing on the caterpillar larvae of the greater wax moth (Galleria mellonella) — known as wax worms — and low-density polyethylene, scientists explored how much plastic is consumed, the relative contributions from the insect and its microbiome to the biodegradation process, and the impacts of plastic ingestion on larval fitness.

Plastics play a fundamental role in modern life, but their resistance to biodegradation makes them very difficult to dispose of.

In 2017, biologists demonstrated that the caterpillars of the greater wax moth can degrade polyethylene plastic.

Polyethylene is the world’s most commonly manufactured plastic, with over 100 million tons of polyethylene produced globally each year.

This plastic is chemically resilient, which makes it resistant to decomposition and can take decades or even hundreds of years to fully degrade.

“Around 2,000 wax worms can break down an entire polyethylene bag in as little as 24 hours, although we believe that co-supplementation with feeding stimulants like sugars can reduce the number of worms considerably,” said Dr. Bryan Cassone, a biologist at Brandon University.

“However, understanding the biological mechanisms and consequences on fitness associated with plastic biodegradation is key to using wax worms for large-scale plastic remediation.”

Utilizing a suite of techniques spanning animal physiology, material science, molecular biology and genomics, Dr. Cassone and colleagues studied the interesting relationship between wax worms, their bacterial microbiome, and their potential for large-scale plastic biodegradation, as well as the possible impacts on wax worm health and survivability.

“This is similar to us eating steak — if we consume too much saturated and unsaturated fat, it becomes stored in adipose tissue as lipid reserves, rather than being used as energy,” Dr. Cassone said.

“While wax worms will readily consume polyethylene, this research also shows that this ultimately ends in a quick death.”

“They do not survive more than a few days on a plastic-only diet and they lose considerable mass.”

“However, we are optimistic that we can formulate a co-supplementation that not only restores their fitness to natural levels but exceeds it.”

The researchers identified two ways in which wax worms could contribute solutions to the ongoing plastic pollution crisis.

“Firstly, we could mass rear wax worms on a co-supplemented polyethylene diet as part of a circular economy,” Dr. Cassone said.

“Secondly, we could explore the re-engineering of the plastic biodegradation pathway outside the animal.”

“As a bonus benefit, the mass production of wax worms would also generate a substantial surplus of insect biomass, which could represent an additional economic opportunity in aquaculture.”

“Our preliminary data suggests that they could become part of a very nutritious diet for commercial food fishes.”

The authors presented their findings today at the Society for Experimental Biology Annual Conference in Antwerp, Belgium.

_____

Bryan J. Cassone et al. Plastic Biodegradation by Insects. SEB 2025, abstract # A17.4