Whether it’s heart murmurs and pipeline oil transport, or bumpy planes and dispersal of pollutants, turbulence plays an important role in many everyday events. But despite being commonplace, scientists still don’t fully understand the seemingly unpredictable behavior of vortices and eddies in turbulent flows.
Today, a new technique for measuring turbulent flows has been developed by an international collaboration of scientists from the Okinawa Institute of Science and Technology Graduate University (OIST) in Japan, with the University of Genoa, Italy, KTH Stockholm, Sweden and ETH Zurich, Switzerland. By using fibers rather than particles – the usual measurement method – researchers could get a more detailed picture of turbulent flows. Their method was reported on September 17 in the newspaper, Physical examination X.
“Turbulence is a very unique and complicated phenomenon, it has even been called the last unsolved problem in classical physics,” said Dr Stefano Olivieri, postdoctoral researcher in the Complex Fluids and Flows Unit at OIST, who was the author of the study. “It’s hard to predict, hard to simulate and hard to measure.”
Measuring turbulent flows is an urgent challenge for physicists for many reasons. Not only is turbulence characterized by its chaotic and random nature, it also occurs on multiple scales at once. In turbulent flows, the eddies of swirling fluid break down into smaller and smaller eddies, until finally the eddies are so small and viscous that the kinetic energy of the fluid is transferred to the environment as heat. .
Currently, the most common way to measure turbulent flow is by tracking the movement of particles, called tracers, that are added to the fluid. These particles are tiny and similar in density to fluid, so they move at the same speed and in the same direction as the flow.
But to observe how each vortex of fluid moves, it is not enough to watch how a particle moves. Physicists need to be able to determine how two particles that are a certain distance apart move relative to each other. The smaller the vortex, the closer the two particles must be to characterize the movement of the vortex.
To complicate matters, one of the defining characteristics of turbulence is its diffusivity: a turbulent flow will spread out over time, as will tracers, especially in open flows, such as an ocean current. In many cases, the tracers can quickly extend too far apart to measure the behavior of vortices.
“Each tracer particle moves independently of each other, so you need a lot of tracer particles to find the ones that are the right distance from each other,” explained Prof. Marco Rosti, who heads the Fluids and complex flows of OIST.
“And too many tracer particles can actually disrupt the flow,” he added.
To get around this problem, the research team has developed an innovative and simple solution to the problem: use fibers instead of tracer particles.
The researchers created a computer simulation where fibers of different lengths were added to a turbulent flow. These fibers were rigid, which kept the ends of each fiber a fixed distance. By tracking how each fiber moved and rotated in the fluid over time, the researchers were able to construct an image that encompassed the full scale and structure of the turbulent flow.
“By using rigid fibers, you can measure the difference in speed and direction of flow at two points a fixed distance apart, and you can see how these differences evolve with the scale of the vortex. The shorter fibers have also allowed us to accurately measure the rate at which the kinetic energy of the fluid is transferred from the largest to the smallest scale, where it is then dissipated by heat. This value, called the rate of energy dissipation, is a crucial quantity in the characterization of turbulent flows, “said Professor Rosti.
The researchers also performed the same experiment in the laboratory. They made two different fibers, one from nylon and the other from a polymer called polydimethylsiloxane. The team tested these two fibers by adding them to a water tank with turbulent water and found that the fibers gave similar results to the simulation.
However, with the use of rigid fibers comes an important caveat, the scientists pointed out, because the overall movement of the fiber ends is limited.
“Due to the stiffness of the fiber, the ends of the fibers cannot move towards each other, even if that is the direction of flow. This means that a fiber cannot fully represent the movement of flow in the same way that tracer particles can, ”explained Dr Olivieri. “So before even starting any simulations or laboratory experiments, we first had to develop an appropriate theory that took into account these movement limitations. This was perhaps the most difficult part of the project. “
The researchers also measured the same turbulent flow in the laboratory in a conventional manner, adding a high concentration of tracer particles to the water tank. The results obtained from the two different methods were similar, verifying that the fiber method and the newly developed theory gave accurate information.
In the future, the researchers hope to expand their method to incorporate flexible fibers that have fewer restrictions on how they move. They also plan to develop a theory that can help measure turbulence in more complex non-Newtonian fluids that behave differently from water or air.
“This new technique has a lot of exciting potential, especially for scientists studying turbulence in large open flows such as ocean currents,” said Professor Rosti. “And being able to easily measure quantities that were previously difficult to obtain takes us one step closer to fully understanding turbulence.”
