Fiber tracking method delivers important new insights into turbulence

There are many scales that swirls and eddies can occur in turbulent flows. Credit: Complex Fluids and Flows Unit (OIST).
Turbulence is an integral part of many daily events, including heart murmurs, pipeline transport of oil, and bumpy planes and the dispersal and distribution of pollutants. Scientists still haven't figured out how to predict the unpredictable behavior of turbulent flows and the swirls.

A new method for measuring turbulent flow has been developed by a global collaboration of scientists from Okinawa Institute of Science and Technology Graduate University in Japan and the University of Genova (Italy), KTH Stockholm, Sweden, and ETH Zurich (Switzerland). Researchers could obtain a better picture of turbulent flow patterns by using fibers instead of particles, a new method of measuring them. The journal Physical Review X published their method on 17 September.

Dr. Stefano Olivieri was a postdoctoral researcher at OIST's Complex Fluids and Flows Unit. He was also the author of the study. It's hard to predict, simulate, and measure.

For many reasons, physicists face a difficult task in measuring turbulent flows. Turbulence is chaotic and random in its nature. It also happens at multiple scales simultaneously. The turbulent flow of fluid breaks down into smaller and smaller eddies, until the kinetic energy from the fluid is transferred to the surrounding environment as heat.

The most effective way to measure turbulent flow is to track the movement of particles (called tracers) that are added to fluid. These tiny particles have a similar density to fluid and move in the same direction and speed as the flow.

Researchers created a simulation in which fibers were added into a turbulent flow. To extract information about the flow, the motion of the fibers could be used. Credit: Complex Fluids and Flows Unit (OIST).

However, it is not enough to see how each particle moves in fluid swirls. Physics must be able to identify how particles at a certain distance move relative to one another. To characterize the vortex's motion, the particles must be closer together than the eddy.

Diffusivity is a key feature of turbulent flow. Turbulence will eventually spread apart, just like an ocean current. Many times, tracer can spread far too fast to accurately measure the behavior of the eddies.

Professor Marco Rosti who heads the OIST Complex Fluids and Flows Unit explained that each tracer particle moves independently from each other. Therefore, you need many tracer particles to find the ones that are right distance apart.

He said, "And too many tracer particle can actually disrupt flow."

The research team devised a simple and innovative solution to this problem by using fibers as tracer particles instead.

Researchers created a computer simulation in which fibers of different lengths were incorporated into turbulent flows. The fibers were made of rigid fibers, which separated the ends of the fibers at a fixed distance. Researchers were able to track how each fiber moved within the fluid over time and create a picture of the entire structure and scale of turbulent flow.

The ETH Zurich research team created a turbulent flow in a closed water tank to track the movement of fibers. Credit: ETH Zurich

We can use rigid fibers to measure the speed and direction of flow at two points that are a fixed distance apart. This allows us to see how the differences change depending upon the size of the eddy. We could also measure accurately the rate at which fluid's kinetic energy is transferred from the largest scale to the smallest, and then heat dissipated. Professor Rosti said that this value, known as the energy dissipation ratio, is crucial in the characterization of turbulent flows.

Researchers also conducted the same experiment in the lab. They manufactured two different fibers, one made from nylon and the other from a polymer called polydimethylsiloxane. Both fibers were tested by the team in a water tank with turbulent water. The fibers produced similar results to the simulation.

The scientists stressed that rigid fibers have one caveat: the fiber ends are restricted in movement.

"Due to fiber rigidity, fiber ends cannot move towards each other even if the flow direction is in the same direction." Dr. Olivieri explained that fibers cannot accurately represent the flow's movement in the same way tracer particles can. Before we could do simulations or run lab experiments, we had to first develop a theory that would account for these limitations. This was the most difficult part of the project.

Researchers also used a conventional method to measure turbulent flow in the laboratory, adding tracer particles to the water tank. Both the fiber method as well as the new theory provided similar results.

The researchers plan to improve their method by incorporating flexible fibers with less restriction in how they move. The researchers also plan to develop a theory for measuring turbulence in non-Newtonian fluids with different behavior than water and air.

Prof. Rosti stated that "this new technique has a lot potential, especially for scientists who study turbulence within large, open flows such as ocean currents." "And being able easily to measure quantities that were previously hard to obtain takes us one step closer towards understanding turbulence."

Continue reading to discover the secrets of COVID-19 transmission in turbulent puffs

More information: Stefano Brizzolara et., Fiber Tracking Velocimetry to Two-Point Statistics on Turbulence, Physical Review X (2021). Information from the Journal: Physical Review X Stefano Brizzolara et. al. Fiber Tracking Velocimetry For Two-Point Statistics Of Turbulence, (2021). DOI: 10.1103/PhysRevX.11.031060