Turbulent flows can be difficult to measure and characterize due to their chaotic nature and the fact they occur across many scales at once. Particle tracking velocimetry (PTV) has traditionally been used as a method for studying turbulence, but has several limitations; particles move independently and often won’t maintain a fixed distance from each other long enough to reliably probe two-point statistics, and using too many particles could even disrupt the flow. Studying natural turbulent flows in large open areas, like the ocean, can result in tracers being dispersed too far apart too quickly to effectively measure eddy behavior. Researchers at the Okinawa Institute of Science and Technology (OIST) Graduate University, in collaboration with international partners, have designed a new method for two-point statistics of turbulence using rigid fibers of different sizes, instead of particles, to ensure a fixed distance and track kinetic energy transfer from the largest to smallest scales.
The researchers first ran computer simulations of fibers introduced to a turbulent flow, observing how the fibers would move and rotate to provide insight about the scale and structure of the flow. The researchers then brought the experiment from the computer to the lab, creating two different types of fibers made from nylon or the polymer polydimethylsiloxane and adding them to a tank with turbulent water. The researchers found the results of the experiments to be similar to those yielded from the simulation, and also in agreement with data obtained by PTV.
Video: The researchers created a simulation where fibers were added to a turbulent flow. The motion of the fibers was then used to extract information about the flow. Credit: Complex Fluids and Flows Unit, OIST
The fixed distance between the ends of the rigid fibers made it easier to measure the differences in speed and the direction of the flow between the two points, and the different fiber lengths provided insights into changes depending on the size of the eddy, according to co-author Marco Rosti, who leads the Complex Fluids and Flows Unit at OIST. The shortest fibers also allowed the team to measure the energy dissipation rate of the flow, a key metric for characterizing turbulence, Rosti said. This research was published in Physical Review X.
“This new technique has a lot of exciting potential, especially for scientists studying turbulence in large, open flows like ocean currents,” said Rosti. “And being able to easily measure quantities that were previously difficult to obtain moves us one step closer to fully understanding turbulence.”
One limitation of the new method is that the overall movement of the fiber ends are restricted, as they cannot move toward each other like particles can, explained co-author Stefano Olivieri. The researchers took these limitations into account while conducting their simulations and experiments, and plan to further develop the method using flexible fibers that are less restricted in their movement. The team is also interested in developing a way to measure turbulence in more complex, non-Newtonian fluids.