Fiber tracking method delivers important new insights into turbulence


Date:
September 17, 2021
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
A new technique for measuring turbulent flows has been developed. By
using fibers rather than particles -- the usual method of
measurement - - the researchers could get a more detailed picture
of turbulent flows.



FULL STORY ========================================================================== Whether it's heart murmurs and pipeline transport of oil, or bumpy
airplanes and the 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
the swirls and eddies in turbulent flows.


==========================================================================
Now, 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, along with
the University of Genova, Italy, KTH Stockholm, Sweden and ETH Zurich, Switzerland. By using fibers rather than particles -- the usual method
of measurement -- the researchers could get a more detailed picture
of turbulent flows. Their method was reported on 17th September in the
journal, Physical Review X.

"Turbulence is a very unique and complicated phenomena, it's even been
called the last unsolved problem in classical physics," said Dr. Stefano Olivieri, a postdoctoral researcher from the Complex Fluids and Flows
Unit at OIST, who was an author of the study. "It's difficult to predict, difficult to simulate, and difficult to measure." Measuring turbulent
flows is a pressing challenge for physicists for numerous reasons. Not
only is turbulence characterized by its chaotic and random nature, but it
also occurs across many scales at once. In turbulent flows, the swirling vortices of fluid break down into eddies that are smaller and smaller
in scale, until eventually 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 flows is by
tracking the movement of particles, called tracers, that are added to
the fluid. These particles are tiny and of similar density to the fluid,
and so move at the same speed and in the same direction as the flow.

But in order to observe how each swirl of fluid is moving, looking at how
one particle moves isn't enough. Physicists need to be able to determine
how two particles that are a specific distance apart move in relation
to each other.

The smaller the eddy, the closer together the two particles need to be
to characterize the motion of the vortex.



==========================================================================
To make matters more challenging, one of the defining features of
turbulence is its diffusivity -- a turbulent flow will spread apart over
time, and so too will the tracers, especially in open flows, like an
ocean current. In many cases, tracers can quickly spread too far apart
to measure how the eddies are behaving.

"Every tracer particle is moving independently of each other, so you
need lots of tracer particles in order to find ones that are the right
distance apart," explained Professor Marco Rosti, who leads the OIST
Complex Fluids and Flows Unit.

"And too many tracer particles can actually disrupt the flow," he added.

To circumvent this issue, the research team developed an innovative and
easy solution to the problem: using 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 apart. By tracking how each
fiber moved and rotated within the fluid over time, the researchers were
able to build up a picture that encompassed the full scale and structure
of the turbulent flow.



==========================================================================
"By using rigid fibers, we can measure the difference in the speed
and the direction of the flow at two points a fixed distance apart,
and we can see how these differences change depending on the scale of
the eddy. The shortest fibers also allowed us to accurately measure the
rate at which the kinetic energy of the fluid is transferred from the
largest to the smallest scales, where it is then dissipated by heat. This value, called the energy dissipation rate, is a crucial quantity in the characterization of turbulent flows," said Prof. Rosti.

The researchers also performed the same experiment in the laboratory. They manufactured two different fibers, one made from nylon and the other
from a polymer called polydimethylsiloxane. The team tested both these
fibers by adding them to water tank containing turbulent water and found
that the fibers gave similar results to the simulation.

However, using rigid fibers comes with one important caveat, the
scientists emphasized, as the overall movement of the fiber ends is
restricted.

"Due to the fiber rigidity, the fiber ends can't move towards each
other, even if that's the direction of the flow. That means that a fiber
cannot fully represent the movement of the flow in the same way that
tracer particles can," explained Dr. Olivieri. "So before we even began simulations or lab experiments, we first needed to develop a suitable
theory that took these limitations of movement into account. This was
perhaps the most challenging part of the project." The researchers also measured the same turbulent flow in the laboratory the conventional
way, by 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.

Moving forward, the researchers hope to expand their method to incorporate flexible fibers that have less restriction 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 like ocean currents,"
said Prof.

Rosti. "And being able to easily measure quantities
that were previously difficult to obtain moves
us one step closer to fully understanding turbulence." ========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Dani Ellenby. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Stefano Brizzolara, Marco Edoardo Rosti, Stefano Olivieri,
Luca Brandt,
Markus Holzner, Andrea Mazzino. Fiber Tracking Velocimetry for
Two-Point Statistics of Turbulence. Physical Review X, 2021; 11
(3) DOI: 10.1103/ PhysRevX.11.031060 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210917122049.htm

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