Turbulent Times
One hallmark of fluid turbulence is the cascade of energy from large-scale fluid motions to smaller flow structures: Stirring a cup of coffee ends up causing a multitude of tiny eddies going in every direction. And this cascade goes one way: Tiny eddies do not add up to spin the coffee in a large coherent motion; instead, they are dissipated by viscosity. Researchers have developed various statistical models in which time asymmetry is predicted to be related to an asymmetry in the microscopic motion of particles: The expression for the distance between two particles involves an odd order term in time ( ), which breaks symmetry. But no direct experimental probes could back up this conclusion.
Now, in a paper in Physical Review Letters, a group in the laboratory of Eberhard Bodenschatz at the Max Planck Institute for Dynamics and Self-Organization, Germany, report a controlled laboratory study of particle movements in a turbulent flow. The researchers use a water tank with rotating blades at the top and bottom that create a turbulent fluid section, and they track the motion of suspended polystyrene microspheres with high-speed cameras.
The setup allowed the researchers to track the separation of pairs of particles as a function of time. For short times, the results confirm that the time asymmetry in pair separation depends on a term: two particles separate more slowly in the forward than in the backward direction, a clear manifestation of the breaking of time symmetry. But the authors see a stronger, linear dependence of time asymmetry when they look at how groups of four particles deform in the flow. This allows them to connect the irreversibility to a fundamental property—the rate of strain of the fluid—and suggests that multiparticle tracking might be a powerful way to study turbulence. – David Voss