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