From the mixing of milk in coffee to the formation of raindrops in clouds, the physics of turbulent flow is everywhere. Researchers at Cornell have discovered that seemingly random turbulent flows, which are the flow of a fluid in which velocity varies rapidly and irregularly, actually have an astonishing structure.
The work, published online June 5 in the journal Nature Physics, was led by Eberhard Bodenschatz, Cornell adjunct professor of physics and of mechanical and aerospace engineering. The work was in collaboration with researchers Haitao Xu of the Max Planck Institute for Dynamics and Self-Organization in Germany and Alain Pumir of Ecole Normale Supérieure de Lyon in France.
The researchers used three high-speed cameras to monitor stereoscopically at 20,000 pictures per second hundreds of polystyrene tracer particles in a turbulent water flow. They concluded that vortices behave similarly to an ice skater performing a pirouette -- whereby the skater bends his or her arms to increase the speed of rotation.
The tracer particles they observed were initially separated by equal distances so that they formed a tetrahedron (a shape composed of four triangles). The team observed the flow stretching the tetrahedron so that it became thinner. In addition, the tetrahedron's axis of rotation moved to align parallel with the original stretching direction of the flow. The stretched tetrahedron's speed of rotation ultimately increased. Initially, said Bodenschatz, the angular momentum was conserved.
In this respect, the observed dynamics was very similar to a pirouette of a spinning ice skater. The fact that the angular momentum was conserved in a turbulent liquid was something that surprised the physicists.
"We do not yet understand why this is the case," Bodenschatz said. Their discovery unveils a surprising simple structure in the turbulent flow that one day may lead to simpler computer modeling of turbulent processes and help scientists understand, for example, the simulation of clouds in climate models, he said.
The work was supported by the Max Planck Society.