14 DAY TRIAL // Turbulence is part of the daily experience: no microscopes or telescopes are needed to notice the meanders of cigarette smoke, the gracious arabesques of cream poured into coffee or turbulent whirls in a mountain torrent. The word "turbulence" indicated first the incoherent movements of the crowd (Latin: turba), then the whirls of leaves or dust. Since Leonardo da Vinci (around the year 1500), the term took its modern meaning of "incoherent and disordered movements of air or water".
Turbulence is important in virtually all phenomena involving fluid flow, such as air and gas mixing in an engine, ocean waves breaking on a cliff and air whipping across the surface of a vehicle. However, a comprehensive description of turbulent fluid motion remains one of physics' major unsolved problems.
Researchers have long suspected that there is a hidden but coherent structure underlying turbulence's messy complexity, but there has been no objective way of identifying it, said MIT research group leader George Haller, professor of mechanical engineering. MIT researchers have illustrated for the first time a convoluted tangle underlying turbulence, which may ultimately help engineers design better planes, cars, submarines and engines.
The group used water jets to force water from below into a rotating tank of fluid. They seeded the resulting complicated flow with luminescent buoyant particles. When illuminated with a laser, the minuscule polystyrene spheres were visible as they raced around the vortexes and jets.
'With this approach, we isolated the very source of turbulent mixing, not just its effect on dye or smoke as earlier studies did,' said MIT mechanical engineering graduate student Manikandan Mathur.
In turbulent flow, unsteady vortices appear on many scales and interact with each other. The team discovered that the complicated, constantly evolving flow patterns are driven by two competing armies of particles constantly being pulled together and pushed apart.
The MIT researchers call their discovery the "Lagrangian skeleton" of turbulence because their particle-based approach is motivated by the work of 19th-century mathematician Joseph-Louis Lagrange. "Lagrange developed mathematical tools still used today for calculating mechanical and fluid motion," said Thomas Peacock, assistant professor of mechanical engineering at MIT.
Among many applications, the new results promise to aid the early detection of clear air turbulence that causes those unexpected jolts in airplanes. They may also help control the spread of oceanic pollution and possibly to improve the future aerodynamic design of cars.