Doctoral student Yi Li and Professor Charles Meneveau conduct turbulence experiments in a wind tunnel at the Johns Hopkins Homewood campus. Photo: Will Kirk/Johns Hopkins University
"This equation gives us a mathematical shortcut to describe a complex characteristic of turbulence called intermittency," said Charles Meneveau, professor of mechanical engineering and director of the Center for Environmental and Applied Fluid Mechanics at Johns Hopkins. "It solves just one piece of the overall turbulence puzzle, but it's a very important piece."
Intermittency refers to abrupt, concentrated changes in the speed of a moving fluid. If the velocity of a fluid is plotted on a graph, these changes look like sharp drop-offs or cliffs, rather than smooth, gentle slopes. These sharp changes occur infrequently within a turbulent flow. But when they arise, they can be quite violent.
This characteristic has been particularly tough to include in computer models of turbulence because representing it numerically requires a huge number of calculations and a mammoth amount of computing power. "Conceptually, we could do it," Meneveau said. "But it's not practical."
Meneveau and doctorial student Yi Li devised a shortcut by tracking two particles as they move with a turbulent flow like two balloons tossed along by a gust of wind. The resulting equation gave them a tool to predict intermittency by merely solving this simple equation rather than having to solve complicated computer models of turbulence. "Ultimately, we believe this will help researchers put together more precise models that could be used to predict weather patterns, movement within bodies of water and even some turbulent events that take place within an internal combustion engine," Meneveau said.
He and his students have been measuring turbulence and intermittency in a Johns Hopkins wind tunnel.