For decades, scientists believed that in a turbulent system, energy cascades in only one direction—from larger scales to smaller ones or vice versa—depending on the system's dimensions. But researchers in Lei Fang's lab at the University of Pittsburgh found that this rule can be broken with a tiny adjustment: by inserting a small obstacle at just the right angle, they reversed the direction of the energy cascade.
“The geometry matters,” Fang said.
The discovery came from watching brine shrimp, which are about a centimeter long and swim upside down, beating their legs and trailing their abdomens. The tiny swimmers served as visible tracers in the turbulent fluid, revealing the unexpected reversal. The finding challenges a fundamental tenet of turbulence theory and suggests that geometry, not just scale, can control energy transfer.
The study focused on two-dimensional turbulence, a simplified yet important system. Whether the effect extends to three dimensions remains to be seen, but the result already points to new possibilities for manipulating turbulent flows in engineering and environmental contexts.