For decades, physicists thought that in a turbulent two-dimensional system, energy always cascades from small eddies up to large ones—the process behind Jupiter's Great Red Spot. But researchers at the University of Pittsburgh, studying brine shrimp as a model of active matter, found that this flow is not fixed. By placing a small obstacle in the fluid at just the right angle, they could reverse the cascade, sending energy from larger scales down to smaller ones.
"The geometry matters," said Lei Fang, the engineer who led the study. The team, including student Xinyu Si, had been investigating how biological swimmers mix fluids in turbulent environments. Their unexpected result suggests that the direction of energy transfer in a turbulent system can be controlled by simple physical disruptions, not just by the system's dimensions.
The work was praised by Gregory Falkovich, a physicist at the Weizmann Institute of Science, as "a beautiful and skillful experimental work." While the experiments were in two dimensions, the findings may extend to larger, more chaotic three-dimensional systems, with potential applications in pollution control and drug design. The source article reports no conflicting views, and the main caveat is that the effect was demonstrated in a controlled laboratory setting rather than in natural environments.