Physicists reversed the way energy flows through turbulence, bending an 80-year rule

In a thin sheet of swirling water, physicists made energy run the wrong way. Instead of cascading toward ever-smaller eddies, or toward ever-larger ones, the flow did whichever the researchers chose, depending on how they arranged the forces stirring it. The result challenges an assumption that has shaped fluid physics for more than eighty years.

For more than eighty years, the working picture of turbulence has been a one-way cascade. In the three-dimensional flows of a river or the open ocean, energy was thought to move steadily from large whirls down to small ones, where it finally dissipates as heat. In thin, nearly two-dimensional layers, the cascade was believed to reverse, with small eddies feeding larger ones. Either way, the direction looked fixed by the geometry of the space the fluid lived in.

The new work pulls that direction loose from the dimensions of the flow. What sets the direction, the researchers found, is the alignment between two quantities at every point in the fluid: the stress squeezing it and the deformation it undergoes in response. Tune the angle between force and displacement, and energy can be pushed up the scale ladder or down it. The shape of the container stops being destiny.

To show it, the team drove a shallow layer of electrically conducting liquid with magnetic forces, sprinkled it with tracer particles, and filmed how the particles moved. By reshaping the pattern of forcing, they produced flows with forward energy transfer and flows with inverse transfer in the same apparatus. Computer simulations of the same setup reproduced the switch, which is the kind of agreement that turns a surprising image into a measurement.

Controlling the cascade has practical reach. The direction energy flows in a fluid governs how things spread through it, so steering it could change how a coastline disperses a plume of wastewater, how a microfluidic chip mixes tiny volumes of fluid for a medical test, or how energy moves through the layered flows that climate models try to capture.

The demonstration lives in a carefully controlled, essentially two-dimensional system, not in the messy three-dimensional turbulence of a storm or a deep current. Whether the same handle on direction survives in fully three-dimensional, high-energy flows is an open question, and the leap from a laboratory tray to the ocean is large. The principle is now established; its range is not.

The research was carried out by a team led from the University of Pittsburgh, working with collaborators at the University of Turin, and was published in the journal Science Advances. The group’s next step is to test how far the tensor-alignment control extends as flows grow thicker and more energetic, the regime where most real turbulence actually lives.

Tags: physics