Simulations suggest a breakthrough in understanding how turbulence develops

Often seen in nature, from whirlpools to the shape of galaxies, vortices are one of the main flow structures that drive turbulence. The dominant idea over the last 100 years is that large swirling motions in a fluid break apart into smaller and smaller swirls, passing energy down the chain until it finally disappears – a process known as the forward cascade.

Johan Hoffman, professor of numerical analysis at KTH Royal Institute of Technology, says there is no reason to abandon that explanation, but now science has to make room for the possibility of another.

The study was published in Scientific Reports.

Hoffman and co-author Joel Kronborg, a PhD student at KTH, began the simulation with two large, counter‑rotating vortices. These big vortices created a very strong strain field — a region where the fluid is stretched in one direction and squeezed in another. That strain field generated the small vortices.

These small-scale vortices then reorganized themselves into patterns whose combined motion produces flow on larger scales. Hoffman says the team used advanced math to break down the flow into basic parts and to explain why the tiny vortices appeared and how they behaved.

Hoffman says the finding doesn’t rule out that energy is transferred also in the forward direction, from big to small vortices.

“Sometimes both can happen together,” he says.

“In fact, this is also the first step in the new mechanism, where energy is transferred from big vortices to vortices on the finest possible scale, before the transfer of energy is reversed to flow from small to larger scales,” he says.

The result could eventually impact a number of industries and applications where turbulence and vortex dynamics are important, he says.

“The aerodynamic performance of airplanes and vehicles is one such example, with potential gains in safety and fuel efficiency,” Hoffman says.

“Another example from our own research is the design of mechanical heart valves, and planning of clinical interventions to treat heart valve disease.”