Limited length changes stratified shear layer turbulence: why extra-long structures matter

What is it about?

We studied turbulence that forms where two fluid layers of different density slide past each other—like in parts of the atmosphere or ocean. Using supercomputer simulations, we changed how much “room” the flow had in the streamwise direction. When there is enough space, very long, meandering bands of fast and slow flow appear and grow; when space is tight, these extra-long structures cannot form. Despite that, the averages—such as the mean speed and density profiles—and the small-scale churning that controls dissipation stay largely the same. The work explains which turbulent features truly depend on having a long domain and which do not.

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Photo by Kirari Shteyn on Unsplash

Photo by Kirari Shteyn on Unsplash

Why is it important?

Engineers and geophysical scientists often simulate stratified shear layers to understand mixing and predict weather or ocean conditions. Our results show when simulations must be run in very long domains to capture the extra-long structures—and when shorter domains still give reliable averages and small-scale behaviour. This helps avoid misleading results caused by artificial confinement, guides the design of future simulations and facilities, and sharpens turbulence models by separating the roles of typical large eddies from those sprawling, slow-to-evolve patterns that organise the flow at the largest scales.