• *Physics* 17, s77

A statistical tool tests the long-held assumption that small-scale turbulence is isotropic.

Large-scale turbulence is causing new headaches for anxious aviators in a warming atmosphere, but small-scale turbulence has physicists scratching their heads. The cascading process that transfers energy from the largest eddies down to the smaller ones results in a small-scale turbulent flow that retains no memory of the large-scale flow structure. Because of this loss of memory, researchers often assume that small-scale turbulence is isotropic. Subharthi Chowdhuri and Tirtha Banerjee of the University of California, Irvine, now introduce a framework for investigating turbulent flows and show that anisotropy persists even at small scales (1).

If you measure the statistical properties of a large-scale turbulent flow relative to the total size of the flow, you will get different values in different directions. Determining whether the same is true at small scales, where the effect of viscosity becomes dominant, is difficult because obtaining 3D information of the flow field at a sufficiently high resolution is impossible. Instead of trying to directly measure small-scale turbulence, Chowdhuri and Banerjee quantified its anisotropy by modeling how sudden, short-lived perturbations transmit kinetic energy in different directions. They applied their model to previously obtained numerical and experimental data sets covering a wide range of flow conditions. They found that in each case they could successfully identify small-scale anisotropy from sequences of measurements taken at a single point in space—a more feasible task than capturing the entire 3D image.

Chowdhuri says their results imply a universal relationship between the large-scale conditions of a turbulent flow and its small-scale anisotropy. Incorporating this relationship will help make next-generation turbulence models more accurate.

– Rachel Berkowitz

Rachel Berkowitz is a corresponding editor for *Physics Magazine* based in Vancouver, Canada.

## reference

- S. Chowdhuri and T. Banerjee, “Quantification of small-scale anisotropy in turbulent flows”, Phys. rev. Fluids
**9**074604 (2024).

## Thematic Areas