# The coherent structure of the kinetic energy transfer in shear turbulence

## TL;DR

This paper reveals the three-dimensional structure of kinetic energy transfer in shear turbulence, identifying vortex hairpins as key features and explaining the asymmetry between forward and backward cascades.

## Contribution

It uncovers the detailed 3D vortex structures associated with energy transfer in shear turbulence, advancing understanding of turbulence dynamics.

## Key findings

- Vortex hairpins are central to energy transfer structures.
- Asymmetry between forward and backward cascades is due to flow circulation differences.
- Flow patterns reverse depending on cascade direction.

## Abstract

The cascade of energy in turbulent flows, i.e., the transfer of kinetic energy from large to small flow scales or vice versa (backward cascade), is the cornerstone of most theories and models of turbulence since the 1940s. Yet, understanding the spatial organisation of kinetic energy transfer remains an outstanding challenge in fluid mechanics. Here, we unveil the three-dimensional structure of the energy cascade across the shear-dominated scales using numerical data of homogeneous shear turbulence. We show that the characteristic flow structure associated with the energy transfer is a vortex shaped as an inverted hairpin followed by an upright hairpin. The asymmetry between the forward and backward cascade arises from the opposite flow circulation within the hairpins, which triggers reversed patterns in the flow.