  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.
