Shear layers appear when two adjacent and tangential streams moving at different velocities meet at some location; the shear layer is then the flow downstream of that location. The vorticity layer between the two streams then undergoes a classical hydrodynamic instability, known as the Kelvin-Helmholtz instability, which leads to the formation of two-dimensional rollers.
The video presents the large eddy simulation of a shear layer where the velocity of the lower stream is 38% that of the upper stream, and where some synthetic turbulence is injected at the inlet of the simulation domain. Although very weak, this specific turbulence excites 3D instabilities in addition to the development of the Kelvin-Helmholtz instability. As a result, one here observes a faster transition to a 3D flow and then to a fully turbulent state, through the growth of complex and stretched flow structures governed by vortex dynamics.