Generation of coherent structures in free shear layers

The transition to turbulence of various free-shear flows is studied using simulations involving small-amplitude broad-band random perturbations. In the case of an incompressible mixing layer, two regimes are found. When the initial disturbances are purely three-dimensional, a vortex lattice is obtained, and when the initial disturbances are rather, but not completely, two-dimensional, the same periods are found but the development of the streamwise subharmonic overwhelms that of the spanwise mode. These results suggest that several mechanisms lead to three-dimensionalization of mixing layers and that they may coexist and interact in experimental or practical situations where the level of residual turbulence or perturbations is not low. In all cases, the transition to turbulence is triggered by random noise superimposed on a unidirectional basic flow.