Secondary instability of free shear flows

The three-dimensional stability of saturated two-dimensional vortical states of planar mixing layers and jets is studied by direct integration of the Navier-Stokes equations. Small-scale instabilities are shown to exist for spanwise scales at which classical linear modes are stable. These modes grow on convective time scales, extract their energy from the mean flow, and persist to moderately low Reynolds numbers. Their growth rates are comparable to those of the most rapidly growing inviscid instability and of two-dimensional subharmonic (pairing) modes. The three-dimensional modes do not appear to saturate in quasi-steady states. Indeed, they seem to lead directly to chaos. Results are presented for the resulting three-dimensional turbulent states.