Flow instability and turbulence

The problem of flow instability and turbulence is reexamined for the case of free mixing layers and jets. It is argued that some rather simple stability considerations can describe the flow development whether the flow is fully turbulent or not. Particular consideration is given to the subharmonic feedback mechanism, in which the pressure field induced by a pairing process feeds back to the origin of the shear layer (the region most sensitive to outside disturbances), thus perturbing the layer in the form of a subharmonic (or multiple subharmonic) to the self-excited frequency of the initial layer.