Local and global instabilities in spatially developing flows

The paper reviews recent developments in the hydrodynamic stability theory of spatially developing flows pertaining to absolute/convective and local/global instability concepts. It is demonstrated how these notions can be used effectively to obtain a qualitative and quantitative description of the spatial-temporal dynamics of open shear flow, such as mixing layers, jets, wakes, boundary layers, and plane Poiseuille flow. Only 'open flows' are considered, where fluid particles do not remain within the physical domain of interest, but are advected through downstream flow boundaries. In addition, the implications of local/global and absolute/convective instability concepts for geophysical flows are alluded to briefly.