GALCIT Colloquium

The dynamics of vortical structures in shear flows such as jets, wakes, boundary layers is traditionally described by resorting to local hydrodynamic instability analyses, whereby the basic undisturbed flow is assumed to be slowly developing in the stream-wise direction. In such a framework, local parallel flow concepts have been quite successful in accounting for the "amplifier" or "oscillator" behavior of various classes of shear flows and in predicting the global frequency of flows that behave as "oscillators". More recently, powerful numerical algorithms have been developed, which allow for the full determination of the global frequency spectrum of complex flows, without invoking the slowly-varying assumption. The relative merits of the local and global approaches will be discussed and assessed for several prototypical flows: wakes, isothermal or hot jets, swirling jets undergoing vortex breakdown and swept wing boundary layers.