Coherent structures and studies of perturbed and unperturbed jets.

The following aspects of large-scale coherent structures are addressed: definition and identification of the coherent structure, difficulties in the eduction of naturally occurring structures, need for studies of structures induced via controlled excitations, relevance of induced structures to natural structures, the effects of initial and boundary conditions including characterization of the initial condition, and some analytical and experimental considerations in investigations of coherent structures. The significance of coherent structures vis-a-vis the background turbulence is discussed, and on the basis of available evidence it is suggested that although coherent structures are most probably inherent to all turbulent shear flows, their predominance in fully developed flows may have been exaggerated. (from authors abstract)