Coherent structures and studies of perturbed and unperturbed jets

Abstract

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. Progress made in the understanding of large-scale coherent structures in the following experimental studies of perturbed and unperturbed axisymmetric turbulent jets are then summarized: coherent structures in the jet near field induced by impulsive and sinusoidal excitations; coherent structure distortion due to the Taylor hypothesis and applicability of the hypothesis to turbulent shear flows; the sensitivity of the jet 'preferred mode' coherent structure to the Reynolds number and initial condition; and coherent structures in the jet near and far fields in the absence of any excitation. The signigicance 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.