Characteristics of flow instabilities from unstable stratification of density in channel shear layers at low Reynolds numbers

The instabilities which form due to globally unstable stratification of den sity in the shear layer which develops between two coincident streams of di fferent velocity in a channel are described. A portion of the flow is conta minated with dye to visualize flow features. The channel employed is 1.59 c m thick, Reynolds numbers range from 10(0)-10(1) and Richardson number rang es from 10(-3) 2 x 10(1). A variety of flow features are evident in unstabl e, marginally stable and unstable flow domains, including streamwise vortic es, hairpin vortices, spanwise structures, and localized turbulent motions. Spanwise and streamwise wave number variations in these structures are als o described, along with the mixing onset boundary, and a scaling factor whi ch accounts for the dependence of the mixing onset boundary on channel thic kness. (C) 1998 Elsevier Science Ltd. All rights reserved.