Structures in stratified plane mixing layers and the effects of cross-shear

A two-dimensional temporal mixing layer is generated in a stratified tilting tank similar to that used by Thorpe (1968). Extensive flow dynamics visualization is carried out using, for the top and bottom layers, fluids of different densities but of the same index of refraction. The two-dimensional density field is measured with the laser-induced fluorescence technique (LIF). The study examines further the classical problem of the two-dimensional mixing layer and explores the effects of cross-shear on a nominally two-dimensional mixing layer, a situation widespread in complex industrial and natural flows. Cross-shear is another component of shear, in plane with but perpendicular to the main shear of the base flow, generated by tilting the tank around a second axis. In the two-dimensional mixing layer, the pairing process is found not only to govern the growth of the mixing layer as is commonly known, but also to play a critical role in the mixing transition. The flow region between pairing vortices exhibits a complex topography of stretches and folds in the fluid interface, the length of which is measured to grow exponentially in time. But as higher stratification increasingly inhibits the pairing process, the flow topography becomes less complex, with the material interface growing less rapidly (linearly). Also, the total yield of mixed fluid, as calculated from the measurements of the density field, is reduced with higher stratification. The reduced mixing is due in part to the reduction in the fluid entrainment into Kelvin-Helmholtz vortices (both in the overall volume and in the portion of the bottom fluid to the overall volume), the reduced frequency of pairing of those vortices, and the subsequent arrest of turbulence during flow restratification. The stratified mixing layer also exhibits many interesting secondary features which have been previously documented to various degrees-the baroclinic shear-induced instability in the braid region, gravitational convective instability within the cores, vortex tearing, and vortex dislocations of the Kelvin-Helmholtz vortices. The introduction of a critical level of cross-shear to a plane shear layer results in a new type of 'co-rotating' streamwise vortices in the braid region of the primary Kelvin-Helmholtz instability and an appreciable gain in the total yield of mixed fluid. The appearance and dynamics of the secondary streamwise vortices are very similar to those of the primary Kelvin-Helmholtz vortices, both qualitatively (dynamics of roll-up and pairing) and quantitatively (normalized length and time scales). It is also found that if cross-shear is introduced to the shear layer while it is still planar, the resulting flow behaves simply as a normal but oblique two-dimensional. mixing layer. The corotating streamwise vortices and the corresponding added mixing result only when cross-shear is introduced after the primary shear layer has started to roll up. There is also evidence that even in the absence of 'global' cross-shear, the co-rotating streamwise vortices can develop locally where a high curvature of the density interface baroclinically induces strong local cross-shear.