STRUCTURES IN STRATIFIED PLANE MIXING LAYERS AND THE EFFECTS OF CROSS-SHEAR

A two-dimensional temporal mixing layer is generated in a stratified t ilting tank similar to that used by Thorpe (1968). Extensive flow dyna mics visualization is carried out using, for the top and bottom layers , fluids of different densities but of the same index of refraction. T he two-dimensional density field is measured with the laser-induced fl uorescence technique (LIF). The study examines further the classical p roblem 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 ano ther component of shear, in plane with but perpendicular to the main s hear of the base flow, generated by tilting the tank around a second a xis. In the two-dimensional mixing layer, the pairing process is found not only to govern the growth of the mixing layer as is commonly know n, but also to play a critical role in the mixing transition. The flow region between pairing vortices exhibits a complex topography of stre tches and folds in the fluid interface, the length of which is measure d to grow exponentially in time. But as higher stratification increasi ngly inhibits the pairing process, the flow topography becomes less co mplex, with the material interface growing less rapidly (linearly). Al so, the total yield of mixed fluid, as calculated from the measurement s of the density field, is reduced with higher stratification. The red uced mixing is due in part to the reduction in the fluid entrainment i nto Kelvin-Helmholtz vortices (both in the overall volume and in the p ortion of the bottom fluid to the overall volume), the reduced frequen cy of pairing of those vortices, and the subsequent arrest of turbulen ce during flow restratification. The stratified mixing layer also exhi bits many interesting secondary features which have been previously do cumented to various degrees-the baroclinic shear-induced instability i n the braid region, gravitational convective instability within the co res, vortex tearing, and vortex dislocations of the Kelvin-Helmholtz v ortices. The introduction of a critical level of cross-shear to a plan e shear layer results in a new type of 'co-rotating' streamwise vortic es 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 (d ynamics of roll-up and pairing) and quantitatively (normalized length and time scales). It is also found that if cross-shear is introduced t o the shear layer while it is still planar, the resulting flow behaves simply as a normal but oblique two-dimensional. mixing layer. The cor otating 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 baroclinicall y induces strong local cross-shear.