Turbulent gas mixing in strong density stratified shear and non-shear flows

Turbulent mixing in the presence of density stratifications is doubtlessly the most efficient mixing process in practice. Despite its technical relevance in fields of safety in nuclear reactors, chemical process engineering and the transport of liquefied gases, as well in the simulation of fires, the phenomenon mostly rules the dynamics in atmospheres and ocean currents. To study the fundamental turbulent mixing phenomena in the presence of high density gradients and shear, an experimental facility was built, consisting of an open gas mixing loop supplying a mixing channel with square cross section, in which measurements are taken. Two gases with a range of density ratios from \ ...1/7 flow past a horizontal splitter plate, dividing the inlet section into an upper and a lower leg. Reynolds numbers of each leg, range from 2∗500 to 20'000. The density difference was created by using pure nitrogen and pure helium. The gas supply loop feeds the test section to establish isokinetic or shear flow conditions. Planar Particle Image Velocimctry is used to investigate the flow field in the developing mixing layer. In addition, Computational Fluid Dynamics analysis was performed making use of Large Eddy Simulation methods for model validation and the resulting flow fields are quantitatively compared. The results are presented in terms of turbulent statistics. An asymmetric development of the mixing layer is observed for all stratified cases, and with increased stratification we observe a production of turbulent kinetic energy (TKE) across the mixing layer from the denser stream towards the lighter gas. The strong formation of shear, due to an asymmetric development of momentum thickness, is found to produce TKE within the developing mixing layer and dominates the damping effect of buoyancy for stable stratification by approximately two orders of magnitude.