Resolved scalar mixing in large eddy simulations of the laboratory mixing layer

We report a study of passive scalar mixing in simulations of the plane turbulent mixing layer that are representative of those found in experiments originating from clean laminar upstream conditions. An inflow condition generator is used to produce inflow data that mimics the residual spanwise non-uniformities present in real experiments. The presence of non-uniformities in the upstream conditions is essential to capture the development of the spatially stationary streamwise structure in the flow. Validation of the subgrid-scale model is performed, and the effect of the spanwise domain extent and imposed spanwise boundary conditions is determined. It is observed that the simulated mixing layers display features characteristic of the natural development of the flow to a fully developed, self-similar state. Analysis of the vortex structures present in the simulations demonstrates that the “Brown–Roshko” structures are present, where quasi-two-dimensional spanwise structures co-exist with statistically stationary streamwise vortices. The transition of the scalar concentration probability density function (PDF) from a “non-marching” type to a “tilted” type as a function of the so-called “pairing parameter” is captured by the simulations, and the PDF transition is linked to the evolution of the stationary streamwise vortex structure in the layer. The streamwise vortices are also responsible for preferential local entrainment across the span of the layer, up to the highest values of the pairing parameter recorded in the simulations. The findings are discussed in the context of the available experimental data in the archival literature, and their impact on currently available entrainment models is noted.