Analysis of the variable density mixing layer using Large Eddy Simulation

In this paper Large Eddy Simulations of mixing layer flows with Reynolds numbers of up to 80,000, based on the velocity difference across the layer and its visual thickness are presented. Mixing layers over a wide range of density ratios are simulated. Several of the simulations are performed at conditions which match reference experimental data. Analysis of the mean velocity statistics shows that the simulations produce very reasonable predictions when compared to the experimental data. In addition the mean growth rates of the simulated flows agree well with the accepted entrainment model of the flow. Spanwise- and cross-stream-averaged flow visualisations are used to illustrate the quasi-two-dimensional coherent structures present in the flow. The simulated flows are observed to undergo the mixing transition, and that coherent structures persist in the turbulent flow for all density ratios. The topography of the flow is analysed in detail, and it is shown that the mean visual thickness, coherent structure convection velocity and coherent structure spacing are all functions of the density ratio between the freestreams. The simulation results provide evidence that the variation in the spatial growth rate with density ratio is a consequence of the density ratio dependence of the convection velocity, whilst the temporal growth rates for all density ratios are the same.