In this paper we perform Large Eddy Simulations of the plane turbulent mixing layer, to assess the effect of the velocity ratio parameter on the evolution of the streamwise vortex structure in the flow. The mixing layers originate from a physically-correlated inflow condition. A simulation performed against reference experimental flow conditions produces reliable flow statistics, and the velocity ratio parameter is subsequently varied by changing the low-speed freestream velocity of the flow. It is found that the residual streamwise vorticity in the flow upstream of the splitter plate is amplified, and subsequently realigns into a single row of spatially stationary streamwise vortices. The distance required for this realignment process decreases with increasing values of the velocity ratio parameter. Once formed, the average spacing of the spatially stationary streamwise vortices evolves in a stepwise fashion, with the overall spanwise wavelength of the streamwise structure scaling linearly with the local vorticity thickness. This scaling is independent of the velocity ratio parameter for all values tested here.

In this paper we perform large eddy simulations of variable density mixing layers, which originate from initially laminar conditions. The aim of this work is to capture the salient flow physics present in the laboratory flow. This is achieved through varying the nature of the inflow condition, and assessing the vortex structure present in the flow. Two distinct inflow condition types are studied; the first is an idealised case obtained from a mean inflow velocity profile with superimposed pseudo-white-noise, and the second is obtained from an inflow generation technique. The inflow conditions generated have matching mean and root mean squared statistics. Validation of the simulations is achieved through grid dependency and subgrid-scale model testing. Regardless of the inflow condition type used, the change in growth rate of the mixing layer caused by the density ratio is captured. It is found that the spacing of the large-scale spanwise structure is a function of the density ratio of the flow. Detailed interrogation of the simulations shows that the streamwise vortex structure present in the mixing layer depends on the nature of the imposed inflow condition. Where white-noise fluctuations provide the inflow disturbances, a spatially-stationary streamwise structure is absent. Where the inflow generator is used, a spatially stationary streamwise structure is present, which appears as streaks in plan-view visualisations. The stationary streamwise structure evolves such that the ratio of streamwise structure wavelength to local vorticity thickness asymptotes to unity, independent of the density ratio. This value is in agreement with previous experimental studies. Recommendations are made on the requirements of inflow condition modelling for accurate mixing layer simulations.

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In this paper we present Large Eddy Simulations of a low Reynolds number plane mixing layer. The purpose of the research is to assess the effect of inflow conditions on the scalar mixing in the flow. A high-resolution grid is used to study a pre-transition mixing layer originating from initially laminar conditions. Both pseudo-random white noise and physically-correlated fluctuations produced by an inflow generation technique provide the background fluctuations for the simulations. It is found that the scalar statistics are largely spanwise invariant when pseudo-random fluctuations are employed, whilst large spanwise variations in the scalar statistics are observed when physically-correlated fluctuations provide the background disturbances. The large fluctuations are attributed to the presence of organised streamwise vortices in the mixing layer. The implications of this research in terms of the documentation of experimental initial conditions is discussed.

Large Eddy Simulations of the plane mixing layer have been performed, for the purpose of educing the streamwise vortex structure that may exist in these flows. Both an initiallylaminar and initially-turbulent mixing layer are considered in this study. The initiallylaminar flow originates from Blasius profiles with a white noise fluctuation environment, whilst the initially turbulent flow has an inflow condition obtained from an inflow turbulence generation method. Both simulations produce good mean flow statistics when compared with reference experimental data. The simulations capture the change in growth rate when the initial conditions are either laminar or turbulent. Flow visualisation images demonstrate that both mixing layers contain organised turbulent coherent structures, and that the structures contain rows of streamwise vortices distributed across the span of the mixing layer. Ensemble averaging of the cross-plane data, however, shows no evidence for statistically stationary streamwise vortices in either simulation.