On streamwise vortices in Large Eddy Simulations of initially laminar plane mixing layers

•The simulated mixing layer is sensitive to the nature of the imposed inflow fluctuations.•White noise inflow disturbances produce pre-transition vortices that undergo-localised pairings. There is no evidence for spatially stationary streamwise vortices in simulations of this type.•Physically-correlated inflow disturbances produces a mixing layer that contains statistically stationary streamwise vortices. The properties of these vortices agree well with experimental data. This paper details the influence of the nature of imposed inflow fluctuations on Large Eddy Simulations of a spatially developing turbulent mixing layer originating from laminar boundary layers. A simulation with imposed white-noise random fluctuations, commonly used in numerical simulations, produces mean-flow statistics that agree well with reference experimental data. Whilst flow visualisation images show evidence for streamwise vorticity in this simulation, quantitative statistics do not reveal the presence of statistically stationary streamwise vortices. A further simulation that uses physically-correlated inflow fluctuations also produces good mean-flow statistical agreement with reference data. From secondary shear stress contours it can be inferred that this simulation does, however, predict the presence of statistically stationary streamwise vortices. The properties of the streamwise vortices are in good agreement with experimental data. The data presented here indicate that, even for initially laminar conditions, plane mixing layer simulations require accurate physically correlated inflow conditions in order to reproduce the flow features found experimentally.