Initial condition effects on large scale structure in numerical simulations of plane mixing layers

Abstract

In this paper Large Eddy Simulations are performed on the spatially developing plane turbulent mixing layer. The simulated mixing layers originate from initially laminar conditions. The focus of this research is on the e↵ect of the nature of the imposed fluctuations on the large-scale spanwise and streamwise structures in the flow. Two simulations are performed; one with low-level three-dimensional inflow fluctuations obtained from pseudo-random numbers, the other with physically-correlated fluctuations of the same magnitude obtained from an inflow generation technique. Where white-noise fluctuations provide the inflow disturbances, no spatially stationary streamwise vortex structure is observed, and the large-scale spanwise turbulent vortical structures grow continuously and linearly. These structures are observed to have a three-dimensional internal geometry with branches and dislocations. Where physically-correlated provide the inflow disturbances a 'streaky' streamwise structure that is spatially stationary is observed, with the large-scale turbulent vortical structures growing with the square-root of time. These large-scale structures are quasi-two-dimensional, on top of which the secondary structure rides. The simulation results are discussed in the context of the varying interpretations of mixing layer growth that have been postulated. Recommendations are made concerning the data required from experiments in order to produce accurate numerical simulation recreations of real flows.

The influence of velocity ratio on the evolution of streamwise vortices in the simulated plane mixing layer

“…At station 6 the normalised streamwise distance of x/θ i = 1180 is sufficiently far downstream to be in a region of the flow normally considered to be representative of the fully developed flow [37], and is substantially further downstream than the distance required to attain self-similarity in the mean statistical information presented in Section 4.1. In agreement with other experimental and numerical investigations [13,24], the current findings indicate that a re-evaluation of the streamwise distance required to attain mixing layer self-similarity may be required.…”

“…When the statistically stationary streamwise structure is present, its properties agree well with comparable experimental data [13]. If uncorrelated white noise disturbances provide the fluctuation environment, the simulated mixing layer does not contain a statistically stationary streamwise structure, and the streamwise vorticity is generated through localised pairings of the primary spanwise rollers [23,24].…”

“…The three-tiered clusters of streamwise vorticity are amplified, and subsequently unwrap to form a single row of alternating sign spatially stationary streamwise vortices. This unwrapping process has been observed in both experimental [13], and numerical [24] investigations of the plane mixing layer originating from initially laminar conditions. The formation of the spatially-stationary streamwise vortex structure is dependent on the velocity ratio parameter of the flow, such that the streamwise distance required to produce a single row of alternating sign streamwise vortices decreases with increasing values of the velocity ratio parameter.…”

“…The span of the mixing layer is sufficient to prevent artificial confinement of the streamwise vortex structure [32]. This grid has been extensively validated elsewhere, with the salient flow physics correctly captured in simulations [24].…”

“…The research of McMullan & Garrett assessed the statistically stationary streamwise structure present in the simulated mixing layer for only a single value of the velocity ratio parameter [24]. As described above, experimental evidence suggests that the ratio of the spanwise wavelength of the streamwise structure to the local vorticity thickness will asymptote to a particular value, independent of velocity and density ratio [4].…”