Abstract
Large Eddy Simulation is used to assess the influence of the spanwise domain extent on the evolution of the spatially stationary streamwise structure that exists in the simulated plane turbulent mixing layer. The mixing layers originate from a physically-correlated inflow condition, which produces accurate mixing layer mean flow statistics. For all three spanwise domains considered a spatially stationary streamwise structure is present. The streamwise structure is artificially confined when its wavelength matches that of the spanwise domain extent, and a criterion for confinement is postulated. The confinement has no significant negative impact on either the computed flow statistics, or the growth of the large-scale spanwise structures. These results demonstrate that the streamwise structure rides passively on the large-scale spanwise vortex structure. A simulation lacking in a spanwise direction produces poor turbulence statistics, and is not a reliable representation of the real mixing layer flow.
Highlights
Numerical simulation of the plane turbulent mixing layer has been a topic of academic research for over thirty years
The flow conditions reported by Browand and Latigo [24] are a good candidate for numerical simulation as a large amount of statistical information is available for comparison
The decrease in low-speed side freestream velocity with increasing streamwise distance caused by the adverse pressure gradient requires that the normalisation of flow statistics by the velocity difference across the mixing layer, ∆U = U1 − U2, is performed with local freestream velocity values
Summary
Numerical simulation of the plane turbulent mixing layer has been a topic of academic research for over thirty years. When low-level three-dimensional random perturbations are superposed on a mean inflow velocity profile, a helical structure is observed in the mixing layer [14] These oblique primary rollers undergo localised or helical pairings, which can lead to the vortex structure attaining a chain-link fence type appearance [15]. Recent research by McMullan and Garrett [16] has shown that the imposition of low-level, physically-correlated fluctuations in an initially-laminar mixing layer, leads to the formation of spatially stationary streamwise vortices. The evolution of these streamwise vortices agreed extremely well with comparable experimental data [4], and their origin was associated with residual streamwise vorticity in the upstream laminar boundary layers.
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