The growth of the initially turbulent mixing layer: A large eddy simulation study

This article presents new information on the growth of a mixing layer originating from turbulent upstream conditions, obtained through large eddy simulation. The mixing layer develops from a turbulent high-speed side boundary layer and a laminar low-speed side boundary layer. Validation of the grid resolution, subgrid-scale model, and spanwise domain extent is performed. It is observed that the initially turbulent mixing layer undergoes three distinct phases in its evolution: a sub-shear region, where an internal shear layer develops immediately downstream of the splitter plate trailing edge and entrains the vorticity from the high-speed side boundary layer; a relaxation region, where the mixing layer develops to a point where the influence of the upstream conditions is forgotten; and a fully developed region, where the flow behaves in a self-preserving manner. The simulations display a discrepancy in downstream distances for the mean field, and turbulent stresses, to attain a self-preserving state, respectively, which is attributed to the spanwise integral length scale requiring a longer distance to attain an equilibrium value when compared to its streamwise and vertical counterparts. Large-scale, spanwise-orientated coherent structures are found to be a constituent part of the fully developed flow, and there is an absence of a spatially stationary streamwise structure in the initially turbulent mixing layer. The findings of the simulations are used to reconcile discrepancies found in experiments available in the archival literature.