Self-similar states in turbulent mixing layers

Large-eddy simulations of temporally evolving turbulent mixing layers have been carried out. The effect of the initial conditions and the size of the computational box on the turbulent statistics and structures is examined in detail. A series of calculations was initialized using two different reali zations of a spatially developing turbulent boundary-layer with their free streams moving in opposite directions. Computations initialized with mean f low plus random perturbations with prescribed moments were also conducted. In all cases, the initial transitional stage, from boundary-layer turbulenc e or random noise to mixing-layer turbulence, was followed by a self-simila r period. The self-similar periods, however, differed considerably:: the gr owth rates and turbulence intensities showed differences, and were affected both by the initial condition and by the computational domain size. In all simulations the presence of quasi-two-dimensional spanwise rollers was cle ar, together with 'braid' regions with quasi-streamwise vortices. The devel opment of these structures, however, was different: if strong rollers were formed early (as in the cases initialized by random noise), a well-organize d pattern persisted throughout the self-similar period. The presence of bou ndary layer turbulence, on the other hand, inhibited the growth of the invi scid instability, and delayed the formation of the roller-braid patterns. I ncreasing the domain size tended to make the flow more three-dimensional.