Streamwise confinement effects in a temporally developing stably stratified shear layer

Direct numerical simulations are conducted to investigate the large-scale features of a stably stratified shear layer. The fully-developed turbulent shear layer exhibits two distinct large-scale structures: one is a typical large-scale structure (LSS) with a scale proportional to the shear layer thickness, and the other is an elongated large-scale structure (ELSS) with a streamwise length much greater than that of the LSS. Simulations employ computational domains with varying streamwise lengths. Auto-correlation functions of velocity reveal that the ELSS meanders in the horizontal plane. This meandering is altered in smaller domains, where confinement effects eventually suppress ELSS growth. Comparisons across domain sizes highlight the role of the ELSS in flow evolution. The mean and root mean square fluctuations of velocity and density remain unaffected by the growth of the ELSS. The LSS length scale consistently scales with the shear layer thickness and is not influenced by the ELSS. The behavior of the dissipation coefficient indicates that energy transfer from large to small scales is predominantly driven by the LSS rather than the ELSS. Counter-gradient diffusion of momentum and density is known to occur at scales between the LSS and ELSS; this feature is shown to be linked to the development of the ELSS. The results indicate that the ELSS has minimal influence on flow properties at scales smaller than the LSS, which govern the averages and variances of velocity and density, while it plays a significant role at scales larger than the LSS.