Within the framework of the present investigation we perform Direct Numerical Simulations to study the role of streamwise dynamics in the jet spreading and mixing, and provide a clear understanding about the source of these important vortices in two-step rectangular sudden expansion flows. The present setup is designed to generate higher mass entrainment, and the system generated passive forcing provided the necessary impetus for the sustained growth/evolution of the streamwise vertices, which played the key role in the mixing enhancement. On the other hand, by suitably placing two tiny rectangular ‘tabs’ on the inlet channel walls, the nature of growth and the dynamics of the streamwise extended threedimensional vortical rollers could be effectively controlled. These vortices through their inflow/outflow type dynamics are found to effectively control the downstream jet spreading; and the associated physical processes either lead to quick axis switching of the jet section, or stop axis switching altogether. In addition, an extensive pressure analysis, as presented here, suggests that the transverse pressure gradient skewing is probably the major source of streamwise vorticity generation for the flow

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