Effects Of Initial Conditions On Mixing In Wall-Bounded Turbulent Flow

Scalar mixing occurs in a wide variety of environmental and engineering flows. Given that the majority of these flows are turbulent, a more complete understanding of the mechanisms that underlie the scalar mixing process is required to predict and control these flows and the evolution of the concentration(s) of the scalar(s) therein. Direct numerical simulations (DNSs) are particularly useful in the study of turbulence since DNSs resolve the full range of (length and time) scales in turbulent flows. The present work employs a spectral approach for simulating the hydrodynamic field, and a flux integral method for computing the advection and diffusion of the passive scalar. Mixing of the latter is studied in a fully developed turbulent channel flow, and particular attention is paid to the evolution of the scalar dissipation rate and the differences that arise from different scalar-field initial conditions. Observed differences in the evolutions for the three initial conditions investigated are explained by an analysis of the different terms that comprise the scalar disspation rate budget. To promote mixing in a wall-bounded flow, it is recommended that scalar interfaces be aligned normal to the mean velocity vector, to enhance the stretching of the interface.