CAREER: Measurements of Subgrid-Scale Scalar Mixing to Study Large-Eddy Simulation of Turbulent Combustion

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This is an investigation of the subgrid-scale (SGS) mixing of conserved scalars which play a critical role in large-eddy simulation (LES) of nonpremixed turbulent combustion. Mixing of conserved scalars is generally considered to be dominated by a large-to-small-scale cascade process based on Kolmogorovs hypothesis. Current mixing models give qualitatively incorrect results for flows where the cascade process should dominate. Also, it is now recognized that large-scale velocity and scalar fields have significant influence on small-scale scalar mixing. In this project, the effects of the large-scale structure and of the cascade process on the filtered density function (the SGS distribution) of a conserved scalar is studied. High-resolution measurements of velocity and of conserved scalars are made in high-Reynolds-number turbulent jets with isotropic grid turbulence. Hot-wire and cold-wire arrays, particle image velocimetry (PIV), and planar laser Rayleigh scattering are used for the measurements. Issues in modeling the filtered density function of a conserved scalar and the filtered joint density function of a conserved scalar and its gradient are investigated. Both conventional statistical analyses and those based on conditional sampling and averaging are performed to investigate the effects of turbulent advection, turbulent strain and rotation, and molecular mixing on the filtered density function and other variables.