Analysis of scalar mixing dynamics in LES using high-resolution imaging of laser Rayleigh scattering in turbulent non-reacting jets and non-premixed jet flames

Abstract

Imaging of turbulent jet flames has provided insights into the dynamic structure of the thermal dissipation field. These insights motivated studies of scalar mixing in non-reacting jets, with emphasis on comparing measurements to large eddy simulations (LES). We use a progressively refined set of calculations to investigate how filter size affects the LES representations of scalar mixing. Results are analyzed using three grids that are successively refined by a factor of two in space and time. The coarsest grid contains 1.3-million cells and provides resolution similar to that used in current LES. The refined grids contain 10- and 82-million cells. Imaging measurements of the instantaneous mixture fraction and dissipation fields provide insights into scalar mixing dynamics and key length scales. They show that dissipation structures reside on the resolved- and subgrid-scales of the LES, and they provide order of magnitude estimates of the relationship between the dissipation layer thickness (the smallest relevant mixing scale), the integral scale (the largest relevant mixing scale), and the filter width. For all grid distributions, the longitudinal dimension of the dissipation layers spans multiple cells, but the ratio of the layer thickness to filter width is of order 1, which is not consistent with typical modeling assumptions. Measurements of mixture fraction were filtered to isolate the effects of spatial averaging on the dissipation field from the combined effects of spatial and temporal averaging. Statistical analysis reveals that the average mixture fraction and dissipation fields for the coarsest grid deviate significantly from the measured profiles as the flow evolves. On the finest grid, however, good agreement is obtained. Trends suggest that temporal damping and dispersion errors compound as the flow evolves downstream, which is directly related to the relationship between the dissipation layer thickness, filter width, and integral scale of the local mixing field.