Four-Dimensional Laser Induced Fluorescence Measurements of Conserved Scalar Mixing in Turbulent Flows

Abstract

We deal with conserved scalar mixing in turbulent flows, and present a newly developed laser imaging diagnostic for obtaining highly detailed, four-dimensional measurements of the full space and time varying conserved scalar field ζ (x,t) and the associated scalar energy dissipation rate field ∇ζ·∇ζ (x,t) in a turbulent flow. The method is based on high-speed, high-resolution, successive planar laser induced fluorescence imaging of a synchronized raster swept laser beam, combined with high-speed data acquisition of gigabyte-sized data sets using very fast computer disk ranks. The measurement resolution reaches down to the local strain-limited molecular diffusion scale in the flow, so that the resulting four-dimensional data are directly differentiable in all three space dimensions and in time. These data spaces are numerically analyzed to determine the time evolution of all three components of the instantaneous scalar gradient vector field ∇ζ (x,t) and the resulting instantaneous scalar energy dissipation rate field. Typical results are presented in the form of spatial sequences of adjacent two-dimensional data planes within a particular three-dimensional data volume, as well as temporal sequences of spatial data planes from three-dimensional data volumes acquired successively in time, allowing the evolution of the true scalar dissipation rate to be examined in detail throughout the four-dimensional data space.