Three-dimensional measurements of scalar dissipation rates in a zero-mean-shear turbulence

High-resolution, high-speed successive planar laser-induced fluorescence imaging of a synchronized swept laser beam, combined with a fast image-processing system, is used to measure a three-dimensional conserved scalar field ζ(x, y, t) and/or ζ(x, y, z) on the inner scale in an aqueous zero-mean-shear turbulent flow generated by vibrating grids. Each measuring image (1 cm x 1 cm) contains 256 x 256 pixels of which the measurement resolution reaches down to the local strain-limited molecular diffusion scale of the flow. Thus we can directly differentiate the resulting scalar field data to obtain the scalar energy dissipation rate field Δζ•Δ ζ (x, t), allowing the local rate of molecular mixing in the flow to be investigated. It is found that for zero-mean-shear vibrating grid turbulent flows, fine scale mixing occurs in both strained laminar diffusion sheet-like layers and line-like tubes (roughly circular interactions are also observed) at which essentially all the dissipation is concentrated. This result is somewhat different from that of Dahm et al. (1991) in which no line-like structures are observed in a free shear turbulent jet flow, probably due to different types of turbulence applied.