INSTANTANEOUS 3-DIMENSIONAL CONCENTRATION MEASUREMENTS IN THE SELF-SIMILAR REGION OF A ROUND HIGH-SCHMIDT-NUMBER JET

The virtually instantaneous three-dimensional concentration fields in the self-similar region of natural or unexcited, circularly excited an d weakly buoyant round jets of Reynolds number based on nozzle diamete r of 1000 to 4000 are measured experimentally at a spatial resolution of the order of the Kolmogorov length scale. Isoconcentration surfaces are extracted from the concentration held. These surfaces along with their geometrical parameters are used to deduce the structure and moda l composition of the jet. The concentration gradient held is calculate d, and its local topology is classified using critical-point concepts. Large-scale structure is evident in the form of 'clumps' of higher-co ncentration jet fluid. The structure, which has a downstream extent of about the local jet diameter, is roughly axisymmetric with a conical downstream end. This structure appears to be present only in fully tur bulent jets. The antisymmetric two-dimensional images previously thoug ht to be axial slices of an expanding spiral turn out in our data to i nstead be slices of a simple sinusoid in three dimensions. This result suggests that the helical mode, when present, is in the form of a pai r of counter-rotating spirals, or that the +1 and -1 modes are simulta neously present in the flow, with their relative phase set by initial conditions. In terms of local structure, regions with a large magnitud e in concentration gradient are shown to have a local topology which i s roughly axisymmetric and compressed along the axis of symmetry. Such regions, which would be locally planar and sheet-like, may correspond to the superposition of several of the layer-like structures which ar e the basic structure of the fine-scale passive scalar field (Buch and Dahm 1991; Ruetsch and Maxey 1991).