| The aim of this work is to characterize the far-field structure of the nearly and fully turbulent (Reynolds number {dollar}le{dollar}5000) round water (high Schmidt number) jet. This thesis describes the acquisition and analysis of three-dimensional concentration data using a laser-induced fluorescence technique. Isoconcentration surfaces are used to deduce the large-scale structure and modal composition of the jet.; The evolution of two-dimensional ({dollar}x - y - t{dollar}) axial concentration fields in fully turbulent natural, circularly and axially excited water jets is visualized. Large-scale structure is found in the form of "ridges" on the {dollar}x - y - t{dollar} isoconcentration surfaces; they are common and robust, or long-lived. Assuming that these structures correspond to the instability modes predicted by linear theory, and that symmetry arguments can be used to distinguish these modes, cross-correlation techniques are used to determine whether these structures are in the axisymmetric and helical modes. Both symmetric (axisymmetric mode) and antisymmetric (helical mode) structures are found in the natural jet; only antisymmetric structures are found in both types of excited jets.; The instantaneous three-dimensional ({dollar}x - y - z{dollar}) concentration fields of nearly and fully turbulent (Reynolds numbers 1000, 2000 and 4000) natural and circularly excited jets are measured. Large-scale structure, which appears to be present only in fully turbulent jets, is found in the form of axisymmetric "clumps" of higher concentration jet fluid along the flow direction x. These structures are of a downstream extent approximately equal to the local jet diameter. The antisymmetric two-dimensional images previously thought to be "slices" of a spiral in three dimensions turn out instead to be slices of a simple sinusoid. This result suggests that the helical instability mode, if present, is in the form of a pair of counterrotating spiral vortex filaments.; The local structure of the concentration gradient field is analyzed using critical point concepts. The concentration field is shown to consist of axisymmetric filamentary structures stretched along the axis of symmetry, which tends to be inclined at an angle of {dollar}pm{dollar}135{dollar}spcirc{dollar} to x, or the expected intermediate (extensive) strain direction. These results and results from direct numerical simulations suggest that the small-scale flow consists of intense vortex filaments aligned with this direction; these filaments draw in and "swirl" the passive scalar around their axis, while the strain stretches the scalar along the filament axis. Finally, the scalar dissipation is calculated and visualized. It is shown that much of the mixing in the flow is taking place in a small fraction of the flow volume, in regions that geometrically appear to be convoluted and sheetlike. |
