Three-dimensional vorticity dynamics in a coflowing turbulent jet subjected to axial and azimuthal perturbations

Department of Applied Mechanics and Engineering Sciences University of California, San Diego La Jolla, CA 92093-0411, USA

Abstract

The effect of axial and azimuthal forcing on the spreading and mixing characteristics in the near field (x/D<13) of a coflowing, turbulent (Re>2000) water jet is studied by means of planar, laser induced fluorescence (PLIF) visualizations. Measurements of both the initial spreading rate of the jet and the decay of the concentration along the centerline of the jet show a marked increase in the mixing of the jet as a result of the axial forcing. The mixing enhancement, which is shown to depend on the wavenumber of the azimuthal forcing, the frequency of the axial forcing, and the Reynolds number of the jet, results from the formation of vortex loops caused by the global induction of the jet's vorticity. This vortex induction mechanism appears to be similar to the vorticity modes earlier found in laminar coflowing jets (Lasheras et al., Lect. Appl. Math., 28, 1991).