The vorticity dynamics in the near field of laminar (Re = 103) co-flowing jets subjected to the single or combined effeets of axial and azimuthal forcing is analyzed. It is shown that the interaction of the three-dimensional vortex structure resulting from the growth of the two and three-dimensional instabilities may result in large changes in the entrainment and mixing characteristics of the jet. For each azimuthal forcing, and for a fixed velocity ratio between the inner and outer jet, we show the existence of several instability modes leading to a pattern of lateral ejections of closed vortex loops. These modes and their topological changes are analyzed in view of the three-dimensional inviscid induction of the two concentric array of vortex rings emanating from the j e t s exit nozzle. For the case of methane-air diffusion flames, fiow visualizations revealed the existence of qualitatively similar patterns of closed fíame cells and fingers.
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