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We carry out direct numerical simulation (DNS) of scalar transport and mixing in a coaxial round jet issued into a small model combustor. The Reynolds number based on the diameter and bulk mean velocity of the outer annular jet is 1320. The outer-to-inner bulk mean velocity ratio is fixed at 6.4. Analysis is made on the detailed mechanism of scalar transport modulated by an active control of the near-field large-scale vortical structure. The main interest lies in dynamics of the vortical structure created by the present active control method, growth of streamwise vorticity, and the associated scalar transport process downstream of the nozzle exit. The mixing enhancement is found to be due to three-dimensional breakdown of the primary vortex rings in the inner shear layer. This breakdown process is caused by the streamwise vortical structure. Budget analysis reveals different dynamic processes taking place in the evolution of streamwise structure in the inner and outer shear layers. The process in the outer shear layer is essentially similar to that in plane mixing layers, while the structure in the inner shear layer is convected toward the central axis before stretched axially by the inner vortex rings.

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