“Lock-in” phenomenon in coaxial jets

Flow visualizations have been performed in order to study the vortex interactions in the near field of a coaxial jet configuration. Smoke has been injected in the outer stream and a continuous Argon Laser sheet has been used as a light source. Instantaneous pictures have been acquired at a rate of 1 kHz by means of DANTEC Nanosense MK1 camera. In order to remove the background noise the average of all the available images has been subtracted from each snapshot. Different tests have been performed at moderately large Reynolds number, for different velocity ratios, ru = Uo/Ui (Segalini 2010), where Ui and Uo are the inner and outer jet velocity, respectively. Three main regimes have emerged depending on the velocity ratio. Although for ru ! 1 the coaxial jets dynamics may be related to the independent instability of the inner and outer shear layer (see Fig. 1a), in this type of configuration (visible for ru " 1 in Fig. 1c) it is typically the stronger outer shear layer that dominates the near field vortex dynamics (Dahm et al. 1992). However, for ru nearly unitary, and for a thick enough separating wall, a clear vortex shedding behind the separating wall is produced as a result of a global instability mechanism. Figure 1b shows that the generated vortices impose their own dynamics on the entire jet near field synchronizing the instability of the outer shear layer. This ‘‘lock-in’’ phenomenon can be used as a ‘‘passive’’ mean, i.e. requiring no energy input, to control the mixing between the two streams and between the jet and the ambient fluid.