2 research outputs found
“Lock-in” phenomenon in coaxial jets
none2Flow 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.noneA. Segalini; A. TalamelliA. Segalini; A. Talamell