Transition in high velocity ratio coaxial jets analyzed direct numerical simulations
Résumé
Direct numerical simulations are performed to analyse the instability, transition scenario and resulting topology from high velocity ratio coaxial jets (ru = 3.3 and 23.5). The inner and outer shear layers roll up into axisymmetric vortex rings due to the Kelvin–Helmholtz instability. For ru = 3.3 the outer primary vortices evolve according to the theory considering an isolated mixing layer profile, and impose their evolution upon the inner structures which are ‘locked’ into the outer ones. For ru = 23.5 there is a big recirculation region that does not affect the development of the Kelvin–Helmholtz instabilities. The preferred mode for simple (non-coaxial) round jets is well recovered at the end of the potential core region in the case ru = 3.3 but not when ru = 23.5 due to the presence of the backflow region. The structure of the preferred mode is the same in both cases, however, and consists in a helical arrangement (m = 1). Finally, when the bubble is present one can see that the inner streamwise structures, corresponding to the secondary instabilities, are stretched by the presence of the bubble which acts as an additional source of axial vorticity production.
