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Carlos B. da Silva
Department of Mechanical Engineering (DEM) IDMEC/IST, Technical University of Lisbon Av. Rovisco Pais, 1049-001 Lisbon, Portugal

Guillaume Balarac
Grenoble-INP/CNRS/UJF-Grenoble 1, LEGI UMR 5519, Grenoble, F-38041, France

Patrick Begou
Equipe MoST/LEGI, Institut de Mécanique de Grenoble, B.P. 53, 38041 Grenoble Cedex 09, France

Olivier Metais
Grenoble-INP/CNRS/UJF-Grenoble 1, LEGI UMR 5519, Grenoble, F-38041, France


Direct Numerical Simulations (DNS) are performed to analyze the instability, transition scenario and resulting topology from high velocity ratio coaxial jets (ru = 3.3 and ru = 23.5). The inner and outer shear layers roll up into axissymmetric 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 which affects only lightly 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 an 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.