Abstract : This work is devoted to the study of round coaxial jets with high velocity ratios using numerical simulations. The work focuses on three particular points: the transition towards fully-developed turbulence state, the mixing properties in this flow configuration, and the influence of the Reynolds number upon the two previous points.
First, direct numerical simulations of coaxial jets with moderate Reynolds numbers are carried out. It was observed that coaxial jets develop a back-flow region when the ratio between the outer and the inner jet velocities exceeds a critical value. The development of the inner and outer Kelvin-Helmholtz rings, arising from the initial instabilities is not independent. The inner rings are controlled by the outer shear layer as they travel downstream. Streamwise counter-rotating vortices develop further downstream, as in the transition scenario of the single round jet, and lead to an high three-dimensionalization of the turbulent field. When it is present, the back-flow region tends to slow down the inner Kelvin-Helmholtz rings and to stretch them in the streamwise direction. Two theoretical models were developed in order to predict some global quantities of the jet, as the inner potential core length and the critical velocity ratio for the appearance of the back-flow region. Both models underline the influence of the inner momentum thickness for the initial jet development.
A tracer transport equation was solved simultaneously with the Navier-Stokes equations to study the mixing properties of coaxial jets. The coherent structures control the mixing process. The streamwise vortices improve therefore the mixing by ejections of tracer around the jet. However spots of unmixed fluid persist at the end of the transition with the initial flow configuration. Modification of the upstream conditions allows the intense or early streamwise vortices to diminish the amount of unmixed fluid. The same effect is caused by the back-flow region due to its increased streamwise vortex stretching phenomena. For the same reason, an azimuthal forcing of the outer shear layer improves the mixing and seems to be more suited to the near-field than an axisymmetric forcing.
Finally, we perform large-eddy simulations of coaxial jets with high Reynolds numbers. The self-similarity allows to validate our simulations by comparison with experimental data in the fully-developed turbulence region. The Reynolds number influences the global quantities of the jet as long as it is smaller than 10000. For larger Reynolds numbers these quantities are almost Reynolds number independent because of the ``mixing transition'' phenomenon caused by a three-dimensional disturbance from the beginning of the jet. The early disturbance of the shear layers causes the back-flow region to be non-stationary. Moreover, this allows to improve the mixing with the ejections phenomenon near the entrance of the jet.