Mode selection in swirling coaxial jets

In order to improve the mixing properties of injectors, we investigate the potential synergy between decrease in circulation and axial shear. For this purpose, we examine the linear modal stability of a simplified analytical model which consists of a temporally evolving swirling jet surrounded by an annular jet with a different axial velocity. The present study extends the results of Gallaire & Chomaz where a single swirling jet was considered. From an experimental point of view this set-up has been studied in particular by Ivanic et al. Their study, and others on coaxial jets, show that the mean axial velocity profiles close to the nozzle exit are characterized by a strong velocity deficit at the interface between the inner and outer jets. To account for this behaviour we considered the analytical profiles proposed by Hairoud and Talamelli & Gavarini. In this last study, the authors carry out a linear stability analysis of coaxial jets, linking the instability frequency to the different wake parameters. When we add swirl to the mix, the choice of shear layer thickness and minimum velocity in the wake has a strong impact on the outcome of the linear stability analysis, affecting not only the intensity of the instabilities, but also which particular azimuthal modes are most amplified. For a given wake model we then compute the maximum instability growth rate over a wide range of azimuthal and axial wavenumbers, for different values of swirl intensity and jet velocity ratio. We identify the dominant azimuthal wavenumber for each combination. Finally we look at the topology of the most unstable modes, identifying the respective roles of centrifugal and shear instabilities