Interactions of large scale structures in turbulent round jets

The dynamical significance of vortical structures in the near field of round jets, viz. the vortex rings and the streamwise vortices, has been known for several decades (Ho & Huerre, 1984). A fundamental understanding of their interactions, especially in turbulent jets at high Reynolds number (Re), is expected to provide an impetus to devising control strategies; for instance in improving combustion efficiency and reducing jet noise. One such possible interaction was recently observed in Davoust et al. (2012) that involved production of additional streamwise vorticity, as a result of an influence of the fluctuations of streamwise vorticity on those of azimuthal vorticity in high Re jets. This additional vorticity showed up in streamwise vorticity correlations in the form of radial organization as opposed to its azimuthal organization classically found. In further exploring this possibility, we have observed in our experiments, that strengthening the vortex rings relative to the streamwise vortices, renders them less susceptible to an influence from the streamwise vortices. This paper presents the results for a Re = 1.5 × 10⁵ and low Mach number round jet in which the most energetic vortex rings are acoustically excited at different excitation amplitudes. A comparison of the relative strengths of the two structures with increasing excitation levels, corroborates the interaction proposed in Davoust et al. (2012).