Azimuthal forcing of an axisymmetrically oscillating self-excited jet*

At a sufficiently low density, an open jet flow can become globally unstable, transitioning from a steady state to a self-excited state characterized by axisymmetric oscillations at a discrete natural frequency. However, if such a flow is then forced sufficiently strongly at a different frequency, it can oscillate at that frequency instead, leaving no evidence of the original natural mode. In low-density jets, this forced synchronization process has been explored for longitudinal forcing, but relatively little is known about how such jets respond to azimuthal forcing. In this experimental study, we examine the effect of azimuthal forcing on a self-excited low-density jet, with the aim of characterizing the bifurcations and nonlinear dynamics leading up to complete synchronization. We force the jet acoustically over a wide range of frequencies and amplitudes, and measure its unsteady response using hot-wire anemometry and high-speed Schlieren imaging. The results should offer new insight into the way self-excited axisymmetric hydrodynamic modes respond to external azimuthal acoustic forcing.