Instability Amplitude Suppression in a Double Orifice Flow Through External Periodic Forcing

Various afterburners, ramjets, and combustors exhibit vortex shedding lock-in with an acoustic mode of the duct. In the present study, we experimentally analyse the effect of external forcing on the flow through a square duct containing two circular orifices (double-orifice cavity). As the mean flow is altered, the system transitions from a noisy state to periodic oscillations (lock-in) through a quasiperiodic regime. Periodic forcing of the system close to its instability frequency leads to amplitude suppression via synchronous quenching and 1:1 lock-in. The phase lock-in happens via phase drifting and phase slipping. We also observe asynchronous quenching on forcing the system far from the instability frequency. However, the phase lock-in asynchronous quenching happens via phase drifting and phase trapping. These approaches serve as effective active control strategies for instability control. Synchronisation maps are presented and the dynamics of transition is explored using return maps. In this study, we introduce phenomenological oscillatory models of these phenomena that effectively depict the transition between different lock-in regimes, including the occurrence of a saddle-node bifurcation near the natural frequency and torus death for forcing at a frequency far from the natural frequency. We also study a model that transforms the continuous domain into a discrete map. Synchronisation maps obtained from both models are compared and validated with the experimental results.