Characterization of dynamic actuation effects on coherent vortical structures in synthetic jets

Synthetic jets (SJs) enable tunable flow control across diverse applications by dynamically modulating the periodic formation and interaction of coherent vortical structures, such as vortex rings. This study experimentally investigates the effects of dynamic modulation—specifically actuation frequency and duty cycle (DC)—on the evolution of these structures. The modulation impacts on key SJ parameters, including jet strength, are carefully analyzed. Time-resolved particle image velocimetry, flow pathline visualization, and phase-averaged analysis were employed to characterize the spatiotemporal features of the unsteady SJ flows. Additionally, spectral proper orthogonal decomposition was used to extract frequency-dependent modal characteristics. Three distinct regimes of vortical behavior were identified depending on actuation conditions: (1) mutually isolated vortex rings with minimal interaction, (2) weakly interacting rings merging in the far-field, and (3) early merged, axially symmetric vortical structures near the orifice. These regimes correspond to transitions from vortex-dominated quasi-steady jets to phase-independent continuous jets. At low frequencies (3–5 Hz), SJs exhibit isolated vortex rings with extended travel due to long blowing phases and large stroke lengths, though the time-averaged jet strength is limited. At mid-range frequencies (10 Hz), SJs show strong transitional behavior sensitive to DC, producing distinct rings at low DC and merged rings at higher DC, with the strongest jets observed. At high frequencies (25 Hz), incomplete vortex ring formation leads to continuous jets insensitive to DC modulation. These results offer deeper insight into fine-tuning SJ dynamics and optimizing actuator designs for targeted flow control applications.