In recent years it has been demonstrated that direct microjet injection into the shear layer of the main jet disrupts the feedback loop inherent in high speed impinging jet flows, thereby significantly reducing the adverse effects. The amount of noise reduced by microjet actuation is known to be dependent on nozzle operating conditions. In this book, two active control strategies using microjets are suggested to maintain a uniform, reliable, and optimal reduction of these tones over the entire range of operating conditions. In order to obtain an optimal performance of the actuator, a two-mode feedback model that captures both the low and high-frequency Rossiter mode was suggested to investigate the role of pulsed microjet in the feedback loop. Due to the fact that a low frequency pulsing brought about additional reduction compared to high frequency pulsing, the presence of low frequency mode is identified. In the context of the analytic model, the effect of pulsing is modeled using a input-shaping controller that accomplishes noise-reduction through a suitable redistribution of the acoustic excitation over the high and low frequencies.

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