Active noise control in supersonic impinging jets using pulsed microjets : actuator design, reduced-order modeling

Abstract

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 reduces the adverse effects. The amount of noise reduced by microjet actuation is known to be dependent on nozzle operating conditions. In this paper, 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 the first method, a quasi-closed loop control strategy is proposed using steady microjet injection and the Proper Orthogonal Decomposition (POD) algorithm. The most energetic spatial mode of the unsteady pressure along the nozzle diameter is captured using the POD, which in turn is used to determine the distribution of microjet intensity along the nozzle exit. Preliminary experimental results from a STOVL supersonic jet facility at Mach 1.5 show that the quasi-closed loop control strategy, in some cases, provides an additional 8,10 dB reduction compared to axisymmetric injection at the desired operating conditions. The second method consists of a pulsed microjet injection, motivated by the need to further improve the noise suppression.

(cont.) It was observed that the pulsed microjet was able to bring about the same noise reduction as steady injection using approximately 40% of the corresponding mass flow rate of the steady microjet case. Moreover, as the duty cycle increased, the performance of pulsed injection was further enhanced and was observed to completely eliminate the impinging tones at all operating conditions. In order to obtain an optimal performance of the actuator, a new model of the impinging jet flow field is suggested based on a collision model of two identical vortices. In addition to the colliding vortex model, 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 (16.4 Hz) 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.