# Noise Reduction Using Synthetic Microjet Excitation in Supersonic Rectangular Jets
###### Abstract  
This work explores a potential methodology for rectangular jet noise reduction that employs nozzle unsteady microjet excitation. Using high-fidelity computational studies and spectral analyses, major jet noise sources impacted by the applied actuation are identified. A heated supersonic rectangular jet is considered with a nozzle aspect ratio of 2:1 at a Mach number of 1.5. The current study essentially validates the hypothesis of a previous reduced-order analysis that predicted jet noise reduction through jet excitation at the harmonic or subharmonic of the dominant frequency associated with jets' large-scale structures. Such noise reduction was attributed to the excitation-induced nonlinear energy exchange between the coherent modes. In the current study, the synthetic microjet actuation of the jet plume shear layer using 1% of the jet mass flow rate is implemented at the excitation ports located at the nozzle lip and directed along the jet axis. A resulting jet noise reduction of up to 4 dB at the peak radiation angle is predicted. An analysis of the near-field Spectral Proper Orthogonal Decomposition (SPOD) results provides further insights into the impact of jet actuation on the modification of jet flow structures, thus addressing the effectiveness of the proposed noise control methodology. rectangular; jet; noise; control; synthetic; microjet; high-fidelity; study 2020 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Vol., 2020, pp. 1-10. IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained. Permission from IEEE must be obtained. the advent of jet engines in the 1940s, with significant advancements in the 1950s when Sir [PERSON] introduced the Acoustic Analogy by transforming the Navier-Stokes Equations into an acoustic wave equation [4; 5]. Such an analysis revealed the scaling laws for acoustic power. Large coherent turbulent structures in jet shear layers, emerging from nozzle exit instabilities, dominate the noise generation mechanism. These structures evolve downstream. In axisymmetric laminar jets, a shift from ring vortices to 3D turbulent formations downstream is evident, with a peak noise frequency around \(St_{D}=0.3\)[6]. A noise source model based on fine and large-scale turbulent structures has received validation from multiple studies [7]. In supersonic jets, distinct shock cell structures interact with these turbulent structures, producing broadband shock-associated noise (BBSAN). Earlier research in the 1970s indicated shock-expansion wave reflections as acoustic sources [8], with subsequent studies highlighting a spectrum of disturbances linked to turbulent scales [9]. Predicting screech noise amplitude, influenced by various factors, remains elusive, despite new insights into the wave modes in supersonic jets [10; 11]. Stringent community noise regulations have catalyzed innovative jet noise suppression methods without impeding aircraft performance [12]. These techniques can be broadly classified into passive or active control measures. Passive strategies, including bypass flow alterations, the use of chevron nozzles, and nozzle corrugations, introduce design changes to modify airflow and influence far-field acoustics. Active methods, such as fluid injection, plasma actuation, and microjet excitation, utilize feedback control systems for dynamic and optimized noise reduction [13; 14; 15; 16; 17]. An advantage of active techniques is their ability to turn them off when they are not needed. Analyzing sound sources requires capturing unsteady fluctuations in the flow field. Direct Numerical Simulation (DNS) is a high-fidelity approach to simulate coupled aerodynamic and acoustic effects in \"virtual laboratories\". DNS does not require subgrid-scale models or filtering, as it resolves all scales of turbulence directly. However, the computational demands of DNS are exceptionally high, making it impractical for high-speed and complex flows.