Jet noise has been a major source of concern for commercial and military aviation sectors alike. The need to assuage the adverse impact of jet noise on human health has led to increased interest in jet noise source identification and noise level minimization/mitigation. Most previous works on common round supersonic jets have primarily explored the ideal case of simple, perfectly expanded jet configurations. In real world scenarios however, many of these simplifications do not hold. Two such considerations are examined in this work. The first is a simple configuration operating at complex operating conditions, specifically an imperfect expansion i.e. where the jets are operating at off-design conditions. The second concerns a complex configuration at simple conditions: specifically two jets (twin-jets such as those on fighter aircraft) operating in close proximity to each other. In this work, Large Eddy Simulation (LES) based high-fidelity computations are used to understand the dynamics of imperfectly expanded and twin-jets respectively, with the following objectives: 1) Identify the impact of active flow control techniques on the plume dynamics and acoustic characteristics of underexpanded jets, and 2) Investigate the interaction dynamics of the twin-jet plumes and study its associated sound field which exhibits complex radiation characteristics.
To meet the first objective, a single-jet with a fully expanded Mach 1.3 jet, issuing from a converging nozzle operating at underexpanded conditions is considered. After selecting the appropriate grid based on mesh resolution studies, flow validation is conducted which indicates an excellent qualitative and quantitative agreement of the computed jet plume characteristics with the experimental observations. The analysis of underexpanded flow-field at two different Reynolds numbers indicates a relative independence of the jet flow characteristics and downstream plume evolution to the variation in Reynolds number. A detailed investigation of the near-field pressure fluctuations at different polar angles confirms the presence of three distinct noise sources i.e. downstream directed mixing noise, side-line directed broadband shock associated noise, and upstream directed screech tone noise. The frequency of the screech tone, generated as a result of a feedback process, is observed to be consistent with theoretical and previous experimental works. To investigate the effect of active flow control on the flow and acoustic features of the underexpanded jet, Localized Arc Filament Plasma Actuators (LAFPAs) are employed, which are modeled using a semi-empirical surface heating technique. For the control simulations, axisymmetric mode pulsing is considered at two different Strouhal numbers of St=0.3 and St=0.9. These simulations show that the response of the jet to flow control is a strong function of the actuation frequency. Relative to the baseline uncontrolled case, actuating at the column mode instability frequency (St=0.3) results in an increase in the rate of spreading of the shear-layer. Furthermore, analysis of the phase-averaged results reveals the formation of large-scale toroidal structures that are generated due to the excitation of jet column instabilities at this actuation frequency. As a consequence of the formation of these large-scale features, the St=0.3 case exhibits increased noise levels, relative to the uncontrolled case. On the other hand, the higher frequency actuation affects the initial shear-layer instability and interferes with the formation of the toroidal events, that are observed for the St=0.3 actuation case, and further appears to weaken even the naturally occurring turbulent structures. As a result, noise level reduction is observed, relative to the uncontrolled case. In spite of the absence of axisymmetric toroidal events, detailed integral azimuthal length scale analyses reveal the dominance of the axisymmetric (m=0) mode, even at large distances from the nozzle exit. This behavior indicates that flow control methods need not always have a visual signature of their influence on the system.
To achieve the second objective of this work of computationally investigating the flow and acoustic characteristics of complex propulsion systems, a Mach 1.23 circular twin-jet configuration, at perfectly expanded conditions, with an inter-nozzle spacing of two jet diameters is considered. The validity of the simulations is established by comparing the jet flow structure and the near-field linear-array noise levels to the experimental results, which exhibit a good match. A qualitative investigation of the jet plume structure reveals the dominance of a helical (m=1) mode on both the jet plumes, which is manifested in the form of cork-screw type features encompassing the supersonic jet core of each of the plumes. Analysis of the inter-nozzle region using mean velocity profiles indicates the formation of a secondary flow, which extends a significant influence on the jet plume shear-layers in its proximity. However, this impact is confined to small azimuthal angles and as a result, many of the potential core properties are almost similar to those of a single-jet at identical flow conditions. Analysis of the near-field pressure fluctuations along the plane containing the jet centers, at various polar angles, reveals the presence of jet noise shielding, where the noise levels imposed by a twin-jet is less than the sum of two incoherent single-jets. This shielding effect is observed to diminish with increasing polar angles. On the other hand, no such shielding is observed along the plane perpendicular to that containing the jet axes and instead noise level amplification, relative to the sum of two incoherent single-jets is noticed. This is attributed to the unabated sound radiation of the twin-jet plume in this direction.
To aid in the identification and understanding of the noise generation mechanisms, in addition to predicting the radiated noise levels and directivity, a decomposition of the flow-field into constituent fluid-thermal (FT) modes based on Momentum Potential Theory (MPT) is performed. The decomposed hydrodynamic mode is observed to be a true representation of the unsteady turbulence, which is evident from the identification of and m=1 helical mode patterns in the iso-levels of this variable. The noise signature associated with these turbulent features is observed to be contained within the decomposed acoustic mode, which exhibits a pronounced wavepacket structure in the jet core with an apparent m=0 axisymmetric toroidal mode dominance. Consistent with recent works on perfectly expanded supersonic jets, the acoustic wavepacket in the core of the jet exhibits significant temporal and axial modulation, which play a major role in determining acoustic radiation characteristics such as directivity and intermittency. Comparing the near-field noise level predicted by the acoustic mode with that of the overall pressure fluctuations, an excellent match is obtained both qualitatively and quantitatively. The decomposed acoustic mode correctly predicts the shielding and magnification phenomena, which signifies its ability to predict acoustic behavior of even severely complex configurations.