The effects of excitation on the coherent structures in a supersonic rectangular jet with a design Mach number of [Formula: see text] are investigated. Planar laser-sheet imaging and time-resolved schlieren imaging, combined with spectral proper orthogonal decomposition (SPOD), are used to investigate the jet over three flow regimes. Coherent structures within the jet shear layers are clearly visualized and are shown to dominate the first SPOD mode at both the natural screech frequency and the excitation frequency. The robust response of the jet is over a large range of frequencies consistent with the overlap between the jet column mode and the shear layer mode. Excitation at frequencies higher than the weak screech frequency of the design condition disrupts the naturally occurring coherent structures. Furthermore, excitation of different azimuthal modes alters both the organization and energy content of these structures within the jet. In the presence of screech staging, excitation can also change the dominant tonal peak within the jet. Excitation also changes the overall entrainment and mixing characteristics of the jet.

This paper summarizes the results of high-speed schlieren experiments on a supersonic rectangular jet of sharp throated design. The rectangular nozzle is of aspect ratio two and has a design Mach number of 1.50. A strongly screeching overexpanded case as well as the weakly screeching nozzle design case are investigated. LAFPA based excitation is used to explore the inherent physics of the jet as well as to control the generation and development of coherent structures (CS). The baseline jet displays a screech signature with a natural flapping mode behavior. LAFPA excitation at the screech frequency is able to reinforce the natural behavior or shift some energy towards another azimuthal pattern, depending on the excitation mode employed. Exciting the jet at a frequency above the natural screech produces smaller CS in an azimuthal mode reflecting that of the excitation. The smaller structures entrain less ambient air resulting in thinner shear layers and a lengthened jet core. The excitation is able to disrupt the natural screech loop when the jet is screeching weakly (operated at the design condition), resulting in the LAFPA introduced structures dominating the flow.

The far-field acoustics of a supersonic rectangular single jet (SRSJ) were studied, and the flow was subjected to active flow control by plasma actuators. The nozzle design is similar to those used in previous twin jet experiments by our group in the Gas Dynamics and Turbulence Laboratory at The Ohio State University. It is a converging-diverging nozzle with a design Mach number of 1.50, an aspect ratio of 2, and features a biconic inner contour. The far-field noise of the baseline jet was characterized from equivalent jet Mach numbers (M-j) 1.30-1.70, then a wide variety of excitation frequencies and modes were explored at strongly screeching M-j = 1.40 (Case 1) and weakly screeching M-j = 1.50 (Case 2) to observe their effect on the jet acoustics. Excitation near the peak shear layer mode frequency (Ste similar to 1.50) produced significant noise reduction across all polar angles. In Case 1, over 2.5 dB of reduction around the peak far-field noise angle was observed, and in Case 2, the reduction was more modest (up to 1.7 dB) and occurred at higher polar angles.

A new supersonic rectangular single jet was designed and installed at the Gas Dynamics and Turbulence Laboratory at The Ohio State University. The geometry is of similar design as previous twin jet experiments. The converging-diverging nozzle is biconic with a design Mach number of 1.50 and an aspect ratio of 2. Planar laser-sheet imaging was performed on the jet in the overexpanded regime, focusing on a fully expanded jet Mach number of 1.40. Phase-locked images were taken to directly visualize the coherent structures within the jet. Localized arc-filament plasma actuators (LAFPAs) were utilized to generate thermal perturbations to excite the Kelvin-Helmholtz instability in the shear layers of the jet. The baseline jet exhibited a flapping behavior but responded to the LAFPA excitation for all excitation modes and frequencies assessed. At excitation frequencies above the natural screech frequency, the excitation was able to switch the azimuthal mode of the jet to a symmetric mode. The size and wavelength of the coherent structures decreased as the excitation frequency increased, as expected, until they were too small to be resolved and visualized with the imaging setup at the highest tested frequency. By significantly modifying the coherent structures within the jet, and thus their entrainment and mixing characteristics, the overall phase-averaged jet behavior was significantly altered, revealing the nonlinear nature of the processes involving active flow control of the jet.

The near-field (NF) acoustics of supersonic rectangular twin jets (SRTJ) under active flow control were studied. The nozzles had a design Mach number of 1.50 and an aspect ratio of 2, operated at an overexpanded condition (M-j = 1.30). While previous works used all installed actuators to excite the flow, this work offers a preliminary look at the effect of actuation patterns with only some of the actuators active, which could significantly reduce the power required for this already very low power active control technique. The actuation patterns in this work explored two aspects of the physics that can reduce the NF overall sound pressure level (OASPL): destructive interference between the jets and the three-dimensionality of coherent structures in each jet shear layer. The best performance among the new cases took advantage of both effects to achieve OASPL reductions up to 4.5 dB. Although an actuation pattern using the full set of actuators (as in the previous work) gave the best reduction in OASPL as expected, significant benefits can still be achieved with a reduced number of actuators (i.e., with approximately 67% actuator power savings).

The experimental campaign of heated supersonic rectangular twin jets (SRTJ) was continued into the underexpanded flow regime. The nozzles had a design Mach number of 1.5 and an aspect ratio of 2. The nozzle pressure ratio was varied from design to underexpanded conditions (Mj=1.5-1.7) using a total temperature ratio of 2. Both baseline and excited cases using LAFPAs were investigated. Results focused primarily on the near-field (NF) acoustics/pressure results. Experiments further confirmed the influence of jet coupling on NF pressure fluctuations with out-of-phase (OOP) coupling between the jets resulting in reduced NF pressure fluctuations when compared to in-phase (IP) coupling. Screech “staging” was observed from near the design Mach number through underexpanded baseline cases. The abrupt jump in the screech frequency does not appear related to a change in spatial mode, but rather to the existence and relative strength of two shock systems and therefore the availability of a larger number of feedback locations. Similar baseline coupling behavior and LAFPA control authority were observed as in previous unheated underexpanded results: the LAFPAs’ control authority was reduced in strongly underexpanded flow regimes (also consistent with literature).

The experimental campaign of heated supersonic rectangular twin jets (SRTJ) was continued into the underexpanded flow regime. The nozzles had a design Mach number of 1.5 and an aspect ratio of 2. The nozzle pressure ratio was varied from design to underexpanded conditions (M-j=1.5-1.7) using a total temperature ratio of 2. Both baseline and excited cases using LAFPAs were investigated. Results focused primarily on the near-field (NF) acoustics/pressure results. Experiments further confirmed the influence of jet coupling on NF pressure fluctuations with out-of-phase (OOP) coupling between the jets resulting in reduced NF pressure fluctuations when compared to in-phase (IP) coupling. Screech "staging" was observed from near the design Mach number through underexpanded baseline cases. The abrupt jump in the screech frequency does not appear related to a change in spatial mode, but rather to the existence and relative strength of two shock systems and therefore the availability of a larger number of feedback locations. Similar baseline coupling behavior and LAFPA control authority were observed as in previous unheated underexpanded results: the LAFPAs' control authority was reduced in strongly underexpanded flow regimes (also consistent with literature).

Heated supersonic rectangular twin jets (SRTJ) with a total temperature ratio of 2, using nozzles of design Mach number 1.5 and aspect ratio 2, were investigated in flow regimes from overexpanded to the design condition (Mj = 1.3–1.5). This work complements our recently published work in unheated SRTJ using the same experimental facility (Samimy et al., J. Fluid Mech, vol. 959, 2023, A13). Localized arc filament plasma actuators (LAFPAs) were used to excite the natural instabilities in the jets, thereby controlling the flow and acoustics. The results show that the jets were coupled primarily out-of-phase in overexpanded cases, that the coupling had significant effects on the near-field (NF) pressure fluctuations, and that these fluctuations were considerably higher for in-phase than for out-of-phase coupled cases. The results also revealed that the far-field (FF) overall sound pressure level is significantly higher on the minor axis plane of the SRTJ and that the onset of Mach wave radiation contributes to the increased acoustic radiation at the peak noise direction. The LAFPAs successfully controlled the coupling and were able to reduce the NF pressure fluctuations by 10 dB. However, only 1 to 2 dB FF noise reduction at the peak noise radiation direction was achieved. The overall trends of the baseline results and response of the flow to excitation are qualitatively similar in unheated and heated cases, but the details are significantly different.

Supersonic rectangular twin jets (SRTJ) are of interest for current and future generations of tactical aircraft. However, the adverse effects of screech-loop coupling have previously been documented to significantly increase the near-field (NF) pressure fluctuations. These high-amplitude pressure fluctuations have the potential to fatigue and damage nearby aft components of the aircraft. Previous studies have documented that the NF pressure fluctuation level depends on the coupling of the jets: in-phase coupling along the twin jets’ minor axes produces stronger NF pressure fluctuations than that of out-of-phase coupling. The objective of this work is to further investigate the effects of coupling modes on NF pressure fluctuations in SRTJ and to mitigate the adverse effects of coupling using active flow control. Localized arc filament plasma actuators are employed to alter the SRTJs’ coupling mode by leveraging natural flow instabilities with minimal power input. A NF microphone array is used for simultaneous coupling and NF pressure fluctuation measurements. Schlieren imaging and spectral proper orthogonal decomposition are used to assess the effects of control on the flow field. The effect of excitation at various frequencies and actuation patterns on coupling and NF pressure fluctuations in different flow regimes are explored and discussed.

An empirical closure model of rectangular twin jets’ screech and coupling has been developed to predict their coupling mode at a given flow condition. It builds on Powell’s original screech closure model (Powell, A., “On the Mechanism of Choked Jet Noise,” Proceedings of the Physical Society, Section B, Vol. 66, No. 12, 1953, pp. 1039–1056) of feedback waves produced by the interactions of large-scale structures (LSSs) with each shock cell. Powell’s work is expanded to include nonuniform shock-cell spacing, overexpanded flow regimes, and twin jet configurations. The feedback wave interference pattern strength at the nozzle exit is postulated to determine the realized coupling mode. The required empirical parameters are the shock-cell streamwise locations and the convective velocity of the LSSs. The model was validated using data from an experimental investigation of rectangular twin jets at overexpanded and underexpanded conditions. The coupling mode is correctly predicted for 11 of the 12 nozzle pressure ratios tested. Two datasets from the literature (Jeun et al., “Aeroacoustics of Twin Rectangular Jets Including Screech: Large-Eddy Simulations with Experimental Validation,” AIAA Journal, Vol. 60, No. 11, 2022, pp. 1–21 and Raman, G., and Taghavi, R., “Coupling of Twin Rectangular Supersonic Jets,” Journal of Fluid Mechanics, Vol. 354, Jan. 1998, pp. 123–146) were also examined, and the model made accurate predictions of the coupling mode for all seven reported cases. The response of the twin jets to flow control at various artificially imposed frequencies and excitation modes strongly supports the model predictions, highlighting their usefulness as a guide for the implementation of active flow control in twin jets.

The effects of perturbation-based active flow control on supersonic rectangular twin jets (SRTJ) over a wide range of nozzle pressure ratios (NPR = 2.77 to 6.7, corresponding to fully expanded Mach numbers M j = 1.3 to 1.9) were investigated. The aspect ratio and design Mach number for the bi-conic, converging-diverging nozzles were 2 and 1.5, respectively. The flow and acoustic fields of SRTJ are known to couple, often generating high near-field (NF) pressure fluctuations and elevated far-field (FF) noise levels. Large-scale structures (LSS), or equivalently instability waves or wave packets, are responsible for mixing noise, broadband shock-associated noise, screech and coupling. The primary objective of this research was to manipulate the development of LSS in this complex flow to better understand and mitigate their effects. The organization and passage frequency of the LSS were altered by excitation of instabilities over a wide range of frequencies and modes. Key findings include: (1) the screech mode of each jet was flapping along its minor axis; (2) the jets coupled, out-of-phase primarily in overexpanded cases and in-phase primarily in underexpanded cases, along the minor axis of the SRTJ; (3) coupling has significant effects on the NF pressure fluctuations, but only minor effect on the FF noise; (4) standing waves were observed only on the minor axis plane of the SRTJ; (5) altering or suppressing coupling can significantly reduce NF pressure fluctuations; (6) two high-frequency excitation methods proved effective in reducing the FF noise; and (7) nonlinear interactions between the screech tones and excitation input were observed in controlled cases in which screech was only partially suppressed.

The effect of active control by a nanosecond pulsed dielectric-barrier discharge plasma actuator was studied on a NACA 0012 airfoil, with a 7-inch chord (with 13-inch endplates) and a 14-inch span, for a sinusoidal motion profile from α = 0° to 20° at Rec = 300,000 and k = 0.075. Characterization of the baseline flow highlighted the dominant influence of the dynamic stall vortex (DSV) and the subsequent separation, during the downstroke, on the aerodynamic forces. PIV results confirmed that actuation over a wide range of frequencies generates structures of various size and spacing through the manipulation of the Kelvin–Helmholtz instability. The results showed that the dominant DSV, present in the baseline case, was replaced by the structures induced by actuation. The effects of control on the flow field were used to explain the changes in aerodynamic loading, providing insight into the underlying physics of the observed control authority. Peak aerodynamic loads (lift, drag, and moment) were all reduced by control. Control also augmented the lift during the downstroke (separated flow), reduced lift hysteresis (responsible for vibratory loading), and increased the cycle-averaged lift-to-drag ratio.

View Video Presentation: https://doi.org/10.2514/6.2022-2402.vid The goal of this investigation is to examine the coupling mechanisms in closely-spaced screeching rectangular twin jets (RTJ) with aspect ratio two nozzles. Each nozzle has an effective diameter (De) of 0.758 in whereas the center to center spacing between the two jets is 2.25De. A standard Z-type schlieren system is used to visualize the flow-field along the major and minor axes planes of the RTJ configuration at an over-expanded jet Mach number of 1.35. Irrotational wavepacket structures are filtered from the schlieren data by applying a novel extension of Doak's Momentum Potential Theory to these images. The resulting wavepackets are further examined using Spectral Proper Orthogonal Decomposition.At baseline conditions, the jets are observed to couple intermittently in-phase, with each jet adopting an anti-symmetric screech mode. The response of the jet to excitation with Localized Arc Filament Plasma Actuators is examined. Different test cases are presented that collectively include the effect of frequency and phase of actuation on the jet coupling. It is observed that the jets respond strongly to actuation at the screech frequency resulting in an elimination of intermittency. An in-phase excitation of top and bottom shear layers, opposing the natural anti-symmetric screech tendency of each jet strengthens the coupling between the two jets. In contrast, an out-of-phase excitation matching the natural screech mode of each jet but opposing the coupling tendency of the twin jets results in a loss of coherence.

View Video Presentation: https://doi.org/10.2514/6.2022-2401.vid Rectangular twin jets are quite promising for tactical aircraft. However, the associated screech and coupling phenomena which can produce elevated far-field noise and strong near-field pressure fluctuations (with the potential to damage nearby aircraft components) must be mitigated. The objective of this work is to examine the control authority of localized arc-filament plasma actuators (LAFPAs) over supersonic rectangular twin jets from closely spaced converging-diverging nozzles over a wide range of flow regimes, and to explore the underlying physics of this control technique using perturbations. LAFPAs exert control authority by leveraging flow physics via manipulating the Kelvin-Helmholtz instability to produce significant effects with minimal power input. LAFPAs have previously demonstrated excellent control authority in subsonic and supersonic jets. Spectral proper-orthogonal decomposition of time-resolved schlieren images demonstrate that the twin jets readily respond to excitation by the LAFPAs over a wide range of frequencies, allowing the LAFPAs to control the generation and development of the large-scale structures in the jet's shear layer and thus their interactions with the shock cells. Time-averaged wavelet coherence magnitude and phase from near-field acoustic data were used to assess the twin jets' screech and coupling states. The effects of various actuation parameters on the twin jets' screech and coupling are consistent with empirical predictions based on the classical screech closure model. The LAFPAs' capabilities include altering the coupling and the screech modes, changing the screech and coupling frequency, and suppressing screech and coupling.

Coupling in the closely spaced twin jets employed in advanced tactical aircraft can lead to significant increases in near-field pressure fluctuations, which might result in structural fatigue and failure, and increased far-field noise. The benefits that can be obtained through integrating non-axisymmetric geometries in the latest generation of tactical aircraft have justified the renewed interest in gaining a better understanding of coupling and screech dynamics in such geometries. The simultaneous presence of two feedback loops, namely screech and coupling, in closely spaced coupled jets render the dynamics involved in these processes quite complex. The results of active flow control experiments aimed at better understanding the complex dynamics of the screech and coupling processes in closely spaced, low AR, twin rectangular supersonic jets with a design Mach number of 1.5 are presented in this paper. Experiments were conducted at an overexpanded Mach number of 1.35. The baseline jets at this Mach number demonstrate a tendency to intermittently couple in-phase. The screech mode of the individual jets was anti-symmetric. Two active control experiments, aimed at exploring the effects of altering the screech and coupling loops, are presented and the results are discussed. In the first experiment, the excitation frequency and relative phase between the adjacent jets were matched to the natural screech frequency of the jets and their coupling tendency but the jets were forced to adopt a symmetric screech mode. The results showed that organizing the shedding of large-scale structures (LSS) through excitation results in strengthening of the screech loop, which in turn enhances coupling. In the second experiment, the excitation was set to match the jets’ natural screech frequency and mode but to oppose the coupling preference of the baseline jets. Results showed that such an excitation pattern leads to decoupling of the jets. This is likely due to the disruption of the time of arrival of acoustic feedback waves by out-of-phase shedding of LSS in the shear layers of adjacent jets. Consequently, it appears that altering the feedback time of arrival in the coupling loop - through altering the relative excitation phase of the jets - is effective in decoupling the jets. More work is needed to establish a comprehensive framework for controlling the jets over various fully expanded jet Mach numbers and the effects of control on near-field pressure and far-field acoustics.

Large-eddy simulation of a jet issuing from rectangular nozzles of aspect ratio 2 is performed. The nozzles are operating at their nominal design Mach number of 1.5. This operating condition and the geometry match those of the companion experiment conducted at Ohio State University. The preliminary results show good agreement with near-field and far-field noise measurements in terms of broadband levels and predictions of screech tone frequencies and amplitudes. In particular, the main noise radiation towards the aft angles and the overall sound pressure level directivity are within 1dB for most relevant frequencies and angles. For future simulations of active control, a numerical model of a localized arc filament plasma actuator is implemented and tested in a small test domain inside one of the nozzles. A grid resolution study is conducted to investigate the minimum grid resolution required for correct energy transport within the boundary layer