Supersonic jets, particularly shock-containing jets, often exhibit high-intensity, discrete-frequency acoustic tones. These tones are the signature of an aeroacoustic resonance loop established by ...

https://journals.sagepub.com/doi/pdf/10.1177/1475472X19834521

Aeroacoustic resonance and self-excitation in screeching and impinging supersonic jets – A review:

The closely spaced twin-jet configuration often seen in military aircraft is distinct from the single jet in both the flowfield and acoustic field. The twin-jet plumes interact with each other weakly or strongly, depending upon the distance between the two jets, the jet operating flow regime, and the Mach number. In the current study, the interaction mechanisms and coupling of a supersonic twin jet exhausting from biconical converging–diverging nozzles with a design Mach number of 1.23 at a separation distance of two nozzle exit diameters are investigated using near-field pressure measurements and phase-locked flow visualization. Across a series of jet operating Mach numbers, the twin-jet plumes feature three major jet azimuthal modes: axisymmetric, helical, and flapping. The jet flapping mode is strongly augmented in the twin-jet configuration compared to the single-jet case. Along the twin-jet plane, which is also the plane of the jet flapping direction, jet plumes with coherent flow structures are obse...

Exploring Physics and Control of Twin Supersonic Circular Jets

Free shear layers are building blocks of many flows of interest in applications, including jets, cavity flows, and separated flows. It was found several decades ago that free shear layers are unsta...

Excitation of Free Shear-Layer Instabilities for High-Speed Flow Control

An experimental investigation into the coupling behavior of screeching axisymmetric twin supersonic jets is presented. Acoustic measurements and schlieren photography are used to identify four dist...

Coupling Modes of an Underexpanded Twin Axisymmetric Jet

Experimental study on the mean flow characteristics of a supersonic multiple jet configuration

It has been widely reported in the literature that the jet preferred mode Strouhal number varies over a large range of 0.2–0.6, depending upon the facility where the measurement is made as well as the measurement techniques and the location in the jet plume where the measurement is taken. This study investigates this wide variation and potential explanations for it. Active flow control is used to show that the jet is receptive to excitation over a large range of Strouhal numbers and azimuthal modes. The wide variation in the preferred mode Strouhal number is shown to be tightly linked to the evolution, spacing, and scale of the coherent flow structures, which dominate the jet shear layer’s development. The low-end of the range is determined by the minimum Strouhal number at which structures begin to interact with one another in the jet plume. Below this range, structures have no significant effect on the plume’s statistical properties. For Strouhal numbers at the high-end of the range, the development of coherent flow structures shifts upstream toward the nozzle exit and the structures disintegrate earlier in the jet plume. The earlier development and disintegration prevent these structures from strongly impacting the entire flowfield. The results imply that upstream perturbations in the flow present in various facilities could be responsible for the variations in the measured jet preferred mode Strouhal number. Experimental results from schlieren imaging and near- and far-field microphone measurements are used to investigate the preferred mode Strouhal number across this range.

Effects of excitation around jet preferred mode Strouhal number in high-speed jets

In this article the erratic coupling that can occur in screeching supersonic twin jets is characterised. Non-stationary acoustic analysis is used to investigate the temporal behaviour of the coupling phenomena. The results show that where the phase between the jets is time varying, the screech tone experiences interruptions. The interruptions are either correlated and experienced by both jets or are anti-correlated and only by one. During the anti-correlated interruption, the uninterrupted jet screeches as an isolated jet. The instantaneous velocity field shows that for the majority of snapshots during an acoustic interruption, the jets do not exhibit a coupled oscillation. When the jets are uninterrupted, they are oscillating in either a coupled symmetric or anti-symmetric mode. This behaviour manifests at a condition between two operating points characterised by different coupling modes. It suggests the interruptions arise due to a competition between two global modes of the flow. Despite the existence of multiple acoustic tones in the region where these modes are competing, analysis of the individual jets reveals energetic structures with only a single wavelength. It is found that jets whose own oscillation is characterised by a single wavelength can, through coupling either symmetrically or anti-symmetrically about their symmetry plane, produce different acoustic tones. These findings are consistent across three experimental facilities. The observed modes are a function of the jet spacing and nozzle pressure, therefore future studies investigating other spacings must recharacterise the encountered coupled modes. This article provides the signatures to characterise the behaviour for future studies.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/67820357E5EDB5E8339F313307837484/S002211202000909Xa.pdf/div-class-title-intermittent-modal-coupling-in-screeching-underexpanded-circular-twin-jets-div.pdf

Intermittent modal coupling in screeching underexpanded circular twin jets

The near-field of twin-jets is investigated to study the twin-jet plumes’ interaction at a nozzle separation distance of two nozzle exit diameters. The linear microphone array was located close to the twin-jet plumes with various azimuthal angles relative to the twin-jet axes to record the near-field twin-jet effect. The twin-jet plumes interact and contribute to the near-field pressure amplification at different levels (weak or strong) depending on the dominant azimuthal mode motion (varying with jet Mach number). The level of twin-jet plumes’ interaction was evaluated by comparing to the equivalent single jet (with one of the twin-jet nozzles blocked). Among three azimuthal mode (axisymmetric, helical, and flapping) motions examined in this study, the twin-jet plumes (whose natural mode is determined by the jet Mach number) featuring the dominant flapping motion result in the most amplified near-field pressure response. Given by the nature of the flapping motion, the strong twin-jet plumes’ interaction is found in the preferred directivity along the flapping direction. In the twin jet this preferred direction is in the plane of the twin-jet axes. Localized arc filament plasma actuators were used as a flow diagnostic tool to confirm the understanding of the interaction phenomenon of twin-jet plumes.

An Investigation of Twin Supersonic Jets' Near-field

The closely spaced twin-jet configuration often seen in military aircraft is distinct from the single jet in both the flow and acoustic fields. The twin-jet plumes interact with each other weakly or strongly varying with the jet operating flow regime and Mach number. The interaction mechanisms of the supersonic twin-jet exhausting from bi-conical convergingdiverging nozzles with design Mach number of 1.23 at a separation distance of two nozzle exit diameters are investigated using near-field pressure measurements and phase-locked flow visualization to explore the twin-jet coupling. Across a series of jet operating Mach numbers, the twin-jet plumes feature three major jet motions: axisymmetric, helical, and flapping modes. The jet flapping motion is strongly augmented in the twin-jet configuration compared to the single jet case. Along the twin-jet plane, which is also the jet flapping direction, the coherent flow structures are observed to displace back and forth laterally. Because of the nature of the jet flapping motion, the near-field pressure fluctuations are markedly amplified at low Strouhal numbers, particularly along the twin-jet plane for far downstream locations. Localized arc filament plasma actuators were implemented as an active flow control tool on the twin-jet plumes to explore the coupling mechanisms. The results show that decoupling and coupling of twin-jet plumes can both be achieved, depending on the jet operating condition and the excitation parameters employed.

Jordan D. Cluts

Papers

Exploring Physics and Control of Twin Supersonic Circular Jets

Effects of excitation around jet preferred mode Strouhal number in high-speed jets

Intermittent modal coupling in screeching underexpanded circular twin jets

An Investigation of Twin Supersonic Jets' Near-field

An Investigation of Twin Supersonic Jet Coupling