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.

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.

Mach wave radiation is one of the better understood sources of jet noise. However, the exact conditions of its onset are difficult to determine and the literature to date typically explores Mach wave radiation well above its onset conditions. In order to determine the conditions for the onset of Mach wave radiation and to explore its behaviour during onset and beyond, three ideally expanded jets with Mach numbers Mj = 0.9, 1.3 and 1.65 and stagnation temperature ratios ranging over To/T∞ = 1.0–2.5 (acoustic Mach number 0.83–2.10) were used. Data are collected using a far-field microphone array, schlieren imaging and streamwise two-component particle image velocimetry. Using arc filament plasma actuators to force the jet provides an unprecedented tool for detailed examination of Mach wave radiation. The response of the jet to various forcing parameters (combinations of one azimuthal mode m = 0, 1 and 3 and one Strouhal number StDF = 0.09–3.0) is explored. Phase-averaged schlieren images clearly show the onset and evolution of Mach wave radiation in response to both changes in the jet operating conditions and forcing parameters. It is observed that Mach wave radiation is initiated as a coalescing of the near-field hydrodynamic pressure fluctuations in the immediate vicinity of the large-scale structures. As the jet exit velocity increases, the hydrodynamic pressure fluctuations coalesce, first into a curved wavefront, then flatten into the conical wavefronts commonly associated with Mach wave radiation. The results show that the largest and most coherent structures (e.g. forcing with m = 0 and StDF ~ 0.3) produce the strongest Mach wave radiation. Conversely, Mach wave radiation is weakest when the structures are the least coherent (e.g. forcing with m = 3 and StDF > 1.5).

An axisymmetric perfectly expanded Mach 1.3 jet, with a Reynolds number based on the nozzle exit diameter (ReD) of 1.1 × 106 and turbulent boundary layer at the nozzle exit, was excited using localized arc filament plasma actuators over a wide range of forcing Strouhal numbers (StDF). Eight actuators distributed azimuthally were used to excite azimuthal modes m = 0–3. Far-field acoustic, flow velocity and irrotational near-field pressure were probed with a three-fold objective: (i) to investigate the broadband far-field noise amplification reported in the literature at lower speeds and ReD using excitation of m = 0 at low StDF; (ii) to explore broadband far-field noise suppression using excitation of m = 3 at higher StDF; and (iii) to shed some light on the connection between the flow field and the far-field noise. The broadband far-field noise amplification observed is not as extensive in amplitude or frequency range, but still sufficiently large to be of concern in practical applications. Broadband far-field noise suppression of 4–5 dB at 30° polar angle peak frequency, resulting in approximately 2 dB attenuation in the overall sound pressure level, is achieved with excitation of m = 3 at StDF ~ 0.9. Some of the noteworthy observations and inferences are (a) there is a strong correlation between the far-field broadband noise amplification and the turbulence amplification; (b) far-field noise suppression is achieved when the jet is forced with the maximum jet initial growth rate frequency thus limiting significant dynamics of structures to a shorter region close to the nozzle exit; (c) structure breakdown and dynamic interaction seem to be the dominant source of noise; and (d) coherent structures dominate the forced jet over a wide range of StDF (up to ~ 1.31) with the largest and most organized structures observed around the jet preferred mode StDF.

Development, experimental implementation, and the results of reduced-order model based feedback control of subsonic shallow cavity flows are presented and discussed. Particle image velocimetry (PIV) data and the proper orthogonal decomposition (POD) technique are used to extract the most energetic flow features or POD eigenmodes. The Galerkin projection of the Navier–Stokes equations onto these modes is used to derive a set of nonlinear ordinary differential equations, which govern the time evolution of the eigenmodes, for the controller design. Stochastic estimation is used to correlate surface pressure data with flow-field data and dynamic surface pressure measurements are used to estimate the state of the flow. Five sets of PIV snapshots of a Mach 0.3 cavity flow with a Reynolds number of 105 based on the cavity depth are used to derive five different reduced-order models for the controller design. One model uses only the snapshots from the baseline (unforced) flow while the other four models each use snapshots from the baseline flow combined with snapshots from an open-loop sinusoidal forcing case. Linear-quadratic optimal controllers based on these models are designed to reduce cavity flow resonance and are evaluated experimentally. The results obtained with feedback control show a significant attenuation of the resonant tone and a redistribution of the energy into other modes with smaller energy levels in both the flow and surface pressure spectra. This constitutes a significant improvement in comparison with the results obtained using open-loop forcing. These results affirm that reduced-order model based feedback control represents a formidable alternative to open-loop strategies in cavity flow control problems even in its current state of infancy.

Localized arc filament plasma actuators are used to control an axisymmetric Mach 1.3 ideally expanded jet of 2.54 cm exit diameter and a Reynolds number based on the nozzle exit diameter of about 1.1×106. Measurements of growth and decay of perturbations seeded in the flow by the actuators, laser-based planar flow visualizations, and particle imaging velocimetry measurements are used to evaluate the effects of control. Eight actuators distributed azimuthally inside the nozzle, approximately 1 mm upstream of the nozzle exit, are used to force various azimuthal modes over a large frequency range (StDF of 0.13 to 1.3). The jet responded to the forcing over the entire range of frequencies, but the response was optimum (in terms of the development of large coherent structures and mixing enhancement) around the jet preferred Strouhal number of 0.33 (f = 5 kHz), in good agreement with the results in the literature for low-speed and low-Reynolds-number jets. The jet (with a thin boundary layer, D/θ ∼ 250) also responded to forcing with various azimuthal modes (m = 0 to 3 and m = ±1, ±2, ±4), again in agreement with instability analysis and experimental results in the literature for low-speed and low-Reynolds-number jets. Forcing the jet with the azimuthal mode m = ±1 at the jet preferred-mode frequency provided the maximum mixing enhancement, with a significant reduction in the jet potential core length and a significant increase in the jet centreline velocity decay rate beyond the end of the potential core.

An experimental study involving flow visualization and two-component LDV measurements has been undertaken to elucidate the deformation of an axisymmetric jet (ReD ≈ 1950 and 4160) caused by tabs placed at the nozzle exit. Previous studies have shown the profound distortion of high-speed jets with tabs and have demonstrated that a pair of counter-rotating streamwise vortices generated by each tab are responsible for the deformation of the jet core. This work illustrates the distortion as well as some of the more subtle features of the tab effect. Extensive visualizations taken simultaneously from two perspectives reveal the real-time evolution of complex three-dimensional flow structures. Velocity data show the expected overall distortion, and the existence and strength of the streamwise vortices responsible for this deformation. Furthermore, a second set of weaker streamwise vortices was detected near each tab, the size and location of which was consistent with a horseshoe vortex system. The data showed a widespread increase in both Reynolds normal and shear stresses and generally indicated the accelerated development of the mixing layer when tabs were inserted. A brief analysis employing vortex dynamics - an alternative to previous work which utilized pressure gradient arguments-is used to explain the tab effect, resulting in similar conclusions.