Effect of plasma actuator-based control on flow-field and acoustics of supersonic rectangular jets

Abstract

We perform a computational study on the effects of localized arc filament plasma actuator based control on the flow field and acoustics of a supersonic 2:1 aspect ratio rectangular jet. Post validation of the baseline jet, effects of control in the context of noise reduction are studied at experimentally guided forcing parameters, including frequencies $St=0.3, 1.0$ and $St=2.0$ with duty cycles of $20\,\%$ and $50\,\%$ . In general, high-frequency forcing reduces noise in the downstream direction, with the actuator signature appearing mostly in the sideline direction. Here $St=1$ , ${\rm DC}=50\,\%$ yields an optimum balance between peak noise reduction (of ${\sim }1.5$ dB) and actuator tones, with control being most effective on the major axis plane that bisects the shorter edges of the nozzle. Shear layer response to the most effective forcing includes generation of successive arrays of mutually interacting staggered lambda vortices, which eventually energize streamwise vortical elements. Causal mechanisms of noise mitigation are further elucidated as follows. First, the control reduces the energy within the supersonic phase speed regime of peak radiating frequencies by redistributing a part of it into a high-frequency band. Second, it enhances azimuthal percolation of energy into the first and second helical modes at frequencies where noise reduction is seen, thus weakening the radiatively efficient axisymmetric mode. Finally, sound-producing intermittent events in the jet are significantly reduced, thereby minimizing the high-intensity acoustic emissions. This small-perturbation-based control strategy results in only minor variations in the mean flow properties. However, reduced production and enhanced convection attenuate turbulent kinetic energy within the spreading shear layer in the controlled jet.

Noise mitigation in rectangular jets through plasma actuator-based shear layer control

“…Conversely, the higher frequencies (St ∼ 2 and above) saturate closer to the nozzle exit, as the spreading shear layer becomes less receptive to this spectral range. These observations are also consistent with prior nonlinear simulations (Lakshmi Narasimha Prasad & Unnikrishnan 2023b) that have identified St ∼ 1 as a suitable frequency to perturb this shear layer.…”

“…It highlights the effectiveness of LAFPA in reducing the size of large-scale structures using high-frequency excitation, and increasing their three-dimensionality using various forcing patterns, eventually reducing near-field pressure fluctuations and far-field noise. In line with this, our recent computational effort (Lakshmi Narasimha Prasad & Unnikrishnan 2023b) detailed the impact of LAFPA-based actuators on rectangular supersonic jets, and identified spectral bands that can result in sound mitigation, which is the first influential control parameter for these actuators. The present work builds on that effort, focusing on the following aspects that are motivated by the second control parameter, the circumferential pattern of forcing along the periphery of the nozzle cross-section.…”

“…The simulation of the baseline jet corresponding to the above parameters was extensively validated using published experimental and computational results in Lakshmi Narasimha Prasad & Unnikrishnan (2023b). These results are omitted for brevity.…”