On the Physics and Active Control of Trapped Waves in a Supersonic Axisymmetric Jet

Abstract

Free jets have been widely studied for decades, providing valuable insight into jet flow and acoustic physics. Jet shear layers are susceptible to perturbations over a range of frequencies, leading to the formation of coherent structures (CS) that dominate jet dynamics. In addition to the classical shear-layer mode, spatial instability analyses have identified an additional mode with upstream- and downstream-traveling branches. More recently, trapped acoustic waves and guided-jet waves, have been experimentally observed in high subsonic and supersonic jets across a range of Mach numbers and nozzle geometries, with signatures appearing in near-field and forward-quadrant far-field acoustics. While trapped waves have been documented, there is currently no information on how coherent-structure characteristics affect trapped wave behavior in supersonic jets. The objectives of this study are twofold: to confirm the existence of trapped waves in the near field of a supersonic axisymmetric jet exhausted from a converging–diverging nozzle over a range of operating conditions, and to examine how controlled modification of coherent structures using localized arc filament plasma actuators (LAFPAs) influences trapped wave behavior. Experiments were conducted using a laboratory-scale supersonic axisymmetric jet with a design Mach number of 1.52 operated in an over-expanded and ideally expanded condition. Near-field and far-field acoustic pressure measurements were obtained using calibrated microphone arrays in an anechoic chamber. LAFPAs were used to excite the jet shear layer at select Strouhal numbers to modify coherent-structure dynamics. Pressure spectra were computed using Fourier transformations to identify trapped-wave signatures and assess their sensitivity to operating condition and actuation. Baseline results revealed distinct near-field spectral peaks consistent with trapped wave signatures, with frequencies that vary with operating condition and exhibit relationships to screech tones and their harmonics. Differences between over-expanded and ideally-expanded cases suggest sensitivity to the operating regime. Preliminary excitation results using LAFPAs at select Strouhal numbers demonstrate measurable changes in trapped wave spectral content, including reductions in harmonic amplitudes and shifts in spectral peaks. These results confirm the existence of trapped waves in a supersonic axisymmetric jet and indicate that their characteristics are influenced by coherent-structure dynamics. Future work will incorporate schlieren imaging and spectral proper orthogonal decomposition (SPOD) analysis to further visualize coherent structures and assess the effects of control. The results advance understanding of guided jet-wave phenomena and support the development of active control strategies for mitigating acoustic loads in high-speed jets, which contribute to structural fatigue in tactical aircraft and cabin noise in commercial applications.