Large-eddy simulations of noise reduction via fundamental-harmonic interactions in a supersonic rectangular jet

This work explores the use of bi-modal excitation to reduce the noise in a supersonic rectangular jet. The excitation is guided by previously published reduced-order modelling, suggesting that harmonic interactions can reduce the dominant noise sources in the jet. An unexcited jet is considered first as the baseline case, which showed Strouhal number 0.15 as the most dominant near field coherent structure, which also appeared as a prominent peak in the far field noise at low emission angles and as a secondary peak at the peak emissivity angle, thus it was taken as the fundamental frequency to target with excitation. The jet is then excited with the single-mode harmonic, St 0.30. Other cases are considered with bi-modal excitation at St’s 0.15 and 0.30 with various phase lags. Both single-mode and bi-modal excitation show a considerable reduction of the near field coherent structures at St 0.15 and amplification at 0.30. Reduction of the fundamental in the bi-modal cases shows a dependence on the initial phase lag whereby reduction is maximized when harmonic amplification is maximized. All excited cases are also shown to reduce the eddy convection velocity. Reduction in far field sound pressure levels at St 0.15 is obtained for all excitation cases up to 13 dB. For all metrics, it is shown that bi-modal excitation is more effective than single-mode excitation given an optimized phase lag. All results ultimately support the use of fundamental-harmonic interactions as a noise reduction mechanism.