Analysis of Noise Reduction in Dual Impinging Jets with Microjets

Short take-off and vertical landing result in intense tones from jets impinging on the ground plane. Microjets have been successfully used to attenuate impinging tones. In this work, we use momentum potential theory (MPT) to analyze previously obtained schlieren images, in which suitably placed microjets were shown to reduce the tones in a dual-impinging jet configuration consisting of a supersonic Mach 1.5 and a sonic jet. MPT extracts the irrotational component of the "momentum density'' through a Poisson solver whose source term is proportional to the time derivatives of density fluctuations. When combined with the data-driven Spectral Proper Orthogonal Decomposition (SPOD) method, applied to both raw and filtered schlieren data, various features of noise reduction mechanisms due to microjets are discerned. In the baseline case, large coherent wavepacket structures, different from the shear layer instabilities, are observed in the irrotational component of the supersonic jet. The effect of control is to suppress these along with a significant decrease in the energy of SPOD modes. At the resonant frequency, the principal SPOD mode of the raw schlieren highlights the influence of dominant supersonic jet instabilities on the lower-speed sonic jet. However, higher SPOD modes of the filtered schlieren show that the fountain flow, generated between both the jets due to their interaction, also affects the supersonic jet. The acoustic component extracted from the MPT-filtered schlieren highlights the effects of microjet-induced supersonic hydrodynamic instability attenuation leading to a lower magnitude acoustic component. Further analysis with bispectral mode decomposition captures the modifications in the nonlinear interactions for the raw and filtered schlieren due to microjets.