A numerical study on the zone of silence of the impinging tone from supersonic perpendicular impinging jets

When supersonic jets impinge on plates, high-amplitude impinging tones can be generated, posing threats to structural integrity and the well-being of personnel. However, the impinging tone ceases within particular parameter ranges, termed as the zone of silence. The tone variation when passing through the zone of silence is still unclear. In this study, supersonic perpendicularly impinging tones are investigated using computational aeroacoustics methods, and the zone of silence phenomenon is successfully reproduced. Through spectrum analysis and dynamic mode decomposition on the flow field, impinging tones in the pre-silence and post-silence regions are found to be divisible into three distinct groups. The first group occurs in the pre-silence region, and most are axisymmetric. These tones are related to the interaction between the shear layer and the stand-off shock. The second group occurs in the post-silence region and consists of a dominant tone and its harmonics. The dominant tone is axisymmetric and related to the oscillation of the Mach disk, recirculation bubble, and the wall jet. The third group is mostly helical and occurs in almost all cases, except the zone of silence. This group has constant frequencies regardless of the nozzle pressure ratio. In addition, the analysis reveals that a mode jump occurs in the zone of silence. The jet structure variations are studied, discovering that the left bound of the zone of silence corresponds to the case where the Mach disk forms, while the right bound corresponds to the case where the stand-off shock disappears and the maximum of the recirculation bubble occurs.