Correlation between Mach waves and supersonic jet density field measured using three-dimensional background-oriented schlieren

The Mach waves are the dominant component of noise from supersonic jets and are thought to be generated by the coherent structures in the jet shear layer. One of the challenges in modeling the source of the Mach waves is the difficulty of experimentally observing these coherent structures, which are generally localized in space and time. To address this issue, this study aimed to capture these structures using the three-dimensional background-oriented schlieren (3D-BOS) method. The target for measurement was a laboratory-scale, unheated, Mach 1.8 ideally expanded jet. The instantaneous 3-D density field obtained using the 3D-BOS method showed a clear correlation with the sound pressure signals from an azimuthal microphone array placed in the Mach wave propagation region. This suggested that the measured density field contained large-scale deformation of the density iso-surfaces caused by advection due to the coherent structures. The relationship between the density field and sound waves was also discussed in terms of the wavy wall analogy, which views the coherent structures as a supersonically moving wavy wall that generates the Mach waves.