Evidence of discrete trapped acoustic waves in high subsonic jets

We offer numerical evidence of the frequency-discrete nature of trapped acoustic waves in subsonic turbulent jets. Acoustic waves trapped within the potential core of high subsonic jets were previously investigated by Towne et al. (J. Fluid Mech. vol. 825, 2017) and Schmidt et al. (J. Fluid Mech. vol. 825, 2017), and found to be capable of resonance in a number of frequency bands. Drawing on mean flow-based global stability analysis, Schmidt et al. further hypothesized that the trapped waves exist at closely-spaced, discrete frequencies within each frequency band. However, classical spectral estimation techniques did not allow these discrete waves to be unambiguously distinguished in data. We analyze data from a large-eddy simulation of the same Mach 0.9 jet that was studied by the aforementioned authors. To extract trapped acoustic modes, we use spectral proper orthogonal decomposition (SPOD) with sine tapers, which achieves high frequency resolution. The SPOD spectrum is substantially elevated over the expected frequency bands. Within each band, the spectrum reveals multiple discrete peaks. Each of these peaks corresponds to a mode with distinct spatial support and structure. The educed structures are consistent with the previously predicted global modes. Experimental evidence is presented.