Sources of sound in the near-nozzle region of a subsonic Mach 0.9 jet

We investigate a M = 0.9 turbulent round jet issued from a straight, cylindrical nozzle with the aim of understanding the behavior of near- nozzle noise sources, and their contribution to the radiated far-field spectra. The numerical database for this study is obtained via a Large- Eddy Simulation (LES) of a Mach 0.9 round jet with a diameter-based Reynolds number of 10⁶, where a geometric trip forces the transition of the upstream boundary-layer inside the nozzle. The far-field spectra is computed by solving the permeable Ffowcs Williams-Hawkings (FW-H) equation in the frequency domain. Next, we perform a Spectral Proper Orthogonal Decomposition (SPOD) on the noise sources identified by the FW-H equation. Preliminary results reveal the presence of strong tonal peaks at high-frequencies (St >~ 2) in the SPOD spectra whose corresponding mode shapes show the presence of strong acoustic signatures inside the nozzle.  In fact, once the far-field spectra is recalculated with only the modal structures present outside the nozzle, these high-frequency tones disappear.  Further, some of the SPOD modes also seem to be scattered predominantly in the upstream direction with such scattering originating at the nozzle lip, as expected.  The nature of these latter SPOD modes are different to the ones that do not scatter which we analyze in this work.