On the interpretation of pressure POD modes in the near field of a subsonic jet in terms of hydrodynamic and acoustic pressures

In this work an experimental investigation of the near-field pressure of a compressible jet is presented. The Proper Orthogonal Decomposition (POD) of the pressure fluctuations measured by a microphone array aligned with the jet spreading angle and placed in the linear hydrodynamic regime is performed in order to provide the streamwise evolution of the jet structure. The wavenumber-frequency spectrum of the space-time pressure fields re-constructed using each POD mode is computed in order to provide an interpretation of the physical nature of the mode in terms of hydrodynamic/acoustic pressures. Specifically, non-radiating hydrodynamic, radiating acoustic and ’hybrid’ hydro-acoustic modes are found based on the topology of the wavenumber-frequency spectrum and on account of the phase velocity associated with the spectral energy bumps. Furthermore, the preferential propagation direction in the far field of the radiating POD modes is detected through the cross-correlation with the measured far-field noise.