HYDRODYNAMIC AND ACOUSTIC MODE DECOMPOSITION OF HIGH SUBSONIC TURBULENT JET AND ANALYSIS OF TRAPPED WAVE

The high subsonic turbulent jet noise is the main component of the aircraft propulsion system noise. The jet noise contains both the far field broadband turbulent mixing noise and the recently found near field discrete trapped wave. The trapped wave is buried in the strong hydrodynamic perturbation in the near field, which poses difficulty in revealing the spatial structure and propagation property of the trapped wave. Accurately decomposing the trapped wave in the near field of the jet is the key to reveal the physical property of trapped wave. To better understand trapped wave, the mode decomposition and physical property based HD-SPOD method for hydrodynamic and acoustic mode decomposition is developed. The method is able to separate the hydrodynamic and compressible component of the perturbation velocity field, with the latter being able to characterize the acoustic mode and reveal the main features of acoustic mode. The method provides tool for analyzing flow and acoustic features of the near flow field. The HD-SPOD method is applied to a jet Mach number 0.9 high subsonic turbulent jet. The hydrodynamic and acoustic modes of the leading SPOD modes whose frequencies corresponding to the trapped wave are decomposed. The spatial structure and propagation properties of the hydrodynamic and acoustic modes are analyzed. It is found that in the subsonic turbulent jet potential core, both the acoustic trapped wave and hydrodynamic "trapped wave" exist. The acoustic trapped wave shows multi-radial order structures, while the hydrodynamic "trapped wave" only has single order radial structure. The acoustic trapped wave exists in the front part of potential core and propagates upstream, while the hydrodynamic "trapped wave" exists through the potential core and transports downstream.