Generation of sound waves by nonlinearly evolving ring-mode coherent structures on a turbulent subsonic circular jet: a comparative study of two mechanisms

Coherent structures (CS) are present on a subsonic turbulent jet and are known to constitute an important source of jet noise. With these structures being treated as wavepackets of instability modes supported by the mean flow, two acoustic radiation mechanisms have been identified. The first, referred to as generalised Mach-wave radiation (GMWR), is associated with the fact that a CS undergoing axial amplification and attenuation consists of supersonic components in its spectral tail, which radiate to the far field as sound waves. On the other hand, the nonlinear interaction of the CS generates a temporally and spatially modulated mean-flow distortion, which emits low-frequency sound waves. This second mechanism is referred to as envelope radiation (ER). We investigates, in a common mathematical setting, these two radiation processes for nonlinearly evolving CS of ring modes, which are described by strongly nonlinear critical-layer theory. Nonlinear effect is found to induce jittering, which enhances the GMWR significantly but suppresses ER slightly. The two mechanisms are both viable for CS of moderate amplitude, with GMWR and ER being dominant in the near axis and sideline regions respectively. The acoustic field due to the GMWR is in qualitative agreement with measurements.