| مستخلص: |
Large-scale coherent structures are present on a subsonic turbulent circular 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 time-averaged 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 coherent structure 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 wavepacket generates a mean-flow distortion, which is modulated slowly in time and the axial direction, and emits low-frequency sound waves with the time and length scales comparable with those of the envelope. This second mechanism is referred to as envelope radiation (ER). The present paper investigates, in a common mathematical setting, these two radiation processes for nonlinearly evolving coherent structures (wavepackets) of ring modes, which are described by strongly nonlinear critical-layer theory. In the linear limit, the sound waves emitted by the GMWR exhibit the so-called superdirectivity, but this feature disappears as nonlinearity comes into play. Nonlinear effect is found to induce jittering, which enhances GMWR significantly but suppresses ER slightly. The two mechanisms are both viable for coherent structures of moderate amplitude, with the GMWR and ER being dominant in the near axis and sideline regions respectively. The directivity of the acoustic far field due to the GMWR is in qualitative agreement with measurements. [ABSTRACT FROM AUTHOR] |
