Inner Sound Field Generated by Large-Scale Coherent Structures of Ring Mode in the Near-Nozzle Region of a Subsonic Circular Jet: An Asymptotic Description of Aeroacoustics

Coherent structures (CS) are a significant sound source for jet noise. In the near-nozzle region of a jet, where the nozzle diameter is comparable with the envelope length scale of the wavepacket representing the CS, the CS resides in the thin shear layer developing from the lip line. One of the significant new features in this region is the presence of an inner acoustic field in the potential core. Sound waves with a frequency much lower than that of the CS are emitted via the mechanism of envelope radiation. Similar to that on a planar mixing layer, the outer and inner acoustic fields interact with each other, and the equivalent sound sources have to be determined along with the sound fields. It is shown for the first time that nonlinear interactions of CS in the thin shear layer excite, via receptivity, the so-called discrete `trapped acoustic modes' as well as low-frequency Kelvin--Helmholtz (K--H) modes, which may evolve into preferred modes in the developed region of the jet. The envelope radiation provides a key mechanism coupling the dynamics in the thin shear layer with the dynamics in the jet column regions. In the incompressible limit, this mechanism of excitation also operates, although the pressure decays exponentially towards the axis rather than behaves as sound waves in the potential core.