Reprint of: Contributions to Jet Noise from Instability Waves and their Interactions: From Theory to Modelling

AbstractExperiment and direct numerical simulation provide compelling evidence that nonlinear wave interactions play a significant role in the generation of noise in subsonic jets. A simple ‘difference mode’ approach captures the essential mechanism, which is a much stronger source of large structure sound, particularly at low Mach numbers, than the alternative of direct linear conversion. Significant features of the low frequency end of the jet noise spectrum are predicted correctly, including the shift of the spectral peak to lower frequencies, the directivity pattern and the effect of Mach number. This mechanism distinguishes large structure sound from that associated with the breakdown to small scales. For practical calculations of jet noise a hybrid approach is proposed in which the Reynolds-Averaged Navier-Stokes equations are used to compute a base flow solution and then either a truncated Navier-Stokes approach, reducing to linearised Euler in the far field, or a Parabolised Stability Equations solver coupled to a linearised Euler solver are used to calculate sound sources and propagation. This approach potentially provides a coupling between flow control on the jet nozzle and the radiated sound