Wave Packets and Turbulent Jet Noise

TLDR

Turbulent jet noise remains a longstanding fluid‑mechanical puzzle, and although simulations can now predict turbulence and its sound, the theoretical framework guiding noise control is still incomplete; wave packets—intermittent, long‑range correlated disturbances—are clearly identifiable in vortical, near‑field, and far‑field regions. The study reviews evidence for the existence, energetics, dynamics, and acoustic efficiency of wave packets. The authors show how extensive simulation data and modern measurement techniques can isolate acoustically relevant turbulent motions. They confirm that wave packets act as instability or modal solutions, are acoustically significant in the aft‑angle radiation of high subsonic and supersonic jets, and provide a unified view that can guide control strategies.

Abstract

Turbulent jet noise is a controversial fluid mechanical puzzle that has amused and bewildered researchers for more than half a century. Whereas numerical simulations are now capable of simultaneously predicting turbulence and its radiated sound, the theoretical framework that would guide noise-control efforts is incomplete. Wave packets are intermittent, advecting disturbances that are correlated over distances far exceeding the integral scales of turbulence. Their signatures are readily distinguished in the vortical, turbulent region; the irrotational, evanescent near field; and the propagating far field. We review evidence of the existence, energetics, dynamics, and acoustic efficiency of wave packets. We highlight how extensive data available from simulations and modern measurement techniques can be used to distill acoustically relevant turbulent motions. The evidence supports theories that seek to represent wave packets as instability waves, or more general modal solutions of the governing equations, and confirms the acoustic importance of these structures in the aft-angle radiation of high subsonic and supersonic jets. The resulting unified view of wave packets provides insights that can help guide control strategies.