The high noise levels generated by the jet exhaust from commercial and military aircraft make mixing-noise reduction an important objective. Crucial to this effort will be the availability of robust, rapidly computable noise models that can be used to guide and optimize noise control strategies. Numerous studies have identified large-scale coherent structures in the form of wavepackets within the turbulent flow field as an important noise source. The typical approach to modeling wavepackets is to approximate them as linear modal solutions of the Navier-Stokes equations linearized about the long-time mean of the turbulent flow field. The near-field wavepackets obtained from these linear models show compelling agreement with those educed from experimental and simulation data for both subsonic and supersonic jets, but the associated far-field acoustic radiation is severely under-predicted in the subsonic case. This suggests that nonlinear effects are important in increasing the radiative efficiency of the wavepackets.
In this talk, I will summarize recent efforts to educe and characterize the nonlinear forcing experienced by wavepackets in subsonic jets. The main conclusion is that random turbulent fluctuations, rather than direct nonlinear interactions amongst wavepackets, are primarily responsible for producing acoustically efficient wavepackets. This suggests that the essential ingredients of sound generation in high Reynolds number jets are contained within the linearized Navier-Stokes operator, a conclusion that has important implications for jet noise modeling.