2009 Nonlinear mechanisms of sound radiation in a subsonic jet

Nonlinear mechanisms of sound generation from subsonic jets are investigated by direct numerical simulations. The main goal is to obtain further physical insight into a recently proposed nonlinear mechanism, based on the idea that nonlinear interaction between two primary instability waves results in a difference frequency mode that can be an efficient sound radiator in subsonic jets. The current approach allows linear, weakly nonlinear and highly nonlinear (turbulent) mechanisms to be isolated. The results show that broadband low-frequency sound radiated predominantly in the downstream direction can be attributed to the large-scale structures and described in terms of the nonlinear interaction model. Ad-ditionally, a significant part of the low-frequency sound radiated in the side-line direction is likely to come from the interaction between two helical modes with azimuthal numbers n = ±1. High-frequency noise in both downstream and side-line directions seems to orig-inate from the breakdown of the jet into smaller structures with ultimate transition into turbulence after the end of the potential core. I