Turbulent Mixing Noise
Experiments and analysis of turbulent mixing noise have been ongoing for fifty years. For most of this period, the acoustic analogy, proposed by Lighthill (1952), Lighthill (1954), and its extensions, have dominated analysis and predictions. In the 1970’s, it was recognized that large-scale turbulent structures in the jet mixing layer are very efficient noise radiators in high-speed jets. In addition, with recent increases in computational power, Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) have shown their potential to provide a complete three-dimensional, time – dependent prediction of both the flow and noise of jets.
In Section 3.1 the early acoustic analogy models and their subsequent extensions to include the effects of source convection and mean flow refraction are described. Jet noise prediction schemes based on the acoustic analogy are also introduced. Section 3.2 describes the mechanism of noise generation by large-scale turbulent structures, modeled as instability waves. The associated analytical and numerical predictions are also introduced. A recent empirical correlation of turbulent mixing noise directivity and spectra is described in Section 3.3. Finally, the model of Tam and Auriault (1999) for noise generation and radiation by fine-scale turbulence, as well as more recent improvements in acoustic analogy-based models, are described in Section 3.4.