Acoustic near- to far-field characteristics of nozzle radiation through plug flow jets

This paper reports a theoretical and numerical study of the radiation of sound throughjet exhausts. It focuses on the transition from near field to far field by considering thefeatures of the near field solution and how these features translate to the far field. Themain finding of this work is the importance (in some cases) of lateral waves radiating fromthe jet. While the presence of lateral waves has long been recognized, there has been nosystematic investigation of the practical consequences of these waves in the prediction ofsound propagation through jet mixing layers. The physical mechanisms involved in thegeneration of these waves are presented as well as the conditions under which they becomesignificant. This paper also discusses the validity of the far-field approximation when lateralwaves are present. It is shown that the standard far-field asymptotic can be improved byadding a correction term which accounts for the presence of the lateral waves.