Noise computation using Lighthill's equation with inclusion of mean flow-acoustics interactions

Abstract

Lighthill's equation is used to compute the noise produced by subsonic flows. The objective is to show that, although this equation is based on a wave equation in a medium at rest, meanflow effects on sound propagation, included in Lighthill's source term, can be properly taken into account by numerical approches. The source terms are evaluated from the unsteady compressible flow motion equations, which provide also a reference sound field. By this way, all mean flow - sound waves interactions are included into the source terms. Two-dimensional cases are first considered with the radiation of a monopole in a sheared mean flow and with the sound generated by a mixing layer. They show that mean flow effects on propagation are correctly predicted with Lighthill's equation. The sound produced by pairings of axisymmetric vortices in a three-dimensional circular jet with a Mach number of 0.9 and a Reynolds number of 65000 is then investigated. Solving Lighthill's equation provides an acoustic radiation quite consistent with the sound field obtained directly from LES.