On the vortex sound sources of an artificially excited two-dimensional subsonic mixing layer via a hydrodynamic/acoustic decomposition

Artificially excited two-dimensional mixing layers are frequently used as model problems to examine the noise source characteristics of strong sheared flows, such as the aircraft engine jets. Previous publications have identified the vortex pairing event as the dominating acoustic sources through various acoustic analogy theories or hydrodynamic/acoustic decomposition techniques. However, criticisms also rose due to the (partial) inclusion of near-field acoustic fluctuations in their equivalent source terms. In this study, direct flow and acoustic simulations are performed for a subsonic mixing layer with two-frequency inlet forcing. A methodology for hydrodynamic/acoustic decomposition is proposed and the acoustic source potential is identified. Based on the unsteady solution samples accumulated in the high-fidelity numerical simulations, a reasonable and more accurate decomposition of hydrodynamic and acoustic fluctuations is achieved as compared to previous approaches. Analysis shows that the acoustic source potentials represent well the noise sources and that tonal sound waves emerge when the acoustic beams produced by local minima/maxima of the acoustic source potentials join as arcs during the vortex pairing process.