Investigation of acoustic sources in the unsteady flow using spectral acoustic analogy

A two-dimensional spectral acoustic analogy (AA) theory is used to study the acoustic-source physics of a vortical flow. By way of the spectral AA theory and convolution filtering operations, the roles played by the wave packets of a pair of co-rotating vortices in the generation of the sound are thoroughly discussed. It is found that the far-field sound is generated by the linear superimposition of the interactions between the base flow and the remaining wave packets. Moreover, a notable wave packet of the co-rotating vortices, called the acoustic-related wave packet herein, is identified in the spectral domain, which not only contains all radiating components but also contributes to a discernible acoustic source. The capability of the spectral AA in identifying the physical sources will improve the understanding of the mechanisms of sound generated by turbulent or unsteady vortical flows.