Sound generation by a pair of co-rotating vortices using spectral acoustic analogy

The spectral acoustic analogy (AA) theory is used to study the acoustic generation by a pair of co-rotating vortices from the decomposition of wave packets. The use of a two-dimensional spectral AA formulation is based on the data obtained by conducting a direct simulation of unsteady compressible Navier-Stokes equations. By way of the spectral AA theory and convolution filtering operations, the role played by the wave packets of the vortices in the generation of the sound is 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. It potentially bridges the gap between the near-field wave packets and the far-field sound. The present methodology based on the spectral AA theory shows promise in providing insights into the mechanisms of sound generation in turbulent or unsteady vortical flows.