Sound generation by different vortex interactions in mixing layers

To understand the correlation between far-field sound and near-field vortex dynamics, in this paper, we numerically investigate the sound generation from different types of vortex interaction (e.g. vortex pairing, vortex tearing, triple/quadruple-vortex merging). Direct numerical simulation (DNS) of the compressible Navier-Stokes equations is carefully conducted for an accurate description of far-field sound radiation. By choosing different phase delays between the fundamental frequency and its sub-harmonics, the vortex interaction changes from perfect pairing to tearing. The sound generated by vortex tearing is much quieter than the one from vortex pairing at the same excitation level. However, the directivity of sound radiations keeps almost the same. When different combinations of forcing frequencies are chosen, there are more complex triple and quadruple vortex mergings, The sound intensity of such cases is stronger than the one of either vortex pairing or vortex tearing. With more frequencies being involved, multi-directivity appears in triple/quadruple-vortex merging cases. © 2012 by The authors.