Dynamic delayed detached-eddy simulation and acoustic analogy analysis of unsteady flow through a sudden expansion pipe

In the present study, the unsteady flow through a sudden expansion pipe with an expansion ratio of 1:3 is numerically modeled using dynamic delayed detached-eddy simulation (dynamic DDES), and the flow noise mechanism and the downstream noise propagation behaviors are analyzed using Lighthill's acoustic analogy. The numerical results for turbulent flow with a Reynolds number of 15,000 are validated against planar particle image velocimetry measurements. The acoustic sources are identified as acoustic quadrupoles in the high-shear flow region and acoustic dipoles on the expansion and downstream walls. The volume quadrupole acoustic source, which is particularly energetic in the low frequency region, is found to concentrate in the separating and reattaching area. On the downstream wall, the reattaching flow gives rise to the peak acoustic dipole, which attenuates beyond the station x/d = 10. On the expansion wall, the alternating appearance of a peak sound-pressure level (SPL) pair resembles the pipe circumferential sound mode beyond the cut-off frequency. The SPL distributions on the downstream wall are closely related to the noise propagation behavior. In the low-frequency region, the total noise in the far-downstream pipe section is dominated by acoustic dipoles on the downstream wall and volume acoustic quadrupoles in the high-shear flow region, whereas the major contribution in the high-frequency region is from acoustic dipoles on the expansion and downstream walls.