Sound radiation during local laminar breakdown in a low-Mach-number boundary layer

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Abstract
The far-field sound of an unstable wave packet undergoing transition in a low-Mach-number, flat-plate boundary layer is investigated in the framework of Lighthill's acoustic analogy. Detailed accounts of the wave packet evolution are obtained by solving the full incompressible Navier–Stokes equations at Reδ = 1000. The numerically simulated flow structures show qualitative agreement with experimental observations of the fundamental breakdown type. The acoustic calculations are focused on the quadrupole source functions arising from Reynolds stress fluctuations. The wave packet is shown to produce negligible sound throughout the primary and secondary instability stages. Dramatic amplification of the Reynolds stress quadrupoles occurs as a result of the disintegration of the detached high-shear layer and the associated vortex shedding near the boundary layer edge. The dominant frequency of source oscillations coincides with that of vortex shedding.

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