SOUND RADIATION DURING LOCAL LAMINAR BREAKDOWN IN A LOW-MACH-NUMBER BOUNDARY-LAYER

The far-field sound of an unstable wave packet undergoing transition i n a low-Mach-number, hat-plate boundary layer is investigated in the f ramework of Lighthill's acoustic analogy. Detailed accounts of the wav e packet evolution are obtained by solving the full incompressible Nav ier-Stokes equations at Re-delta = 1000. The numerically simulated flo w structures show qualitative agreement with experimental observations of the fundamental breakdown type. The acoustic calculations are focu sed on the quadrupole source functions arising from Reynolds stress fl uctuations. The wave packet is shown to produce negligible sound throu ghout the primary and secondary instability stages. Dramatic amplifica tion of the Reynolds stress quadrupoles occurs as a result of the disi ntegration of the detached high-shear layer and the associated vortex shedding near the boundary layer edge. The dominant frequency of sourc e oscillations coincides with that of vortex shedding.