Acoustics of Laminar Boundary Layer Breakdown at Low Mach Number

The sound of an unstable wave packet undergoing transition in a low Mach number, flat plate boundary layer is investigated within the framework of Lighthill’s acoustic analogy. Detailed accounts of the wave packet evolution up to laminar breakdown are obtained by solving the full incompressible Navier-Stokes equations at Reδ* = 1000. The simulation results allow an accurate evaluation of the complete near-field acoustic source terms. The wave packet is shown to produce little sound throughout the primary and secondary instability stages. As the detached high-shear layer disintegrates, volume quadrupole sources dominated by Reynolds stress fluctuations amplify dramatically in the frequency range of high-shear layer vortex shedding. A surge in the viscous surface dipole strength due to intensified near-wall vortical activities is found at a later laminar breakdown stage. At low Mach number the wall-shear-stress radiation dominates the far-field.