Direct computation of the sound generated by two-dimensional shear layer

The sound generated by a two dimensional shear layer is investigated by Direct Numerical Simulation (DNS) of the compressible Navier-Stokes equations. A high-order-accurate numerical scheme and non-reflecting boundary conditions are used to accurately compute both the near field hydrodynamics and the acoustic waves produced by the layer. The directly computed acoustic field shows that the acoustic waves which are generated at the fundamental frequency (most unstable mode), and its first two sub-harmonics, originate near the region where the instability wave at these frequencies saturate, i.e. the region where the layer rolls up into vortices for the fundamental frequency, and the first two Vortex pairings for the sub-harmonics. The sound generated by the pairings is most intense in the downstream direction, indicating a superdirective acoustic source.