The interaction between an imposed monochromatic, time-dependent acoustic disturbance and a steady mean shear flow in a two-dimensional duct is studied using direct numerical simulation. Unlike previously reported numerical results, the long-time acoustic response is captured through implementation of accurate nonreflecting boundary conditions. Below a certain cutoff frequency, the acoustic field is a nearly planar traveling wave prop- agating axially along the duct. Above this cutoff frequency, oblique waves are generated due to both acoustic refraction and cross-stream dependent source oscillation, leading to an alternating pattern of higher acoustic pres- sures at the wall and the centerline, downstream of the disturbance source.
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