The effects of initial perturbation to mixing-layer noise

The far-field noise radiated from mixing layers is determined by the near-field flow dynamics which is sensitive to the initial perturbation of instability introduced physically or numerically. This study focuses on the effects of the phase delay in two initial perturbations, one at the fundamental wave number and the other at its subharmonic both calculated from linear instability analysis, on the sound generation in mixing layers. When different phase delays φ1 changing from zero to 2π is applied on the fundamental mode, we observe different vortex merging processes (e.g. vortex pairing or tearing). The strong nonlinear interaction in the merging process generates most of the noise from mixing layers. There shows a pattern in a period of 2π for the response of far-field sound to the change of φ1. Similar effects on the dynamics and acoustics can be achieved by adding different phase delays φ2 to the subharmonic mode instead, however, the response repeats in a period of only π for φ2. The effects of the combination of different phase delays to other parameters, including the amplitude and wave number for each perturbations, are also investigated. All the results indicate a critical role of nonlinearity in the sound generation mechanism of mixing layers.