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Vortex dynamics and aeroacoustic energy transfer, which play essential roles in vortex-excited acoustic resonance inside straight channels with coaxial side-branches, were investigated by phase-locked particle image velocimetry (PIV) and Howe’s acoustic analogy. In the experiments, the periodic acoustic pressure fluctuations at the endplates of the side branches were used to trigger PIV via a field-programmable gate array control system. The results revealed that the spatiotemporal evolution of vortex shedding can be classified into three regions in response to the acoustic standing-wave propagations: the formation region, the convection region, and the collapse region, along with the flapping recirculation zone and the intermittent vertical flow streaks that occur inside the side branches. Further investigation was performed in terms of phase-dependent quantities such as the shear and normal stresses; the normal stress production, which was attributed to the evolution of vortex shedding, was found to be the major contributor to the kinematics and energetics of the self-sustained flow. Finally, Howe’s acoustic analogy was used to determine the instantaneous acoustic power and the accumulated aeroacoustic energy during one acoustic resonance cycle. The aeroacoustic energy extracted from the acoustic standing-wave propagations contributed to the formation and subsequent growth of the shedding vortex, whereas the decreased turbulent kinetic energy of the shedding vortex was transferred to the acoustic standing waves to maintain the longitudinal wave propagations.

The influence of vortex-excited acoustic resonance on flow dynamics in a straight channel with coaxial side branches was experimentally determined. The energetic vortex structures superimposed in the shear layer, formed at the intersection between the straight channel and the side-branches, are closely coupled with the natural acoustic modes of the side-branches, resulting in significantly intensified acoustic standing waves in the side-branches. Two configurations with symmetric (length-to-diameter ratio: L1/D = L2/D = 10) and asymmetric (L1/D = 10, L2/D = 5) arrangements of the side-branches were used to investigate the clearly different flow dynamics with and without acoustic resonance. In the experiments, wall pressure measurements and acoustic modal analyses were first conducted to determine the frequency lock-on range of the vortex-excited acoustic resonance. Subsequently, the flow fields in two configurations were measured with planar particle image velocimetry. The results indicated that the vortex-excited acoustic resonance resulted in significant flow fluctuations inside the branches together with expanded shear layers and the enlarged vortex structures. Vorticity fluctuations and Reynolds shear stresses were also increased significantly when subjected to acoustic modulation. In terms of the proper orthogonal decomposition analysis, two different motions of the energetic flow structures were identified as interacting with the acoustic resonance: the vertical synchronous oscillation mode and the streamwise vortex-shedding mode. Correspondingly, the phase-averaged low-order flow fields, determined by the mode coefficients of these two correlative modes, clearly indicated the flapping motion of the mainstream and the periodic oscillation of the shear layer.