Vortex-excited acoustic resonance in channel with coaxial side-branches: Vortex dynamics and aeroacoustic energy transfer

Abstract

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.