Intensified flow dynamics by second-order acoustic standing-wave mode: Vortex-excited acoustic resonances in channel branches

Abstract

The intensified flow dynamics by the second acoustic standing-wave mode, superimposed with vortex-excited acoustic resonances inside a channel with coaxial side-branches, were experimentally investigated. In the experiments, the frequency lock-on range of vortex-excited acoustic resonances was determined first by comparing wall pressure fluctuations and simulated acoustic standing-wave modes. The intensified flow dynamics by the second acoustic standing-wave mode were measured by particle image velocimetry (PIV); the flow dynamics were also measured when coupled with the first standing-wave mode for comparison. The results demonstrate that shear layer developments over the branch entrance can be classified into three regions, i.e., the developing region, the transition region, and the collapsing region. Both the momentum thickness and the growth rate of the shear layer were significantly intensified in the developing and transition regions by the second standing-wave mode. The subsequent spatiotemporal evolutions of the shedding vortex, recirculation zone, and synchronous flow streaks were identified by the phase-locked PIV measurements. With second-order acoustic modulations, the shedding vortex breaks away from beneath the recirculation zone to impinge the downstream branch corner while directly converging with the recirculation zone during first-order modulation. Finally, the aeroacoustic energy transfer between vortex dynamics with standing waves was revealed using Howe’s aeroacoustic analogy. The energy transferred from the standing-waves contributed to the formation and development of the shedding vortex, while the energy produced by convection and the collapsing of the shedding vortex maintained the standing waves’ propagations. The accumulated aeroacoustic energy produced during one second-order acoustic resonance cycle was found to be up to 150 times that produced during one first-order cycle.