Flow-induced acoustic resonance in a piping system containing closed tandem side-branches was investigated experimentally in this study. Velocity perturbation was induced at the mouth of the cavity using two pumps and a block. An uncommon acoustic mode change, from a higher mode to a lower mode, was observed when the flow rate in the main pipe increased. This phenomenon was examined by high-time-resolved Particle Image Velocimetry (PIV). The instantaneous velocity field in a cross section was visualized two-dimensionally using PIV technique, simultaneously with the pressure measurement at multi-points around the cavity by microphones. The fluid flows at different points in the cavity interact, with some phase differences between them, and the relation between the fluid flows was clarified. The phase difference of the acoustic pressure fluctuation at different points around the cavity was also obtained. Consequently, phase delays of oscillation at different points were obtained two-dimensionally. 2-D phase map was helpful to discuss the feedback mechanism of the self-induced vibration. This is the first research which can obtain this kind contour map using the time sequential instantaneous velocity fields for closed tandem side-branches system which has long side-branches (L/D ≫1) and high inflow velocity at high resonant frequency.

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