Visualization study of flow-excited acoustic resonance in closed tandem side branches using high time-resolved particle image velocimetry

Flow-induced acoustic resonance in a piping system containing closed tandem side branches was investigated experimentally. Velocity perturbation was induced at the mouth of a cavity by using two pumps and a block. An unusual acoustic mode change from a higher 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). It was found that fluid flows at different points in the cavity have some phase differences between them and that they interact with each other. The relationship between the fluid flows was clarified and the phase difference between the acoustic pressure fluctuations at different points in the cavity was obtained. Using these data, the phase delays of oscillations at different points were two-dimensionally mapped. This two-dimensional phase map was helpful in clarifying the feedback mechanism of self-induced vibrations.