Flow visualization study of flow-induced acoustic resonance in closed side branches

The phenomenon of flow-induced acoustic resonance is encountered when an acoustic mode of a side branch is coupled with the flow instability oscillations, which may generate undesirable vibrations of components in pipe systems. In particular, flow-induced acoustic resonance has caused some problems in nuclear power plants. In this study, the characteristics of the flow-induced acoustic resonance in square closed side branches are investigated experimentally. Three different aspect ratios of L/b = 5, 6 and 7, where L is the branch height and b is the branch width, are investigated with Mach number up to 0.22. The flow structures in different acoustic and hydrodynamic modes are visualized using a high speed camera, and the frequencies counted by visual measurement agree well with the frequencies measured by dynamic pressure sensors. The effects of side branch height on the amplitude and frequency characteristics of the acoustic resonance are demonstrated. The results indicate that the maximum normalized pulsation amplitudes in different resonance modes decreases in branch height, acoustic mode and hydrodynamic mode. The branch height has little influence on the Strouhal number at onset of resonance, which is about 0.55 for the first hydrodynamic mode and 0.95 for the second hydrodynamic mode in this work. The maximum normalized fluctuating pressure at the first acoustic mode occurs at a Strouhal number close to 0.43. The hysteresis phenomenon in the evolution of the fluctuating pressure as a function of the velocity is analyzed.