Study on flow-induced vibration of a flexible weir due to fluid discharge: Effect of weir stiffness

A fluid-elastic vibration of a thermal shield induced by the coolant discharge was experienced in the reactor vessel of SPX-1, a French fast breeder reactor. Since a similar structure is considered in the design of the Japanese Demonstration Fast Breeder Reactor, an experimental study was performed on the flow-induced vibration of a flexible weir due to fluid discharge from the upper plenum to the lower plenum. The purposes of this experiment are: (1) to clarify the vibration phenomena and (2) to confirm the effectiveness of several countermeasures for preventing the instability of the weir. In the experiment a 1/5-scale weir-coolant model for the thermal shield of the Japanese demonstration fast breeder reactor was employed as a test apparatus. Three cylindrical weirs were tested to study the effect of the stiffness of weirs. Those were 3mm and 6mm-thick polyvinyl chloride models and 2.5mm-thick aluminum model. Unstable vibration conditions were obtained by altering the fall height and the flow rate. The predominant frequencies and the amplitudes of the weir displacement and the sloshing were measured under the instability conditions. The phase differences between the weir displacement and the sloshing were also obtained. Moreover, several kinds of vibrational mitigation methods were investigated. Concerning the threshold of instability, the stiffness and the natural frequency of the weirs and the frequency ratio between the weir and sloshing were discussed. From this study, the effects of the weir stiffness on the unstable vibration of the flexible weir due to fluid discharge were clarified and the effectiveness of some of the vibrational mitigation methods were confirmed