Self-Excited Vibration of a Flexible Shell Caused by Fluid Discharge

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A new type of self-excited osillation due to the fluid discharge was observed during test operations of the Super Phenix LMFBR in France. Two types of instability modes were observed : one is the sloshing mode the meaning of which is the oscillation of a weir associated with both feeding and restitution collectors ; the other is hydroelastic mode the meaning of which is the osillation of a weir associated with fluid shell modes. In this investigation, the excitation mechanism of both modes is discussed both theoretically and experimentally. In the experiments, rectangular and annular tank were designed and manufactured to investigate the effect of fall hight and supply flow rate on the instability. Both sloshing and hydroelastic modes were observed in a rectangular tank. On the other band, only asloshing mode instability was observed in an annular tank. To establish the theory which can explain the mechanism of this instability, theoretical model was proposed. In this theory, basic equations are composed of the momentum equation of the downstream tank sloshing and conservation of supply flow rate. To formulate the fluid force colliding with free surface, momentum theory was employed. Finally, we succeeded in deriving the simplified governing equation of this instability. Theoretical results show the following ; (l)the frequency of this unstable oscillation is almost equal to the first natural frequency of the downstream tank sloshing, (2)unstable vibration can occur at the range of certain fall height, (3)the range of which becomes narrow with increasing supply flow rate. These theoretical results agree well with experimental results and the validity of the theory was confirmed.