Self-excited oscillations of the structure coupled with inner fluid occurred in the insulation wall cooling system of the fast breeder reactor Super-Phenix-1 (SPX-1) in France during test operations. In this system, lower temperature sodium going up along the inside of the main vessel in a feeding plenum brims over a cylindrical flexible weir shell and flows into a restitution plenum. It is reported that the self-excited oscillations were found to occur at the same natural frequency of the sloshing in the restitution plenum, the free surface of the fluid in both feeding and restitution plenums oscillated at large amplitudes, and the cylindrical shell was found to oscillate with an oval vibration mode instability. Such an instability is called a coupled sloshing mode. In this study, the authors analyzed and performed experiments on the instability of a multiple cylindrical structure system that has dual flexible weir shells. They measured self-excited oscillations changing various parameters such as discharge flow rate, difference of the liquid heights between feeding and restitution plenums, and so on. In the experiment, it was found that dual cylindrical shells oscillate in and out of phase with each other with an oval vibration mode under the specific conditions. Comparison between theoretically and experimentally obtained instability regions was made showing good agreement.
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