A large cylindrical water storage tank typically has a thin sidewall. When such a tank is under an earthquake, the vibrations of the water inside are coupled with the vibrations of the sidewall, producing a phenomenon called fluid-structure coupled vibration. The fluid-structure coupled vibration is an important issue for a tank like this to achieve reasonable seismic-proof design. Even though there have been many studies on fluid-structure coupled vibrations, only a few of them have examined the dynamic fluid pressure and oval vibrations. This paper reports on the investigations into the characteristics of oval vibrations exhibited by a cylindrical water storage tank, in which a vibration test was conducted using a shaking table, the correlation of changes in the excitation force and behaviors of dynamic fluid pressure with the appearance and growth of oval vibrations were analyzed, and the modes of oval vibrations that appeared were identified. The vibration test was conducted using a scale model tank of a large cylindrical water storage tank and a shaking table. The input vibrations were sinusoidal waves of 53 Hz, a frequency that was in the vicinity of the resonance frequency. The test took the form of a large amplitude excitation test, which increased the acceleration of the input vibrations gradually. The response acceleration of the tank and the dynamic fluid pressure were measured. Strain gages attached around the trunk of the tank were used to identify oval vibration modes. The frequency analysis of the dynamic fluid pressure revealed two major peaks, one at 53 Hz which matched the excitation frequency and the other at 106 Hz which was double the excitation frequency. It showed that the dynamic fluid pressure has nonlinear behavior like higher-harmonic resonance. The frequency analysis of the responses on the trunk of the tank arising from oval vibrations also revealed two major peaks, one at 53Hz and the other at 106Hz. The behavior of dynamic fluid pressure and the behavior of oval vibrations were coupled. It was found that a certain magnitude of the response acceleration of the tank that gave rise to oval vibrations were in proportion to the rate of increase of the response acceleration of the tank. In other words, oval vibrations appeared at a relatively low response acceleration if the response acceleration increased slowly, whereas oval vibrations appeared only at a relatively high response acceleration if the response acceleration increased quickly. An analysis of the circumferential distribution of circumferential strains around the trunk of the tank revealed the presence of two oval vibration modes with different circumferential wave numbers: 14 and 16, which have not been predicted by the FEM analysis. None of the natural frequencies determined by the FEM analysis of the two different vibration modes matched 106 Hz; however, a half of the sum of the two natural frequencies was close to 106 Hz. Thus oval vibrations were found to have a nonlinear characteristics experimentally.
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