The liquid structure interaction, in particular the change in mode shapes and corresponding natural frequencies of an open cylindrical tank containing liquids with the level of liquid was studied by FEM and by experimental modal analysis (EMA). It was found that the level of liquid influences greatly the dynamic characteristics of the tank-fluid structure. In the case of water and motor oil, the natural frequencies corresponding to a particular mode shape decreases with level of liquid. The mode shapes themselves are also affected. Some mode shapes exist at all levels of liquid while some would be wiped out at certain level or a range of liquid levels. New mode shapes also emerge, even the original fundamental mode of the empty tank was replaced by one of lower frequency. Some of the new modes are entirely different from previous neighbouring modes but some share their neighbour's features. The FEM analysis shows that for thin wall structures containing liquids, the influence of the liquid level on the dynamic characteristics of the tank-fluid structure is very complex. Some of these features have been confirmed by experimental modal analysis using the impact hammer tests.
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