EFFECT OF FREE SURFACE BOUNDARY AND WALL FLEXIBILITY IN SEISMIC DESIGN OF LIQUID STORAGE STEEL TANKS

Abstract

Fluid Structure Interaction (FSI) itself is a vast and extensive discipline. It originated from studies of aero and hydro-elasticity, which are often related to aeronautics and aerospace as well as nuclear industries. In practice, within the scope of nuclear, civil, aerospace, ocean, chemical and mechanical engineering, there are many terminologies involved, ie., flow induced vibration, aero-elasticity, hydro-elasticity, fluid structure interaction and fluid solid interaction. Typical problems include structure interaction with surface and sound waves and vibrations and stabilities of cables, pipes, plates and shells. In this paper, the effect of fluid structure interaction on the modal characteristics of a cylindrical steel water tank with and without free surface effect is considered. Acoustic structure interaction using unsymmetric pressure based formulation is used to solve the coupled system using FEM and the procedure is validated using results from published literature. Two tank models (shallow and tall) are modeled using ANSYS and modal analysis was done by considering different conditions like with slosh and without slosh. The effect of fluid mass on the convective and impulsive modes of tall and shallow aspect ratio tanks is shown. Parametric study is done for different fluid levels to characterize the variation of slosh frequencies in both rigid and flexible wall conditions. Free surface is considered in fluid alone model to predict the slosh frequencies employing rigid wall boundary. Then slosh frequencies got from both rigid and flexible wall conditions are compared with design data frequency tabulated from the GSDMA Guidelines. From this we can say that the flexibility of tank wall has a greater effect on the slosh frequencies.