WATER TOWERS UNDERGOING DYNAMIC ACTIONS: NUMERICAL SOLUTION OF COUPLED FSI PROBLEM AND APPLICATIONS

Abstract eng:
In the common practice seismic analysis of water tanks is based on the analytical solutions of free surface fluid undergoing sinusoidal actions. The identification of impulsive and convective contributions (frequencies, modal shapes, pressure distribution, etc.) allows the definition of equivalent mechanical models able to represent cisterns on flexible structures. This procedure, however, only applies to a few common shapes of tank and it is based on simplifying assumptions, such as ideal fluid and uncovered tank. In the present study a sound procedure is proposed with the aim of analyzing water towers undergoing dynamic actions inducing sloshing phenomena and their interaction with structural overcoming simplying assumptions. A weakly coupling a fluid and a structural solver is introduced: at each time step structure’s motion becomes the boundary conditions to be applied to the fluid and pressures obtained from this last give back forces to be applied to the structure together with other external actions. Fluid problem is solved thanks to an edgebased level set finite element method for free surface flows developed within the multi-physic code Kratos (CIMNE, Barcelona) while structure, composed by monodimensional linear finite elements, is solved by Newmark method on mass, damping and stiffness matrices. At this stage of development, rigid walls are assumed for the tank. Proposed approach was used to study the dynamic behavior of a 450 m3 capable cylindrical tank undergoing seismic action, both in the case of base on ground and at top of a concrete water tower. Sinusoidal base displacements and accelerograms were applied to the structure: results were compared with those from analytical solution and equivalent mechanical model. Results from different approaches show full agreement among them demonstrating the reliability of the proposed FSI strategy. Future studies will concern different and more complex shapes of tanks and also covered ones which may experiment uplift of the roof due to the slosh wave. Moreover, strong coupling will be implemented within Kratos code to consider also deformable tanks.

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