Numerical Simulation of the Water Impact on Cylindrical Shells Considering Fluid-structure Interaction

Abstract

The present paper proposes a numerical simulation method for analyzing water impact on elastic structures.In order to accurately evaluate the structural strength of marine structures under severe water impact, the characteristics associated with the dynamic structural response to an impact load, the magnitude of which changes rapidly, must be examined. Until now, a simplified two-step method has been widely used for this purpose. This method entails estimation of a time series of the hydrodynamic force on a structure, assuming that the body is rigid. This is followed by response analysis of the structure by applying the previously estimated force history. However, when the structure is relatively flexible, the two-step method may not be applicable because it neglects the coupling effect between the fluid and structure. Thus, a numerical simulation method that considers fluid-structure coupling is prepared. The new method consists of two components, i.e., a 2-D numerical simulation method that utilizes the computational fluid dynamics (CFD) for flow field analysis and a modal analysis technique for solving the dynamic structural response of the cylindrical shell in its transverse section. Information such as fluid pressure, motion and deformation of the structure, is exchanged between the two components at each time-step of the numerical simulation. A series of drop model experiments was also conducted to verify the accuracy of the newly developed numerical simulation method. The experiments consisted of dropping a cylindrical model made of an aluminium plate onto a still water surface. The motion of the model and strain on the shell plate were measured. Experimental data taken from a published report were also used for comparison.The following results were obtained : 1. In the case of a light-weight model, the rigid body motion of the dropped model changed remarkably upon impacting the water surface. This deceleration influenced the pressure field around the model remarkably as well as the elastic response of the model structure, i.e., strain-time history. The accuracy of the numerical simulation was improved substantially by considering the deceleration effect of the rigid body motion during water entry.2. Although the predicted elastic response by the two-step method agreed fairly well with measured response in the case of a cylinder with high rigidity, the method cannot predict the elastic response of a cylindrical shell with low rigidity. By carrying out the above mentioned coupling simulation, the accuracy of the computed elastic response becomes noticeably improved. Thus, the proposed simulation method appears to be effective.