다자유도 세장체의 VIV해석

Abstract

The object of this study is to develop the tool to analyze VIV(vortex-induced vibration) of a slender marine structure which may vibrate in inline and cross-flow directions due to the vortex shedding on the condition of a constant flow speed. The three instability regions in inline direction is defined in the low reduced velocity, and the sinusoidal drag force with two or three times of strouhal frequency is induced in inline direction. But the variable drag force with two times of strouhal frequency applies for most current speeds in case studies because the frequency intervals between adjacent modes of the model in the case study are proportionately very small. On the other hand, the lift force in cross-flow direction based on the Morison equation has the strouhal frequency. Applying dynamic analysis method for a slender marine structure based on the 3D FDM(Finite Difference Method), case studies for a tensioned beam model are carried out under a constant flow speed. The eigenvalue analysis for a tensioned beam considering the effect of tension increase due to the inline and cross-flow drag force according to the flow speeds is performed. It is found that the revision of the eigenvalue due to the tension increase should be considered if a tension along a riser or beam is significantly affected with an environmental or functional load. From the snapshot on the XY plain it is believed that phase angles between inline and cross-flow displacements in high modes are different among the nodes. In this study, the developed numerical method, even the preliminary analysis method, can be a fairly good tool to predict inline and cross-flow vibrations of a slender marine structure with the proper hydrodynamic coefficients in initial design.