Numerical study about the dynamic effect of internal progressive flow on a flexible riser model deformed by external shear current has been carried out. The CAE technology which combines ANSYS structural analysis software with a CFD program (CFX) has been proposed to solve this FSI problem. It has been found that the existence of internal flow does play an important role in determining the vibration mode, vibration intensity and the magnitude of instantaneous vibration amplitude of the curved riser model, when velocity ratio of internal flow against external current is relatively high.


In the field of ocean engineering, practical significance of vortex-induced vibration has led to a large number of fundamental studies. There are so many works carried out for VIV research relating flexible riser systems subject to various kinds of external current, e.g. uniform, shear or even nonlinear-patterned, and a great deal of work has been done on the dynamics of straight pipes conveying fluid over the past 60 years or so. However, relatively less effort has been directed towards the investigation of the fluid-structure interaction (FSI) dynamics and stability of fluid-conveying curved pipes or risers. In fact, flexible riser or piping may be curved into complex spatial forms, when they are imposed to rough ocean environment (Païdoussis, 1998). As it is well known, the curvature of flexible pipes/risers and the relative motion of axial internal flow to the time-dependent pipe/riser motion are crucial factors to the safety design of marine riser structure. According to dynamics principle, the internal progressive flow experiences centrifugal and Coriolis accelerations, when flow progresses along the curved path of deflected pipe/riser. Those accelerations would affect the dynamic behavior of marine pipe/riser structures in turn. It may cause the happening of multi-modal coexistence or even the emergence of vibration modal transition subsequently.

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