A time domain model is presented to study the vibrations of long slender cylinders placed in shear flow. Long slender cylinders such as risers and tension legs are widely used in the field of ocean engineering. They are subjected to vortex-induced vibrations(VIV) when placed within a transverse incident flow. A three dimensional model coupled with wake oscillators is formulated to describe the response of the slender cylinder in cross-flow and in-line directions. The wake oscillators are distributed along the cylinder and the vortex-shedding frequency is derived from the local current velocity. A non-linear fiuid force model is accounted for the coupled effect between cross-flow and in-line vibrations. The comparisons with the published experimental data show that the dynamic features of VIV of long slender cylinder placed in shear flow can be obtained by the proposed model,such as the spanwise average displacement,vibration frequency,dominant mode and the combination of standing and traveling waves. The simulation in a uniform flow is also conducted and the result is compared with the case of nonuniform flow. It is concluded that the flow shear characteristic has significantly changed the cylinder vibration behavior.
In this paper,equations calculating lift force of a rigid circular cylinder at lock-in in uniform flow are deduced in detail. Besides, equations calculating the lift force on a long flexible circular cylinder at lock-in are deduced based on mode analysis of a multi-degree freedom system. The simplified forms of these equations are also given. Furthermore, an approximate method to predict the forces and response of rigid circular cylinders and long flexible circular cylinders at lock-in is introduced in the case of low mass-damping ratio. A method to eliminate one deficiency of these equations is introduced. Comparison with experimental results shows the effectiveness of this approximate method.
This paper investigates the effects of structureparameters on dynamic responses of submerged floatingtunnel (SFT) under hydrodynamic loads. The structureparameters includes buoyancy-weight ratio (BWR), stiffnesscoefficients of the cable systems, tunnel net buoyancy andtunnel length. First, the importance of structural damp inrelation to the dynamic responses of SFT is demonstratedand the mechanism of structural damp effect is discussed.Thereafter, the fundamental structure parameters are investigatedthrough the analysis of SFT dynamic responses underhydrodynamic loads. The results indicate that the BWR ofSFT is a key structure parameter. When BWR is 1.2, there isa remarkable trend change in the vertical dynamic responseof SFT under hydrodynamic loads. The results also indicatethat the ratio of the tunnel net buoyancy to the cable stiffnesscoefficient is not a characteristic factor affecting the dynamicresponses of SFT under hydrodynamic loads.
Xu Long Fei Ge Lei Wang Youshi Hong State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China
In virtue of reference Cartesian coordinates,geometrical relations of spatial curved structure are presented in orthogonal curvilinear coordinates.Dynamic equations for helical girder are derived by Hamilton principle.These equations indicate that four generalized displacements are coupled with each other.When spatial structure degener- ates into planar curvilinear structure,two generalized displacements in two perpendicular planes are coupled with each other.Dynamic equations for arbitrary curvilinear structure may be obtained by the method used in this paper.
