A Study On The Key Tecnologies Of Submerged Floatig Tunnel Under Current-Induced Vortex Effect

It is an important issue among the studies of submerged floating tunnel (SFT) for the vortex-induced vibration (VIV), the'lock-in'phenomenon and the fluid-structure interaction (FSI) of the submerged floating tunnel will occur when it undergoes the ocean current effect and the vortex-induced vibration will induce the submerged floating tunnel to become instable. Both vortex-induced transverse oscillation and in-line oscillation will occur when fluid flows around a structure. For the amplitude of transverse oscillation is larger than that in-line oscillation, in this dissertation, it focused on the dynamic characteristics and stability of submerged floating under transverse vortex-induced vibration and it carried out the main research work in the dissertation as followed:1 The dissertation premised that the amplitude of the oscillating SFT under vortex-induced effect was small, and derived the formula of fluid load based on the kinematics and dynamics characteristics of the vortex. It's easily to determine the fliud load exactly acted on the SFT when its vibration regular had been known. The dissertation made a summary of the mechanism of vortex-induced vibration, and computed the fluid force acted on the SFT by Morison equation.2 The dissertation simplified single-span SFT as an Euler-Bernoulli beam simply suppored at both ends. It used separation of variables to analyze the intrinsic characteristics of the SFT. The study showed that the additional quality coefficient is dominant among those factors impacted on the first-order natural frequency of SFT, the impact of the outside diameter of pipe sections followed by, and the impact of the current density is nearly the least important. It simplified the multi-span SFT supported by tension-leg as a beam supported by linear spring, and it used the Laplace transformation and Laplace inverse transformation to study the intrinsic characteristics. The investigation showed with the equivalent axial stiffness increasing the natural frequency of multi-span SFT declined, and the intrinsic frequency is inversely proportional to the equivalent axial stiffness. With the elastic modulus of the SFT increasing the natural frequency increases, not being a linear relationship as well as the outside diameter of the tube.3 The dissertation derived the stability criterion of single-span SFT under the vortex-induced vibration from using of the Melnikov method of Chaos Theory. And it analyzed the factors affected the stability. The analysis showed that:With the increasing in spanlength, the stability of the SFT goes down. The higher of the vibration order the greater the stability greater of SFT is, therefore it can only consider the first-order stability on the stability analysis. With the increase of current velocity, the stability of the SFT dropped. The stability of the SFT increased linearly with the tube's elastic modulus. It is an upward trend when the stability of the SFT affected by the material density of the SFT. The impact of the SFT's outside diameter on the stability is of spoon-shaped, the impact of fluid damping on the stability is of V-shaped, however the impact of the lift coefficient on the stability is a decline step function. In addition it established the vibration equation of the tension leg on condition that considered the vibration of the SFT as parameters incentive. It derived the stability criterion of the tension leg under vortex-induced vibration from utilizing the Lyapunov function. The investigation showed the stability of tension inceased with vibration order, structural rigidity, or the initial tension, but not being a linear relationship.4 The result of the vortex-induced vibration model test showed that with the increase of current velocity, the dynamic response goes up, and the maximum vibration displacement increases linearly with current velocity. Under the same flow field conditions, the increase in the number of support can reduce the force acted on the SFT and reduce dynamic response amplitude, but the response regularity will be complicated. The axial strain of tension leg will increase follwed by the current velocity. The dynamic response regulerity of two tubes is the same as the one of single-tube, and the single-tube SFT's response amplitude is larger than that of two tubes. Numerical simulation results showed that there are positive pressure zone in upstream surface area and negative pressure zone at the opposite point separately. The value of each area increased with the current velocity. Increasing the current velocity the separation point will prolong and the the vortex frequency will be greater. The wake area formed after the circular areas is the biggest, followed by elliptical cross section, the smallest of multilateral cross-section.5 This dissertation also studied the dynamic response under current effect based on considering the structural non-linearity, dynamic characteristics analysis, stability analysis and model test. The research showed:the first-order harmonic mode displacement is of maximum of the single-span SFT. Increasing the elastic modulus or the outside diameter of the SFT tube it will induce the dynamic response to decrease. With the span-length increasing, the dynamic response amplitude will rise, as well as the current velocity. With the axial stiffness of the tension leg increasing, the displacement response of multi-span SFT will go down.With the tension leg length increasing, the amplitude of displacement response will rise.The decrease of angle between the two tension legs contributed to curbing the displacement response amplitude more obvious when not considered the impact of non-linear factors, while when considered the impact of non-linear factors the smaller the angle between tension legs the greater amplitude of the dynamic response obtained.