Model Test And Numerical Simulation About Flexible Pipe Systems Subject To Vortex-Induced Vibration

As a complicated synchronization phenomenon of Fluid-Structure Interaction (FSI), Vortex-Induced Vibration (VIV) which is related mainly to flexible cylindrical structures acts as the dominant fatigue damage to risers systems widely used in offshore engineering. Although there are many research works have been done concerning model tests and numerical simulation, the aspects related to the efficient method to combine them together and an instantaneous frequency-energy analysis method for oscillation mode identification are still left unsolved. For the sake of actualizing the numerical application of CFD method to VIV study, a sysmatic research related to VIV have been carried out, including the innovation of data analysis method, the interpretation between vortex sheding pattern and pipe oscillation. The model tests and numerical simulations related to single pipe and multi-assembled pipe systems subject to VIV and other hydrodynamic loads has been carried out, in order to provide meaningful reference to optimal design of offshore riser systems and accurate assessment for fatigue damage. This paper is a systematic report about vortex-induced vibration, including the innovation for data analysis method, the combination of model tes and numerical simulation.Firstly, a signal decomposition method based on FFT band-pass filtering technology has been introduced to analyze nonstationary vortex-induced vibration data. In this newly developed signal decomposition method, some critical innovations, that is, a"modeless component"decomposition criteria and a"narrow band"redefinition have been adapt to make this method out for more complicated nonstationary VIV signals. By the way, it has obtained great success when appling it to stationary random wave theory in my previous master degree thesis. By means of those two applicable improvements, the energy dissipation resulting from nonstationary and the disability of instantaneous frequency-energy spectrum has been well conquerd. And then 3D instantaneous frequency-energy Hilbert spectrum based on the improved decomposition method has been developed and applied to instantaneous oscillation modal analysis.Secondly, the model tests and numerical simulation tasks works as supplementary to each other. The former provided the orientation and instrument for the setup of the latter, and acted as reliable criteria for numerical simulation result estimation; in turn, the well-designed post-test numerical simulation offered the former supplementary vortex shedding visualization behind the pipe's wake and the corresponding instantaneous displacement information; taking advantage of the combination between pipe oscillatiom from model tests and vortex shedding pattern from numerical simulation, some coupled phenomena about fluid-structure interaction has been well comprehended.Finally, a series of pre-test numerical simulation tasks aiming at solving more complicated problems have been accomplished. The tentative pre-test simulation tasks mentioned above which have a tight relation with practical engineering application have been carried out, including the self-excited vibration simulation task related to a large-scale pipe system exposed to external shear current and internal flow simultaneously, the forced vibration simulation task related to a short pipe system subject to the coupled dynamic load resulting from wave and current and the dual-assembled pipe systems undergoing. Based on the well-performed simulation tasks, not only some conclusions consistent with the published research works, but also some new discoveries have been observed. The tentative pre-test numerical tasks accomplished in this paper will be an available orientation and reference to the further model tests and even to the practical engineering application.The innovations in this thesis can be concluded as follows: the improvement in data analysis method has provided a powerful tool to extract efficient information in both model tests and numerical simulation tasks; many important conclusions obtained from model tests and numerical simulation tasks are based on them; taking advantage of the combination between model tests and numerical simulation tasks, some problems related to the correlation between vibration mode and amplitude, the vortex shedding modes related to reduced velocity and instantaneous amplitude have been well solved. In addition, the intrinsic principle related to the instantaneous mode jump observed in model test has been concluded, as well as the rules about the coupled oscillation in the multi-assembled pipe systems and the vortex shedding pattern corresponding to different VIV styles.