|Alternative Title||The analysis of impact of evanescent waves on hydrodynamic interactions of truncated cylinders|
|Place of Conferral||北京|
|Keyword||浮式圆柱 相对运动 非传播模态 水动力特性|
The effective exploitation and utilization of marine resources has greatly promoted the development and progress of society. Nowadays, due to the oil and gas resources are decreasing day by day, the development of oil and gas resources are gradually turning from the land to to ocean areas. In order to seek resources independence, resources development from the shallow sea area to the deep-sea area is the inevitable trend. Up to now, the exploitation technology of marine resources is developing rapidly and various large marine floating structures have emerged. The common marine floating structures include the following types: deep-sea platforms for exploring marine oil and gas resources, artificial islands, offshore airports and very large floating structures (VLFS) that utilize sea space effectively, the wave energy generator that uses wave energy and tidal energy to generate electricity.
Most of these marine floating structures can be simplified as floating column structures. The study of the hydrodynamic interaction between the column structures and the wave laid a theoretical foundation for the structural design, strength checking and fatigue analysis for marine floating structures. The key point for forecasting the performance of the floating column structures lies in studying the interaction between columns structures. The interaction mainly includes two aspects, propagating modes and evanescent modes. In general, the influence of the evanescent modes is not considered, but when the number of floating columns is larger and the column space is smaller, the influence of the evanescent modes on the hydrodynamic characteristics is more and more obvious. Besides, for different marine floating structures, the floating columns can be divided into "relative motion between columns" and "no relative motion between columns" due to the differences in the form of connection between structures. For theses structures like oil drilling deep-sea platforms, the floating body and the platform are rigid connections, so the floating column group calculation model can be seen as a whole. And the relative motion between columns is not considered when conducting hydrodynamic analysis. However, for the floating structures with no connection or weak connection, such as wave energy generating device and very large floating body, the hydrodynamic analysis model is mostly simplified as a cylindrical group model with relative motion between columns.
This thesis focuses on the relative motion between the columns, analyzing deeply the influence of evanescent modes on hydrodynamic characteristics and probing into whether taking the existence of evanescent mode under a certain condition into consideration or not. Firstly, the potential flow theory is used to simulate the wave field, and the nonlinear boundary condition and the Laplace control equation are expanded by Taylor expansion and perturbation expansion, and then we could get different order equations. In order to solve the problem of first-order linear water wave, the eigenfunction expansion method is applied to deduce the existence of hydrodynamic interaction between columns with relative motions. Secondly, the corresponding Fortran program was developed according to the theory, and comparing with the examples in the relevant literature to verify the correctness of the theoretical derivation and self-programming. Thirdly, according to the calculation results of the Fortran program under different working conditions, the influence degree and rule of evanescent modes are analyzed in detail. Based on the floating column group containing two columns of heaving motion, the influence mechanism of local mode on hydrodynamic characteristics is analyzed, and the attenuation law of hydrodynamic characteristics with increasing column spacing is discussed. According to the analysis results, the fitting formula of hydrodynamic characteristics which caused by evanescent modes in diffraction is given, which decreases with spacing increasing and is suitable for different working conditions. Finally, in the condition of the water depth is ten times radius of cylinder, the fitting formula is optimized and then extended it to the floating columns of rectangular arrangement.
Through the above research, we found that: The additional mass is sensitive to the presence of evanescent modes between the columns, however, the damping coefficient is not affected by the evanescent modes; The additional mass coefficient is particularly sensitive to the two kind of evanescent modes (the evanescent modes in radiation velocity potential and diffraction velocity potential), and the difference of additional mass which caused by the presence of evanescent modes is mainly caused by the local mode itself; Compared with the diffraction velocity potential, the evanescent modes in the radiation velocity potential have greater influence on the hydrodynamic characteristics of adjacent columns. Moreover, when the wave number is larger, the additional mass caused by the evanescent modes of the column is constant and no longer change with the wave number; As the column spacing decreases or the number of columns increases, the influence of evanescent modes on the additional mass will gradually increase, but the rate of change will be slower and slower, so the influence of evanescent modes between columns must be considered in small spacing or large number of column groups; With the increase of column spacing, the evanescent modes rapidly attenuates, and the additional mass caused by evanescent modes between columns attenuates rapidly in exponential form.
|孙哲. 圆柱群水动力辐射绕射中非传播模态影响规律研究[D]. 北京. 中国科学院大学,2018.|
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