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    Numerical Wave Simulation Using Geometrical VOF Method Based on OpenFOAM
    TIAN Kang, ZHANG Yao, LI Jinlong, ZHANG Xinshu, YOU Yunxiang
    Journal of Shanghai Jiao Tong University    2021, 55 (1): 1-10.   DOI: 10.16183/j.cnki.jsjtu.2020.99.015
    Abstract1608)   HTML441)    PDF(pc) (2416KB)(691)       Save

    Numerical wave simulation is a significant research topic. In this paper, the open source computational fluid dynamics (CFD) platform, OpenFOAM, is utilized to simulate Stokes fifth-order waves. Since geometrical volume-of-fluid (VOF) could better capture free surface due to its geometrical reconstruction step, the free surface simulations are accomplished by applying OpenFOAM built-in geometrical VOF method-isoAdvector, and the relaxation zone scheme is introduced through secondary development for wave absorption. The mesh density and Courant number convergence analyses with geometrical VOF are conducted. The simulation shows that satisfactory results could be obtained with a large Courant number. The algebraic and geometrical VOF simulated data with respect to wave elevation and phase at varied wave steepnesses and frequencies are recorded and compared with the theoretical value of Stokes fifth-order waves, which demonstrates that geometrical VOF is better than algebraic VOF in the prediction of wave elevation. Finally, the lengths and weights of the wave absorption zone are discussed, and the results imply that the best practice for the wave absorption is assigning the wave absorption zone length at least two times of the wave length along with applying exponential weight distribution.

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    Multi-Objective Optimization of Three-Column Semi-Submersible Platforms Based on Surrogate Models
    QIU Wenzhen, SONG Xingyu, ZHANG Xinshu
    Journal of Shanghai Jiao Tong University    2021, 55 (1): 11-20.   DOI: 10.16183/j.cnki.jsjtu.2019.087
    Abstract922)   HTML8)    PDF(pc) (5962KB)(481)       Save

    In the initial design stage of a semi-submersible platform, the main particulars of the platform are the key factor affecting the hydrodynamic performance and construction cost. Therefore, multi-objective optimization of the main particulars of the semi-submersible platform is of great engineering significance. First, the design variables of each platform and sample database are determined by design of experiments. Then, the hydrodynamic performances of the semi-submersible platform are analyzed by using the panel method and Morison’s equation. The distribution of probes for estimating the wave elevations on the calm water surface is arranged, and the airgap can be computed. Based on the database obtained by numerical simulation, the surrogate models based on radial basis function (RBF) are established. Next, the formal parameters in RBF are obtained by using the leave-one-out cross validation method. The surrogate model can greatly improve the optimization efficiency. Finally, by using the multi-objective particle swarm optimization (MOPSO) method, taking safety and economy of offshore platforms as two optimization objectives, and taking platform stability, airgap and horizontal motion performance as constraints, the optimization program for the semi-submersible platform can be obtained. Through the detailed analyses of the optimization program for the semi-submersible platform, the most efficient design strategy for the three-column semi-submersible platform is proposed.

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    Coupled Dynamic Response on a 6 MW Spar-Type Floating Offshore Wind Turbine Under Extreme Conditions
    YANG Jie, HE Yanping, MENG Long, ZHAO Yongsheng, WU Haoyu
    Journal of Shanghai Jiao Tong University    2021, 55 (1): 21-31.   DOI: 10.16183/j.cnki.jsjtu.2019.140
    Abstract1021)   HTML10)    PDF(pc) (3907KB)(487)       Save

    To realize the practical scale application of the spar-type floating offshore wind turbine (FOWT) in the medium depth sea areas, a novel 6 MW spar-type floating offshore wind turbine is analyzed by model test and numerical simulation under extreme conditions. The response of main freedom degrees, the mooring tense and the stress at the danger point are explored by a 1∶65.3 scale model at the State Key Laboratory of Ocean Engineering in Shanghai JiaoTong University. Coupled motion response of the spar-type floating wind turbine is calculated by using numerical simulation software in time domain. The results of the numerical simulation and model test are compared and analyzed in time and frequency domain. The maximum deviation between numerical simulation and model test is less than 12%, which shows that the numerical simulation results are in good agreement with the model test results. The dynamic response energy of the FOWT is mainly concentrated at low frequency and wave frequency. Moreover, the whole FOWT system has an excellent survivability under extreme conditions. Finally, the ultimate load of the wind turbine is predicted, which provides the necessary theoretical basis and calculation parameters for the structural strength calculation.

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    Envelope Features of Response Amplitude of Hanging Underwater Slender Bodies in Upper End Horizontal Periodical Motion
    CONG Shanxue, XU Xuesong
    Journal of Shanghai Jiao Tong University    2021, 55 (1): 32-39.   DOI: 10.16183/j.cnki.jsjtu.2019.225
    Abstract645)   HTML0)    PDF(pc) (3237KB)(250)       Save

    To improve the safety and operating efficiency of underwater slender bodies, the hanging underwater slender body is divided into several micro segments for analysis. The equilibrium equations of each segment are listed according to the mechanical equilibrium and deformation coordination conditions. Then, the equilibrium equations are solved with MATLAB programming. When the upper end moves horizontally and periodically, the upper end motion amplitude, upper end motion period, and lower end weight are changed separately to obtain the amplitude envelope of hanging underwater slender bodies. Based on the results of numerical calculations, the response envelope characteristics of hanging underwater slender bodies are analyzed. The maximum amplitude point on the amplitude envelope of the slender body is generally at the upper end. As the parameters change, it can be converted to the lower end. Changing the amplitude and period of upper end motion has a greater impact on the minimum amplitude point. The lower end weight has a small influence on the minimum amplitude point. By adjusting the ranges of these parameters, the positions of maximum amplitude point and minimum amplitude point on the amplitude envelope of slender body can be controlled.

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    Modeling and Sliding Mode Control for Chaotic Yawing Phenomenon of Large Oil Tanker
    ZHANG Xianku, HAN Xu
    Journal of Shanghai Jiao Tong University    2021, 55 (1): 40-47.   DOI: 10.16183/j.cnki.jsjtu.2019.104
    Abstract736)   HTML2)    PDF(pc) (1255KB)(685)       Save

    In order to explain and control the unexpected yawing phenomenon of large oil tankers, a pilot model is used to replace the original proportional model and is combined with the nonlinear ship responding model to construct a model of the whole closed-loop maneuvering system, which is found to be similar to the chaotic Duffing equation, and to be able to have a positive Lyapunov exponent after parameter adjustment, indicating that the chaotic theory can be used to explain this unexpected yawing phenomenon. In order to realize course keeping control with robustness to parameter uncertainty, based on the model built and the backstepping method, a sliding mode control scheme is proposed. The simulation illustrates that the static state rudder angle is smaller than 5° and course deviation is smaller than 0.07° when the chaotic yawing is at the theoretical maximum. Chaotic yawing is eliminated. The idea of establishing man-in-the-loop chaotic system is novel, and the method of solving backstepping parameter uncertainty through sliding mode is easy and effective.

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