Study On Hydrodynamic Characteristics Of Small Scale Vertical Cylinders Under Internal Solitary Wave

There is a rich storage of resources in South China Sea, and the government pays more and more attention to the exploration and development of resources there. The natural conditions are rough and many internal waves are observed in South Sea. Thus, it is very important to study the damage to ocean engineering induced by internal wave.By means of analyzing the observational data, physical oceanography scientists have studied the generation, propagation and evolution mechanism of internal wave and known much about the distribution of internal wave in the global ocean. The ocean engineering experts have realized that the great destruction may be caused by internal wave on ocean engineering. However, the experts don’t consider the vertical distribution of internal wave load when calculating it, and the hydrodynamic coefficients are selected depending on the data of surface wave and current. They may caused much calculation error. Thus, a series of experiments and numerical simulation are conducted with the aim of researching on the vertical distribution characteristics of internal wave load and the hydrodynamic coefficients in internal wave induced flow field. Based on the results, the dynamic response of top tension riser under internal wave is studied.The experiments are firstly conducted to study the internal solitary wave induced flow field and internal wave loads on vertical cylinders in different depth. A two-immiscible-fluid system is prepared in the internal wave tank. Gravitational collapse is adopted to generate internal solitary waves. Particle Image Velocimetry (PIV) is used to monitor the interface of internal solitary waves and record the time-history curves of horizontal velocities and accelerations in the whole depth. Sex separated cylinders are vertically arranged in different depth, and shear force sensors are used to measure the horizontal forces on each cylinder when internal solitary passes them by. Based on the obtained flow field data and horizontal force of each cylinder, the hydrodynamic coefficients can be calculated by means of least square method. And the selecting method of these two coefficients, drag coefficient and inertia coefficient, are explored tentatively.Computational fluid mechanics (CFD) is a new numerical method to simulate internal wave, and it can overcome some pitfalls in experiments. A three-dimensional internal wave tank is established using FLUENT software, and the simulations about internal solitary wave induced flow field and horizontal forces on cylinders in different depth are conducted. The correctness and stability of simulating results are validated by means of comparing them to the experiment data. The hydrodynamic coefficients are obtained based on the simulation data of flow field and horizontal force on cylinders. A series of conditions are conducted by changing internal solitary wave heights and cylinder diameters, and the relationship of hydrodynamic coefficients and KC number is studied. The research results provide a reference for selecting coefficients when calculating the internal solitary wave force on cylinders.For the small scale structures, Morison formula is a dominant method to calculate wave force. On the basis of the above researching results, this paper builds a theoretical model which is used to calculate internal solitary wave force on vertical cylindrical structure. This model is accurate which is proved by comparing with the experiment data. The time history and vertical distribution of internal solitary wave force are exhibited. The polarity character of internal wave load and the influence of wave height are investigated.The purpose of studying internal solitary wave load is to calculate the dynamic response of ocean structures under internal solitons. Top tensioned riser (TTR) is a kind of common used flexible structure in ocean engineering. The inline governing equation of TTR is established based on work-energy principles, in consideration of internal solitary wave force and flowing fluid inside the pipe. In order to solve the coupled differential equation in time domain iteratively, a Matlab program is compiled. In this way, the stress and displacement on any position of TTR can be get. Based on the observation of internal solitary wave in South China Sea, the response of TTR under the soliton is simulated. The results show that internal soliton will threaten the safety of working TTR seriously, and the wave height, top tension, internal fluid density and velocity will exert an influence.