Study On The Interaction Of Multidirectional Irregular Wave With Array Of Large-scale Cylinders

Circular cylinders are often used in coastal and ocean engineering and many offshore structures are comprised of arrays of cylinders. In general, the large-scale of cylinder with the slenderness parameters D)/?=0.15(D is the diameter of a cylinder and ? is the wave length). For the purpose of developing marine resources, more and more offshore structures are built as the cylinder array, examples include cross-sea bridge and wind turbine foundations, offshore platforms and floating airports. Wave load is the main load on the offshore structure, accurate predictions the wave loads on an array of cylinders are significant in practice engineering design.However, most of the research associated with wave interaction with arrays of cylinders has been focused on unidirectional waves. But in reality, the sea waves are multidirectional, the distribution of the wave energy not only based on the frequency domain, but also is determined by the direction of the wave components. In the study of wave forecast, wave diffraction and refraction, sediment movement and the interaction of wave with structures, the effects of wave directional distribution should be considered. Therefore, the study on the interaction of multidirectional irregular waves with an array of large-scale cylinders has an important practical significance.Wave basin experiment is an important method to investigate the interaction of multidirectional waves with cylinders. At beginning of the thesis, the layout and process of an experiment about multidirectional wave interaction with cylinders are particular stated. Then the multidirectional wave loads on cylinders are studied by the experiment. The normal and transverse forces and the wave run-up are received by analyzing the time process of wave loads. For unidirectional wave, the wave pressure difference on two sizes of the cylinder equals to zero and the transverse force not exist on it. But for multidirectional waves, the incident waves are a group of waves with different direction, the wave energy distributes in a certain range, the wave crest is shorter and random, since the wave pressure difference on two sizes of the cylinder not equals to zero any more, and the transverse force emerged. The effect of the wave directionality is more obvious on the transverse force than on the normal force. The transverse force has a very obvious tendency to increase as directional distribution becomes wider. The wave directionality has a significant effect on the wave run-up around the cylinder. The maximum wave run-up occurs at the position a-0°, which is at the front of the cylinder. But the location of the minimum run-up depends on the directional spreading parameter s, for small values of s, the minimum is at 180° or 135°Moreover, in the following chapters, based on the linear theory of wave interaction with cylinders combined with the superposition method, a method to calculate the multidirectional wave loads on a cylinder or an array of cylinders is developed. The calculation results are verified by the results of physical experiment. For multidirectional random waves, compared with the result by unidirectional wave, the multidirectional wave height at parts of the wave field is 50% more than it. The wave directionality has a significant influence on the transverse force loads on cylinders. It is found that the biggest transverse force in tandem array occurs on the rear cylinder rather than the front one. This is quite different from the results in unidirectional waves and should be paid much more attention in the design of offshore structures. Meanwhile, multidirectional wave interacts with common arrays of cylinders are studied and the group coefficients for different layout are presented. In addition, the near-trapping not only occurs on regular wave condition but also on multiditectional wave condition, and the wave loading on the cylinders would be obviously increased.Further, in order to consider the effect of wave nonlinearity, a numerical wave basin based on OpenFOAM is used to simulate the interation of multirectional irregular wave with large-scale cylinder. Special to the numerical wave basin based on OpenFOAM to generate multi-directional wave which has the insufficient of large amount of calculation, to overcome this problem, the coupling method is used based on the potential theory and OpenFOAM, and the amount of calculation is reduced and the computational efficiency is improved. Later, the coupling modeling is used to simulate the interaction of multidirectional wave with a large-scale cylinder, the numerical results of wave forces and run-up load on the cylinder are more close to experimental results.