On estimation method of diffraction forces on cylindrical structures in multi-directional waves

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Nonlinear wave diffraction due to cylindrical structures in a multidirectional irregular wave field and the wave forces acting on the cylinder are investigated experimentally and theoretically. An estimation technique for the diffraction forces on the cylinders is introduced in this work. To discuss the mechanism of both the wave field and its induced forces on the cylinders, extensive experiments have been conducted in case of a regular wave train and two-crossing waves. The followings can be concluded based on the project work.(1) The theory obtained by MacCamy and Fuchs gives reasonable estimate of the diffraction wave force on a cylinder in case of regular wave field. However, the theory is inadequate in estimating the diffraction wave force on a cylinder in unidirectional irregular wave. In the multidirectional wave field, the estimation of the diffraction wave forces by the theory has low accuracy.(2) The spreading parameter affects the diffraction wave force in general. The wave … More force in the principal direction does not always increase with increasing the spreading parameter. Moreover, it is found that the wave force on a cylinder constructed on a slope shows different characteristics from that in a constant water depth because of wave refraction.(3) Wave forces acting on a cylinder in an array of two cylinders are influenced by the spacing between the cylinders and the incident wave period rather than the spreading parameter in case of weakly directional wave field. As the directional spreading of the incident waves becomes appreciable, the effect of the interaction between diffracted waves from individual cylinders becomes less significant.(4) As the spacing between the cylinders in an array becomes smaller, the wave height in the area between the cylinders increases. This enlarges the wave pressure on the cylinder surface. The wave height outside the cylinder array seems to be independent of the cylinder's spacing, however, this depends on the directionality of incident waves such that it becomes larger when increasing the directional spreading. This may result in larger wave pressures on the cylinder surface in a multidirectional wave field.(5) Except for the impulsive wave breaking force, which is not discussed in this project, it can be judged that the effect of the directional spreading should be properly considered in estimating the design wave force on cylindrical siructures.(6) For more detailed discussions of the nonlinear wave diffraction and wave forces, the time domain analysis may be necessary. Moreover, the velocity field in a multidirectional wave should be investigated with respect to the wave height distribution. These deemed necessary for future investigations of this problem. Less