| In recent years,maritime safety accidents caused by extreme waves have attracted more and more attention to this type of waves that are more destructive in the marine environment.Further research on the formation mechanism and propagation characteristics of extreme waves can better guide the structural design of offshore engineering structures and ensure the safety of the structures.Actually,the formation of extreme waves mainly comes from the simultaneous focusing of wave group energy in the ocean,it is a useful method to analyze the properties of extreme waves by using wave focusing in the experimental environment.As a main component of marine engineering structures,horizontal cylindrical structures are subject to the nonlinear effects of waves when they encounter extreme waves,and more in-depth research is required on the horizontal cylindrical hydrodynamic characteristics under extreme waves.The main research contents of this article are as follows:(1)The research the nonlinear characteristics of focused wave is carried out in an experimental way.The two-phase separation and four-phase separation methods are used to obtain the higher harmonic elevation of the focused wave.The results show that the four-phase separation method has a better separation effect because it can separately extract the first four harmonics of the wave.Based on this,the nonlinear analysis of the wave elevation and velocity of the focused wave is carried out,and it is found that when the wave is focused,its harmonics are also focused at the same time,the wave elevation and energy of higher-order harmonics are smaller than that of the overall wave.The Stokes wave theory can describe the variation characteristics of second-order harmonics with wave number to a certain extent,but for third-order harmonics,it cannot capture the third-order wave interaction occurring during the propagation process.Due to the influence of the wave-wave interaction,the focused wave water particle velocity is lower than the linear theoretical value.(2)The research of evolution and energy characteristics of the harmonic of the focused are carried out.The study shows that the wave group is affected by the nonpropagation mode within a short distance from the wavemaker and the amplitude and energy of high-order harmonics vary widely,and the performance is relatively stable with the increase of the distance.The energy exchange between the linear part and the higher-order harmonics mainly occurs before reaching the focus position.The total energy increases when the wave reaches the focus position.In addition,the linear theoretical solution combined with the coefficients of higher harmonics is used to reconstruct the focused wave,which is in good agreement with the experimental results.(3)A numerical wave flume is constructed based on FLUENT software,and the hydrodynamic effect of the focused wave on the horizontal submerged cylinder was analyzed.Then the characteristics of the wave elevation and flow field of the focused wave in the presence of a horizontal cylinder and the force changes of the horizontal cylinder under the action of different wave steepness are analyzed.The results have shown that: in the presence of a cylinder,the shape of the wave surface remains basically unchanged,and the phase shift of the higher-order harmonics occurs.The flow field around the cylinder generates main vortices with the arrival of the wave peak and then shedding;The four-phase separation method is effective for the decomposition of the horizontal and vertical forces of the cylinder.And it is found that the ratio of its higher-order force to the overall force increases with the increase of the wave steepness,The steepness of the wave has a greater effect on the vertical force of the cylinder than that in the horizontal direction. |
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