Investigation On Wave Loads And Run-up Of Cylindrical Structures Due To Freak Wave

Freak wave(also called rogue wave)is kind of extreme wave on the sea which is attracting people's attention recently.Freak wave can pose devastating damages on ships and offshore structures because of its extreme wave crest height,crest-trough asymmetry and suddenly occurring.The study on freak wave is not developed extensively due to low probability and rare field records of freak wave.The investigations of freak wave mainly focuses on the physical mechanisms and probability prediction.Considering its hazard on ships and offshore structures,it is necessary to investigate freak wave impact on structures.In this dissertation,theoretical analysis,numerical simulation and physical wave tank experiment are coupled together to investigate the interaction between freak wave and simple cylindrical structures.Firstly,based on computational fluid dynamics,numerical wave tank(NWT)is developed.Two types of wave-maker are applied including flap wave-maker and velocity inlet,and comparison of two types of wave-maker is conducted.In order to decrease wave reflection,source terms are added in the momentum equation.The method of mesh sensitivity and time-interval sensitivity are described to ensure the stability of numerical simulations.Under the assumption of random wave theory,the mathematical models of freak wave are introduced which include linear superposition wave model and nonlinear wave model.As to the linear superposition wave model,the influence of wave energy spectrum distribution is discussed.The 3rd order nonlinear Schr?dinger Equation and its Peregrine solution are elaborated.Velocity inlet method is applied to generate wave in two dimensional numerical wave tank.Vertical wave forces on horizontal cylinder due to regular waves are modelled,and the numerical results agree well with experimental results and other numerical data which prove that this numerical model is correct.Following,the hydrodynamics characteristics of focused waves are discussed.The horizontal velocity of regular wave and focused wave group are compared which shows that the horizontal velocity of focused wave is larger than that of regular wave above the static water surface.Vertical wave forces due to regular wave and focused wave are compared.It shows that the maximum value are at the same magnitude,while there is an obvious secondary load cycle for focused wave.A flap numerical wave tank is established.The recorded freak wave New Year Wave is numerically generated in this numerical wave tank coupling with phase optimization method.Horizontal cylinders with different submerging depths are investigated,and the flow field and wave forces are calculated.In order to investigate the wave run-up and wave forces on vertical cylinder due to freak waves,a three dimensional numerical wave tank is developed.Mesh and time-interval are analyzed to ensure the stability.The numerical results of wave forces are compared with the empirical Morison equation.It can be concluded that the Morison equation underestimates the maximum wave forces on vertical cylinder.FFT is utilized to analyzed the wave elevations around the vertical cylinder.It is found that the higher harmonic component is obvious in the wave elevations near the vertical cylinder.The minimum wave run-up is located in the direction of 45° angle with the direction of wave propagation.Freak wave is generated in a physical wave tank,and compared with CFD results.Particle Image Velocimetry is applied to reveal the detail information of freak wave.In a word,computational fluid dynamics is mainly utilized in this dissertation.Numerical wave tank is established.The recorded New Year Wave and defined freak wave sequences are numerically regenerated.Some work has bee conducted about the interaction between freak wave and simple cylindrical structures.The dissertation provides some useful guidelines for revealing freak wave impact on offshore structures.