Study On Harbor Resonance Induced By Solitary Waves Or Wave Groups

Harbor resonance is a traditional and vitally important research orientation in the field of coastal engineering. The phenomenon has been observed in many harbors at home and abroad, which significantly disturbs the operational efficiency of the harbor and safety of the ship moored inside the harbor. At the beginning of this dissertation, the research background and significance of harbor resonance are introduced. A series of harbor oscillation accidents reported in previous literatures are listed. And the wave dynamic characteristics of these harbor resonance events are summarized. The previous study concerning this subject is systematically reviewed. Subsequently, the numerical model used in this dissertation, FUNWAVE 2.0, is introduced, which is based on a set of fully nonlinear Boussinesq equations. The control equations, the numerical discretization, boundary conditions and the wave maker theory are presented. And then the ability of the numerical model used for simulating the harbor resonance phenomenon is varified by reproducing laboratory experiments presented in the previous literatures. On these bases, the following problems are further investigated:In the paper by Sobey (Sobey,2006. Normal mode decomposition for identification of storm tide and tsunami hazard. Coastal Engineering 53,289-301), the author proposed a normal mode decomposition (NMD) method to calculate the eigenfrequencies, the eigenmodes and the response amplitudes of different resonant modes in natural harbors that are subjected to storm tides and tsunamis. However, the numerical method to address the fully-reflective boundary condition in that paper is imprecise, which would lead to inexact eigenfrequencies and eigenmodes. In this dissertation, the mirror-image method was proposed to improve the imprecise handling process. The accuracy of the improved NMD method was verified using three verification tests. For the NMD method, the calculation of the response amplitudes of different resonant modes induced by storm tides or tsunamis is based on the linear wave theory. Using two different numerical methods, the applicability of the NMD method is investigated when this method is used to decompose the response amplitudes of different modes under the weakly nonlinearity wave condition inside the harbor.Harbor oscillations inside a narrow long rectangular harbor is induced by solitary waves. Using the NMD method, the response amplitudes of different resonant modes are decomposed, and then the effects of different incident solitary wave heights and bottom slopes inside the harbor on the relative wave energy distributions are studied systematically. Computed results show that when the incident wave height is small, the resonant wave energy inside the harbor is dominated by the lowest few modes, and the higher modes only possess a very small proportion of the resonant energy; when the incident wave height increases, the relative energy distribution becomes uniform, and the proportion of energy in the higher modes increases. In addition, for the same incident wave height, the change of the bottom slope inside the harbor has a negligible effect on the relative energy distribution within the ranges of the variation in the incident wave heights and bottom slopes studied in this dissertation.The oscillation phenomena inside a narrow long rectangular harbor induced bybichromatic wave groups are simulated by the fully nonlinear Boussinesq model. A separation procedure which is based on the least square method is proposed to decompose the low-frequency components inside the harbor into bound and free long waves. Subsequently, we investigated how bound and free long waves and their relative components change with respect to short wavelengths. For comparison, the non-resonant wave condition is also considered. It shows that the amplitudes of bound and free long waves and their ratio are closely related to the short wavelengths, regardless of whether the harbor is resonant or not. For the given harbor and primary wave frequency ranges studied in this dissertation, when the harbor is at the lowest resonant mode, the amplitudes of bound long waves are always lower than those of free long waves but tend to be larger than half of the latter when the average short wavelengths are larger than 0.66 times the harbor length. When the harbor is non-resonant and the average short wavelengths are larger than 0.56 times the harbor length, the former is inclined to be larger than the latter.