The Study On The Features And Evolution Of Crescent Waves

Crescent waves are three-dimensional nonlinear waves composed of crescent-shaped symmetric waves. Their wave heights often reach the limit of wave height and form a spilling breaker, and can produce large strength action on ships and offshore plat forms. The study on the crescent waves is not developed extensively own to the rare data in remote observations of the actual wave fields. The investigation on crescent waves still lies in its early stage so far. The evolution and effect factors of crescent waves are still unclear. From the ocean science point of view the three-dimensional patterns are very important, since they modify the airflow above the surface and consequently affect the air-sea momentum transfer, while also changing in specific way radar scattering from the sea surface. Thus, it is neeessary to investigate this problem thoroughly. This thesis will conduct experimental study on the development and evolution of crescent waves and its impact on the cylinder. The numerical study using high nonlinear Boussinesq equation on crescent waves is also investigated.(1) Experiments are conducted in three-dimensional wave basin. The effect of different relative water depths on the wave pattern, the horizontal and vertical geometrical parameters and the amplitude spectra of crescent waves is studied. Crescetn wave pattern on large domain is developed through inducing perturbation waves. The distances from the wave maker over which the crescent waves get well-developed, the evolution of amplitude spectra and component waves of crescent waves are discussed detailedly for different relative water depths. A series of sensitive experiments are conducted to investigate the critical condition for triggering crescent waves for different relative water depths.(2) The effect of water depth on interaction of Benjamin&Feir instability (the class Ⅰ instability) with crescent waves (corresponding to the class Ⅱ instability) is discussed. The amplitude spectra of crescent waves are computed from the time series of surface elevation by the Fast Fourier Transform. The evolution of class I and class II instabilities and their effect on the evolution of crescent waves are analyzed. From the amplitude spectra of waves it is condirmed that Benjamin&Feir instability grows as crescent waves develop. The fast growth of Benjamin&Feir instability at the end area of the basin may be the reason of the disappearance of crescent wave at this area.(4) A new method of developing osscilating crescent waves in laboratory is given. The oscillating crescent waves are successfully developed through introducing perturbation waves. The oscillating period is measured and the effect of different relatively water depths is studied. The crescent wave patten is discussed by introducing other types of perturbation waves. (4) Experiments of crescent waves impacting on cylinder are conducted. The nonlinear character of point pressure is analyzed and the difference of point presure between Stokes waves and crescent waves is found out.(5) Crescent waves are simulated numerically in relatively shallow water with a two-dimensional model. In the earlier study, three-dimensional model is adopted to simulate crescent waves limited to relatively small domains and short time scale. In order to overcome this limit, the BouN2D4model which has dispersion accurate to Pade[4,4] and is fully nonlinear up to O (μ=kh and ε-A/h are two parameters denoting dispersion and nonlinear properties, and k, h and A present wave number, characteristic water depth and characteristic wave amplitude respectively) is used to simulate crescent waves in relatively shallow water. The results show that there is a good comparision between numerical simulation and experiment in water depths h=0.2m,0.3m and0.4m.