Freak Wave Simulation And Its Interaction With Nuclear Power Station Ingarage Structure

Freak waves are events that feature unusually large and cause severe damage to mariners and offshore structures. In order to avoid their hazards, it is necessary to obtain much more data to explore their generation mechanisms and occurrence properties. However, the measured data of freak waves in situ are very rare so far because most of them take place under the unknown and unexpected conditions. Therefore, the experimental generation of these giant waves in laboratory seems to be particularly important.Up to now, the formation of freak waves based on the Longuet-Higgins wave model theory is offen used in laboratory. In order to overcome the short-comings of the existed methods, a new numerical method called new phase modulation method is developed. The new method can not only keep the statistical properties of the wave train and the structure of the target wave spectrum, but also keep the statistical properties of the simulated wave train identical with those of the realistic random sea state and generate freak waves at certain time and space with high efficiency. Based on the new method, factors influence freak waves characteristics and simulation efficiency are disscused. Results show the modulation way of high frequency to low frequency is superior to the way of low frequency to high frequency. Waves with high frequency are important factor on formation of freak waves. In the modulation way of high frequency to low frequency, freak wave height and its crest height, the ratio of the freak wave height to the significant wave height and the ratio of the crest height to the freak wave height and also the freak waves simulation efficiency are increased as the number of modulated waves and the spectrum band width increased. The spectrum peak periods (8 to 16s) adopted in the paper have little influence on the freak wave characteristics and simulation efficiency. Through numerical simulation, this paper makes recommendation values of cut frequency to be 3.5 to 4 times the spectrum peak frequency, and wave number components of 50～70 to simulate normal freak waves, wave number components of 70～100 to simulate high nonlinear freak waves. The wave number components and wave number to be modulated are selected according to a tentative simulation.Freak waves recorded in situ are simulated by the new method sucessfully. The simulation results satisfy the wave series statistics and keep identical with the target spectrums; effectiveness and applicability of the new method are validated. Comparing to the existing methods, the new approach can generate a high nonlinear freak wave numerically which height is 3.14 times the significant wave height and crest height is 0.76 times freak wave height. However, this high nonlinear freak wave can not be simulated in physical wave flume because of wave breaking in front of the focal point.Freak waves generation and development process are studied. Numerical and physical simulation results both show that less than half characteristic wavelength in front and back of the focal point there is a big trough (hole) in sea; and less than one characteristic wavelength in front and back of the focal point there is a freak wave occurred physically which height is more than 2.0 times the significant wave height and crest height is more than 0.65 times freak wave height. The above appearances are coincidence with eyewitness and researchers'results. Freak waves life and propagation distance are strongly related linearly. The ratio of life time to spectrum peak period is about two times the ratio of propagation distance to characteristic wavelength. The shortest and longest life time of freak wave are 2 and 10 times the spectrum peak period, respectively; The shortest and longest propagation distance of freak wave are 1 and 5 times the characteristic wavelength, respectively.Nuclear power station is usually located in coastal area. Freak waves may be occurred in front of ingarage structure because of variation of bathymetries and boundaries effects. As to strong destructiveness of freak waves, it is necessary to study the interactions between freak waves and ingarage structure. Freak waves near coastal sea were recorded in an experimental and this proves the existence of coastal freak waves. It is found that abnormal wave pressure on the ingarage structure towards to wave direction is caused by freak wave formed in front of the structure. The abnormal wave pressure caused by freak wave can be 2.28 times wave pressure caused by normal random wave, the total lateral force caused by the former is 2.51 times of the latter, so it is seen that freak wave is quite different from normal random waves and it may cause severe damage to the structure. If wave pressures on the structure are obtained without freak waves, it is suggested that the situation of freak wave existence be considered and wave pressures adopted shoud be enhanced to 2.5 times. As to the bottom pressures on ingarage structure, there is only a slight difference of maximum wave pressures and pressure distribution caused by freak wave and normal random wave.