摘要
海浪的群性是海浪场的一种重要特性,它对海堤的损毁,港湾共震,浮体结构物的动力反应等有明显影响,因此,在海洋工程设计中需要进行波群对建筑物作用的试验,这就要求模拟波群。目前随机波群模拟的研究大多限于单向波,而实际的海浪是多方向的,所以对多向随机波群模拟的研究具有很重要的理论意义和工程应用价值。
论文首先介绍了研究波群的主要方法和目前常用的描述波浪群性的参数,由实测波浪资料,对描述群性的高度与长度特征参数之间的相关性进行了分析。同时应用基于小波变换所得小波波能过程定义了两个新的群性参数,并由数值模拟波浪和实测波浪资料对其与常用的波群参数进行对比分析,结果表明基于小波波能过程定义的群性参数是有效的,从而展示了小波变换用于在时频域上分析波群的能力。并详细探讨了波浪记录长度对群性参数稳定性的影响,分析结果表明波浪观测长度对于波群参数的影响较大,在考虑波浪群性的波浪模拟及分析时,建议模拟时间长度应在400～500个波以上。
根据渤海和珠江口两地的波浪观测数据,采用合理的无因次化方式,拟合得到一个适于风浪的波包谱经验公式。该公式同时考虑了群高与群长两方面的因素,并依据对实测波浪资料波群参数的分析结果给出了较合理的参数取值范围,只要给出波要素和波群的特征参数即可确定波包谱形,便于实际应用。
然后,以建议的经验波包谱作为靶谱,建立了单向不规则波群的数值模拟方法,模拟实例表明,只要给定波要素及描述波浪群性的高度特征参数和长度特征参数,即可通过波包谱方便地模拟出同时满足给定高度和长度两方面群性特征要求的不规则波群。在此基础上,建立了在水槽中指定位置处产生不规则波群的造波机控制信号的计算方法,在实验室成功地模拟了同时满足频谱及群性要求的波浪。并对实验模拟的不同群性的波浪在水槽传播过程中群性的变化进行了分析,结果表明波浪在传播过程中,由于谐波的调制作用,波浪的群性会趋于某一有限的范围,在进行考虑波浪群性的物理模型实验时,需在水槽中指定位置处产生满足模拟要求的波浪。
进一步,在单向不规则波群模拟研究的基础上,引进方向分布函数,建立了可有效地数值模拟多向不规则波群的方法。然后建立了能够应用于模拟多向不规则波群传播的数值计算模型的入射波浪边界条件,对多向不规则波群的传播进行了数值模拟和分析,可以在数值水池指定位置处产生满足给定群性要求的多向不规则波浪。在此基础上,建立了在物理水池中指定位置处产生多向不规则波群的造波机控制信号的计算方法,并开展了多向不规则波群的物理模拟实验,在实验水池中指定位置处成功地模拟了满足给定群性和方向分布要求的多向不规则波浪,同时分析了不同群性的多向不规则波浪在水池传播过程中群性的变化。
最后,由数值模拟的多向波群和实测波浪资料分析了波浪的群性对其特征波高和波高分布的影响,结果表明波浪特征波高随群高增大而增大,且最大波高受群高影响最大,但与群长无明显关系,并分析得到了威布尔分布的参数α、β与群高GFH的关系式。
Wave groupiness is an important characteristics of the ocean waves.It has obvious effects on the damage of the seawall,harbor resonance and the response of floating structures, etc. Hence, tests on the effects of wave groups on the structures have to be involved for the design of the ocean structures, which requires simulating wave groups. So far, the studies on the simulation of random wave groups are mostly confined to the unidirectional waves.However, the real ocean waves are multidirectional waves.So, the study on the simulation of multidirectional random wave groups has very important theoretical importance and application value for engineering.
At the beginning of this paper, the main approaches to research wave groups and the parameters commonly used to describe the wave groupiness degree are introduced. Correlation of the main characterisitic parameters to describe wave group height and group length is analyzed based on the field measured wave data. At the same time, two new factors to describe wave groupiness based on the wave energy history deduced by wavelet transform are defined.Through numerically simulated wave and the field wave data, their effectiveness is confirmed by the comparation with the commonly used wave groupiness parameters. Therefore, the ability of the wavelet transform to analyze wave groupiness in time-frequency domain is demonstrated.Further, the effects of the wave record length on the stability of the wave groupiness factors are discussed in detail.The analyzed results indicate that the wave record length has great effects on the wave groupiness parameters,and it is suggested that, for the simulation and analysis of waves considering wave groupiness,the number of waves during the simulation time length should be at least400to500.
An empirical wave envelope spectrum suitable for wind waves is obtained by fitting to the reasonablely normalized wave envelope spectra of the measured sea waves at the mouth of the Zhu Jiang River and in the Bohai Sea. In the proposed wave envelope spectrum,both the wave group height and group length are considered.In addition, more reasonable parameter selection principle is given according to the analysis results of the wave group chracterisitics of the field wave data. It can be determined provided that the wave parameters and wave groupiness factors are given. So, it is feasible for practical application.
Then, based on the improved empirical wave envelope spectrum, a method to simulate unidirectional irregular wave groups is developed.Simulation examples demonstrate that, irregular wave groups which includes not only its group height but also its length can be conveniently simulated by the empirical wave envelope spectrum,provided that the wave parameters and the parameters for the description of the wave group height and group length are given. Then, the calculation method of the wave maker signals for generating irregular wave groups at the specified position in the wave flume is proposed.Random waves containing expected wave groupiness and wave spectrum are satisfactorily simulated in laboratory wave flume. In addition, the variations of wave groupiness for the simulated waves with different wave groupiness during propagation in the wave flume are investigated.The analyzed results demonstrate that wave groupiness tends to be an limted range along with wave propogation due to the modulation of harmonic waves.It means that, for physical model tests considering wave groupiness, the waves with desired wave groupiness should be generated at the specified position in the wave flume
Further, based on the study of the simulation of unidirectional wave groups,the method to simulate multidirectional wave groups is developed by introducing directional spreading function. The calculation method for the incident boundary condition of the numerical wave basin used to generate multidirectional waves containing expected wave groupiness is proposed. Using the numerical model, the multidirectional wave propogation is numerically simulated and analyzed. Multidirectional wave groups can be generated at the specified position in the numerical wave basin. Then, the calculation method of the wave maker driving signals for generating multidirectional wave groups in physical wave basin is proposed.The physical simulation of multidirectional wave groups is performed in physical wave basin. The multidirectional waves with expected wave groupiness and directional spreading can be satisfactorily generated at the specified position in the physical wave basin. The variations of wave groupiness for the experimentally simulated waves during propogation in the physical wave basin are also analyzed.
Last, the effects of wave groupiness on the characteristic wave heights and wave height distribution are analyzed through numerically simulated multidirectional wave groups and the field wave data. The results show that the characteristic wave heights increase with the wave group height, moreover, the maximum wave height is effected mostly by the wave group height. But, they have little corelation with the wave group length.The relation between the parameters α、β of the Welbull distribution and wave group height is obtained.
引文
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