On diffraction and radiation problem for two cylinders in water of finite depth

The hydrodynamic problem arising form the interaction of linear water waves with a wave energy device consisting of two coaxial vertical cylinders of different radii is investigated. One cylinder is riding in waves, while another is submerged in fluid. By use of the method of separation of variables and the method of matched eigenfunction expansion, analytical expressions for the potentials are obtained. Using the expressions for the potentials, analytical expressions for the hydrodynamic coefficients and exciting forces/moments on the device are obtained. Numerical results of the hydrodynamic coefficients and exciting forces/moments are presented for some ratios of the radius of the submerged cylinder to that of the riding one. It is found that the radius of the submerged cylinder has a significant influence on the hydrodynamic coefficients and exciting forces/moments for relatively bigger radius of the submerged cylinder at low frequencies.     

一种改进的Hu不变矩多源影像匹配方法

Hu不变矩对多源影像进行匹配时,无法对抗大幅度旋转和灰度差异,而且计算量较大无法快速匹配。本文结合多源影像之间的区别和联系,针对Hu不变矩的适用性和计算过程,从匹配窗口设计、影像预处理、亮度因子、计算过程等4个方面进行改进。实验结果表明,改进后的Hu不变矩匹配方法可以抵抗大幅度影像旋转和灰度差异,而且提高了匹配准确度和速度。     

Analysis And Comparison Of Different Estimations Of Mars' Polar Motion

We have summed up three recent modellings of Mars' polar motion (Hilton, 1992; Yoder and Standish, 1997; Zhang, 1998) from which we have studied in particular the Chandler wobble (CW) of the planet and expressions of its period T_CW. We add a fourth study (Kubo, 1991) which deals with the polar motion on the Earth, but whose the results can easily be applied to Mars. To conpare the different expressions of the period T_CW from the authors above, we have chosen common parameters of the planet coming from recent data and we have calculated values of the period corresponding to these parameters after having determined from analytical developments the moments of inertia of the core and of the mantle. We show how the different formula proposed by the authors lead to different values of T_CW and we study in particular the contribution of somc parameters such as the core radius, the mass of the core and the Love number of these values.     

Homogeneity testing: How homogeneous do heterogeneous cross-correlated regions seem?

The homogeneity of the flood frequency regime for a given pooling-group of sites is a fundamental assumption for many regional flood frequency analysis techniques. Assessing regional homogeneity is a critical step, which may be complicated by the presence of cross-correlation among flood sequences. The scientific literature proposes a number of statistical homogeneity tests and documents that inter-site correlation of floods is normally not negligible, but does not specifically address the impact of cross-correlation on such statistical tests. This paper analyzes the effectiveness of a well-known homogeneity test proposed in the scientific literature in the presence of inter-site cross-correlation through a series of Monte Carlo experiments. The numerical experiments enable us to comment on a possible theoretical correction for the test and to identify an empirical tool that accounts for the impact of inter-site cross-correlation of floods.     

岩土工程可靠度分析的神经网络四阶矩法

针对岩土工程的功能函数强非线性且难以显式表达的特点,提出了基于人工神经网络的四阶矩法,充分利用了基本随机变量的统计信息。首先利用神经网络对结构的隐式功能函数进行拟合,求得基本随机变量在均值点处的功能函数值及其偏导数,然后利用泰勒级数展开的方法由基本随机变量的前四阶矩求得功能函数的前四阶矩,并借助于Pearson系统获得功能函数的更高阶矩。在此基础上,通过最大熵原理确定以功能函数各阶矩为约束的功能函数的概率密度函数,最后由一次积分得到结构的失效概率。通过数值算例和工程实例不同方法的对比分析,表明基于神经网络的结构可靠度分析四阶矩方法是可行的,有效的,能够满足岩土工程可靠度分析的要求。     

从重力场识别与提取地壳变形带信息的方法研究

表面形貌识别是指用具体的参数表征表面各区段几何形态及属性并最终对不同类型的表面形貌进行识别,表面刻痕识别是其中的一种.以随机过程理论为基础的表面刻痕识别技术可以确切地识别表面形貌的各项特征,其各阶谱矩及统计不变量可以对表面的刻痕以及各向异性进行详细地刻画.区域重力场上的各向异性刻痕主要反映地壳线状变形带,它们常常是区域大地构造单元的边界.本文用重力场研究地壳变形带信息识别方法,将二阶谱矩、统计不变量等参数赋予一定的地质构造含义,并定义了脊形化系数、边界脊形化系数等,从而更详细地表征地壳变形带及大陆构造单元边界信息.理论模型与实际数据试验结果均表明,地壳变形带信息识别方法不但对重力场上地壳变形产生的线形刻痕进行了有效的刻画,而且提取出的刻痕信息,可为大陆构造单元划分提供客观依据.     

基于RANS方程的桩式U-OWC装置波浪荷载分析

使用OpenFOAM建立基于雷诺时均N-S方程和SST k-ω湍流模型的三维数值水槽,对桩式U-OWC装置在不同周期和波高的正向入射规则波作用下的受力情况进行分析。首先将数模结果与物理试验结果进行对比验证校核,然后通过数值模拟对装置的受力情况进行了分析,分别研究了作用在装置上作用力和倾覆力矩特点,装置上作用力和倾覆力矩随周期和波高的变化情况,以及装置稳定性极小状态。研究表明,装置所受到波向力、垂向力相比侧向力数值较大,对倾覆力矩起决定性作用。对装置上作用力和倾覆力矩进行无量纲分析,进一步发现在测试工况范围内,装置所受到的3个方向作用力中侧向力随周期变化不明显,明确了装置前的波浪爬高对装置波浪荷载和力矩的影响,并针对U-OWC模型特点和具体工况应用一个波浪爬高经验公式,获得了较好效果。     

离散小波变换与重力异常多重分解

小波变换多尺度分析是位场分解的有益工具.离散二维小波变换产生的低阶小波细节具有尺度不变的特征,它们不随小波变换的总阶数改变,而总阶数的增加仅增加高阶小波细节的个数和改变最后的高阶逼近.因此,用小波变换分解重力异常时,可根据地质目的来组合小波细节,从而实现有地质意义的异常分解.实验表明,用小波变换分解的重力异常小波细节,同样起到功率谱分解的作用,用分解之后的小波细节功率谱可以精确确定场源的埋深.中国大陆科学钻探场地的重力异常分解给出局部异常分解的典型例子.     

Energetics prediction of frequency-dependent suspended sand transport rates on a macrotidal beach

The on–offshore (cross-shore) transport of sand on beaches is highly time-variable, which has made it difficult to model or predict. In this paper, simple energetics modelling is used to compare velocity moment predictions with field observations of suspended sand transport rates. Separate consideration is given to transport associated with the three main frequency-dependent cross-shore transport processes: that associated with the short (incident) waves, that due to the long (infragravity) waves, and transport associated with the mean flow. Direct comparison between the depth-averaged model predictions, and the in-situ point measurements was facilitated by making the first order assumption that the time-averaged suspension profile is exponential and the wave velocity profile is vertically uniform. An appropriate rippled bed roughness was used to provide the drag coefficient in the energetics model and the vertical length scale of the exponential suspension profile. Despite these simple assumptions, comparison of the velocity moment predictions with the field observations of suspended sand fluxes reveals that this approach has the capacity to predict transport magnitudes due to short wave, long wave, and mean flow components to within about one order of magnitude. However, owing to the limitations of the model, the transport direction of the short wave component could not, on occasion, be correctly determined, probably due to ‘reverse’ transport over ripples. © 1998 John Wiley & Sons, Ltd.     

线性矩法与常规矩法对太湖流域降雨频率分析的比较研究

介绍了线性矩法的基本理论,并与常规矩法进行了初步的比较与分析,说明了其理论上的优越性;以太湖流域的雨量资料为例,选取4个站点分别应用线性矩法和常规矩法估计其不同重现期下年极值降雨频率设计值;最后,利用Monte Carlo方法对太湖流域内96个站点数据进行模拟,比较线性矩法和常规矩法所估计的统计参数。结果表明:线性矩法估计的参数在精确性、不偏性及稳健性方面较常规矩法更优。