赤潮藻类流式图像自动分析算法的研究

分析了Phillips波浪谱结构,指出该谱是由符合Neumann形式的频率谱和国际拖曳水池会议推荐的方向分布函数构成的方向谱,Phillips谱的频率谱与P-M谱接近。参考P-M谱提供的方法给出了Phillips谱风速的确切含义并计算出该谱的常数,解决了其在应用中未尽的问题。比较PM-ITTC方向谱和Phillips谱的绘制结果,发现用Phillips谱绘制的海浪低频长波更加突出。对Phillips谱和PM-ITTC方向谱进行比对,发现Phillips谱的谱峰频率与PM-ITTC方向谱接近,但谱宽更窄,能量更集中于低频部分。为Phillips谱增加了风距参数,使绘制结果能反映风速、风距对海浪的影响。绘制的海浪传播方向、波高随风速变化而变化,与航海实际情况相符,已应用于航海模拟器。     

航海模拟器视景系统中的混合浪绘制

使用快速傅里叶逆变换(IFFT)Gerstner波模型描述海浪运动,在航海模拟器视景系统中实现了混合浪实时绘制。利用六参数谱构造混合浪的波数谱,将该波数谱应用于海面高程和choppy波的傅里叶系数中,通过IFFT获得海面质点的位置,实现混合浪的实时绘制。绘制的混合浪受低频长峰波影响,波形较为平坦光滑,与绘制的风浪有一定的差别,但与真实的海浪更为接近。该方法在航海模拟器视景系统中的使用,表明帧率可以达到实时。     

无粘性海浪实时绘制综述

根据是否考虑液体粘性,对海浪绘制进行分类,并对无粘性海浪实时绘制展开综述。无粘性海浪的绘制忽略了液体粘性,绘制结果能体现风的影响,且绘制实时性好。无粘性海浪的实时绘制可分为基于规则波的方法和基于不规则波的方法,真实的海浪可以视为随机过程,因此基于不规则波的方法较之于基于规则波的方法更接近实际情况。提出基于不规则波方法中可能的改进:对基于不规则波方法中所用的海浪谱进行改进,使绘制结果能体现风速、风距的影响。     

基于谱的三维随机海浪数值模拟研究

为有效地模拟三维波浪,从已有的统计观测结果出发,应用P-M谱和SWOP方向函数的基础上,建立了风浪的三维随机波面数学模型,该模型采用多个随机余弦波叠加来模拟随机波浪曲面。最后利用Matlab实现了该算法,结果表明这种方法能有效地模拟三维随机海浪。     

使用波数谱绘制海浪波幅畸变的校正

针对快速傅里叶逆变换形式Gerstner波绘制海浪出现波幅畸变的问题,提出使用波数谱绘制海浪波幅畸变的校正方法.首先重新推导了包含基元波振幅的快速傅里叶逆变换形式Gerstner波;然后用右侧Riemann求和对波数谱的定积分进行离散,离散积分域的边界值由快速傅里叶逆变换对波数向量的采样方式决定,得到基元波振幅的近似解;最后将该近似解代入上述推导所得的形式中,得到傅里叶系数中包含波数谱和离散积分域面积的快速傅里叶逆变换形式Gerstner波.实验结果表明,采用该方法可以准确地计算海浪势能和基元波振幅,绘制结果波形稳定,有效地解决了波幅畸变的问题.     

基于多波模型和多策略的真实海浪仿真研究

海浪运动随机性大、波谱范围宽,且涌浪波谱窄、能量大,是仿真大规模海浪的难点所在.针对难点问题,采用频率等分方法对海浪波进行区间等分分割,获取海浪真实性有效数据区域,提出了一种在Gerstner软件上扩展多波模型叠加LOD多情况绘制策略的海浪仿真模型.使用P-M波浪谱和SWOP观测数据求解真实海浪一系列参数,完成了海浪模型的真实造型;叠加LOD视点算法和多情况绘制策略解决了海浪模拟实时性问题.基于多波模型,设计了海浪仿真的UML类图,并运用Direct3D技术实现了海浪的实时仿真.实验证明,等分方法能够满足海浪真实场景实时仿真的要求,取得了较好的实验效果.     

双峰海浪谱的SAR图像交叉谱仿真研究

由6参数双峰频谱和cos-2s型分布函数得到了双峰海浪方向谱的波数谱形式,并利用海浪谱到合成孔径雷达(Synthetic Aperture Radar,SAR)图像交叉谱的非线性转换关系,对双峰海浪谱的SAR图像交叉谱进行了仿真,分析了不同海浪参数和SAR平台参数对SAR图像交叉谱的影响,并与SAR图像谱进行了比较。     

基于文氏方向谱的海浪仿真技术研究

研究不同成长状态下的海浪真实性优化问题,海浪方向谱不仅与风速风向有关,还与风浪的成长状态息息相关.为获取海面波高数据,不仅要求正确地反映风速风向的影响,还应能够真实地反映海浪成长状态的变化.为解决上述问题,提出了一种采用能正确反映海浪成长状态的文氏方向谱来作为靶谱的三维海浪仿真方法,利用海洋谱能量散布随角频率以及不同风浪成长状态变化的具体特性,根据频率等分以及能量等分的采样原则,分别对多种成长状态下的海浪进行了数值仿真.仿真结果表明,新方法能够正确地反映海浪成长状态的变化以及风速风向因子的影响,仿真的海面波高数据与实际海况相符.     

基于海浪谱的Gerstner波浪模拟

本文从海洋学现有的观测和研究成果出发,结合海浪的Gerstner模型,提出一种风力作用下的基于海浪谱的三维Gerstner海浪模型。首先,通过分析Gerstner模型各个参数的随机性,利用海浪谱和方向谱相关公式,获得固定风速下模型各随机参数值,建立固定风速下的海浪模型;然后,在此基础上进一步构造了基于海浪谱的风力模型,模拟了风力作用下的海浪;最后,采用基于视点的网格拓扑结构,实现了海浪的实时绘制。实验结果表明,该模型不仅能更真实地模拟海浪,还具有较好的交互性。     

大规模海浪场景的真实感绘制

针对计算机图形学中大规模海浪场景绘制的难题,提出一种实时绘制高真实感海浪的方法.首先提出同心圆网格模型来模拟海浪表面;然后利用基于GPU的快速傅里叶变换方法生成海浪高度图,并模拟了Choppy波,减轻了磁砖效应.分别采用立方体纹理映射、平面反射及Phong镜面反射技术模拟海面对天空、景物、太阳的反射效果,并在海面的折射效果中考虑了海水深度的影响.最后模拟了海浪中的泡沫以及景物在海面上的阴影效果.该方法已成功地应用在航海模拟器中.     

Wind-directional effects on the hydrodynamic modulation of microwave radar images of ocean waves

Abstract

A numerical simulation has been constructed of the hydrodynamic modulation transfer function (HMTF) which defines the spatial and spectral properties of swell waves as observed by an imaging radar. While the simulation is based on the Alpers and Hasselmann (1978) two-scale modulation transfer model, it explicitly takes account of directional properties of the short-wave spectrum and does not necessarily assume an isotropic wind-wave spectrum. Several different spreading functions are modelled and it is demonstrated that an anisotropic wind wave field significantly distorts the HMTF. Depending on the wind direction relative to the radar azimuth, the effect can be to shift the direction and the wave number of the peak of the image spectrum relative to the true swell spectrum. The effect of fetch limitation on the wind-wave spectrum is also examined based on the JONSWAP spectrum, but the consequences of this for the HMTF imaging mechanism are found to be minimal.     

Observations of wind and ocean wave fields using ERS Synthetic Aperture Radar imagery

The aim of the research reported here is to evaluate Synthetic Aperture Radar (SAR) capability to estimate the wind vector and associated directional wave spectrum. Two ERS–2 SAR images of the Mediterranean Sea, one over the Sicily Channel and one over the Ligurian Sea, were selected as case studies. Wind speed was estimated using SAR calibrated backscatter response, in conjunction with empirically derived ERS scatterometer models such as CMOD4 and CMOD–IFREMER. The predictions of these models were then compared with the actual sea surface wave spectra either provided by in situ measurements or resulting from the inversion of the SAR image spectrum. SAR-detected effects of both wind and wave features, induced either by atmospheric boundary layer instability or by land shadowing, were also used as reliable indicators of wind direction.     

利用噪声谱进行风浪声场特征反演

通过建立修正的风浪噪声模型,研究风浪声场特征的反演.在实测数据中提取100段进行频谱分析,建立不同频段噪声谱均值曲线.利用均值曲线风浪模型进行优化,建立修正的风浪声场特征反演模型.对APL和库普曼-费尔拉风浪噪声模型进行修正获得高频段反演模型;对克努森频谱浅海区模型进行修正获得低频段反演模型.对建立的全频段模型进行数据反演,生成一定范围内幅值服从高斯分布、相位服从均匀分布的噪声信号,即风浪信号时域输出.仿真结果表明,利用合理的反演模型,可以通过数据反演获得准确的海洋风浪噪声声场特征信号.为评价反演的准确性,对反演结果进行验证分析.     

飞行模拟器视景系统中水域的研究与实现

对飞行模拟器视景系统中真实水域的生成算法进行了研究.为兼顾水域的渲染逼真度与渲染效率,根据飞机离水面的高度采用两种不同的方法渲染水域:高度大于等于500米时,使用多张纹理连续贴图的方法渲染水域;高度小于500米时,创建水域网格,利用波动方程沿水域网格积分得到每个网格节点实时高度,模拟动态水域.利用凹凸纹理映射实现了水域网格顶点之间的光照细节,同时增加水域的环境映射,使水域效果更加逼真.     

Comparison of wave parameters determined from SILAR images and a pitch and roll buoy

Abstract

The nature of the data recorded by Side-Looking Airborne Radar (SLAR) and that by a pitch and roll buoy differ completely. The radar records a spatial data set at one instant in time, whereas the buoy records a time series on a given point on the sea surface, The spectrum in wave number space of the radar image is transformed to the frequency domain by using the dispersion relation for shallow water waves. With further processing, the radar derived data can be presented in the traditional form of pitch and roll buoy displays a (relative scale) waveheight spectrum, a directional distribution and the directional spreading, all as a function of frequency. Limitations of the method and differences in the results of the sensors are discussed.     

基于贪心优化策略的网格排布算法

针对由存储带宽和数据访问速度导致的复杂数据集绘制性能低下等问题,提出了一种基于贪心优化策略的三角形排布算法,通过对绘制数据集进行重排以改善数据的空间局部性和时间局部性。该算法首先将顶点分为三类,根据改进的代价函数选择代价度量最小的顶点作为活动顶点;然后绘制（即输出）其所有未绘制的邻接三角形,并将相邻顶点压入缓存,算法迭代执行直到所有顶点的邻接三角形都绘制完成,得到重新排列后的三角形序列。实验结果表明,该算法不仅具备较高的顶点缓存命中率,还提高了渲染速度,减少了排序的时间,有效地解决了图形处理器的处理速度不断提升而数据访问速度严重滞后的问题。     

SAR海浪方向谱的Monte Carlo仿真

利用Longuet-Higgins线性海浪模型和JONSWAP海浪谱来表示海浪方向谱。在Bragg波散射模型的假设下,推导了海浪谱与SAR图像谱之间的映射变换,它适用于线性海浪的成像范围。采用Monte Carlo方法,可产生具有随机性的海浪方向谱。对于时不变的海面来讲,该方法具有计算效率适中的特点。仿真结果表明,海浪谱与SAR图像谱之间存在着谱的畸变、谱的分裂和方位向谱的移位等特点。     

HIERARCHAL CONTROL SYSTEM FOR A VARIABLE SPEED CAGE MACHINE WIND GENERATION UNIT USING NEURAL NETWORKS

A hierarchal control strategy, that addresses three control objectives for a wind generation system, is proposed in this paper. It controls the local bus voltage (to avoid voltage rise), captures the maximum power in the wind and also minimizes the power loss in the induction generator. In the first level, given the instantaneous wind speed, electrical torque and output power, the designed neural networks calculate the desired rotor speed, air‐gap flux and the grid side reactive power. In the second level, the desired current wave shapes (instantaneous three‐phase currents) of the rectifier and the inverter in a double‐sided PWM converter system are calculated. In the third level, the PWM controller guides the system towards the optimum operating conditions. Simulation results show that even as the wind speed changes randomly, the proposed control strategy leads the system to the optimum operating conditions.     

An analytical algorithm with a wave age factor for altimeter wind speed retrieval

Based on the specular reflection theory of electromagnetic waves at rough sea surface and the wind wave spectrum model with a wave age factor, the sea surface wind speeds are retrieved from the normalized radar backscatter cross‐section (NRCS) measured by TOPEX/Poseidon (T/P) Ku‐band altimeter using the mean square slope (MSS) calculated from the spectrum models of the wind waves and the gravity‐capillary waves. A relationship between wave age and non‐dimensional wave height is applied to compute the wave age factor using the significant wave height (SWH) and wind speeds obtained from buoy or altimeter simultaneously. The study indicates that the wave age factor has a significant impact on the retrieval of altimeter wind speed. Compared with the operational algorithm for retrieving altimeter wind speed, the wind speed retrieved from the new analytical algorithm based on the wind wave spectrum model with the wave age factor, proposed in this study, can match the buoy measurements better. The effects of the wave age factor on altimeter wind speed retrieval are also shown quantitatively through a series of experiments and measurements. The comparison with the operational algorithm indicates that both the bias and root mean square error (RMSE) between wind speeds retrieved by the proposed analytical algorithm and those observed by the buoy decrease significantly. In the Gulf of Mexico, with the new analytical algorithm, more accurate altimeter wind speeds are retrieved.