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《中国海洋大学学报(自然科学版)》2016,(4)
针对经验模式分解(EMD)易受噪声影响的缺点,提出采用小波消噪结合Hilbert-Huang变换的结构损伤检测方法。首先对含噪声振动信号进行小波消噪预处理实现信噪分离,再进行EMD分解得到若干固有模态函数(IMF),利用希尔伯特(Hilbert)变换得到其瞬时频率。提出了一个基于低阶瞬时频率变化率的损伤判定指标,利用此指标可以判断损伤是否发生。数值研究结果表明小波消噪结合HHT的方法是进行损伤检测比较有效的方法。 相似文献
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利用2010年4月3日在舟山外海观测的25hLADCP海流数据与CTD连续观测数据,综合利用Thorpe方法、功率谱分析、交叉谱分析等方法对测站所在海域的小尺度湍混合参数进行估计,分析并讨论了其时空分布特征及影响因子。研究结果表明,上混合层湍流所致的垂向翻转尺度普遍大于下混合层,较大尺度翻转均出现在潮位峰值附近,涨潮时段混合明显大于落潮时段且半日周期,1/4周期显著。该站点存在明显的"上强下弱"双密度跃层现象,湍动能耗散率、湍混合率也呈现出"表强底弱"特征。弱跃层中近惯性频率的内波和近半日潮频率的内潮信号最为显著,而强跃层中则是高频内波和近半日潮频率的内潮信号明显,上下跃层及其之间伴有间歇性强湍流发生。上混合层平均的湍混合对风应力的响应要快于对海流的响应,底应力是下混合层水体湍混合的重要因子。 相似文献
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水面水下无人系统的运动控制、航路规划算法设计和仿真中均需要考虑海流影响,定常海流动力学模型应用最为广泛。该模型基于流体力学原理简化推导得到,缺乏验证试验支撑其可信度和准确度。针对这一问题,设计了一种验证试验。试验中记录海流信息和无人潜航器多个航向的实航试验数据,同时设计具有与实航试验相同海流工况的半实物仿真对比试验。最后将实航数据与半实物仿真试验数据对比,对定常海流动力学模型的准确性进行了评估。 相似文献
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为了掌握南海北部海区的海流及潮流情况，利用2000年8-11月在南海北部海区75天的ADCP定点流速观测资料，对海流的观测结果、海流前进矢量图、海流的日平均流速、海流随时间和深度的变化情况、正压流速的矢量旋转谱和斜压流速的二维矢量频率波数谱以及正压潮流进行了分析研究。结果表明，此处海流主要为逆时针方向旋转，并且K1和M2为主要分量。这说明南海北部海区的海流及潮流变化比较复杂，需要大范围的长期观测才能更好她掌握其特征与变化规律。 相似文献
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以2000年为例,采用SWAN波浪数值模型对浙江近海海域的波浪进行了全年模拟计算,并计算获得年、月平均波功率密度分布。研究表明,浙江近岸海域年平均波功率密度约为2~6 kW·m-1,往外海逐渐增大;同时季节变化明显,秋、冬季节波功率密度较大,春、夏季节较小。另外,通过对浙北、浙中和浙南3个近海海区的波浪出现频率和波功率密度随波高和周期变化的分析可知,浙北海域波功率密度比较高的波高及周期范围和波浪出现频率较高范围较为接近,而其对应平均波功率密度相对较低;浙南海域波功率密度比较高的范围所对应的平均波功率密度较高,而与波浪出现频率较高的范围则略有差异;浙中海域居两者之间。总体而言,浙江近海波浪能资源丰富,且全年中可开发与利用的波浪能出现频率较高。 相似文献
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《中国海洋大学学报(自然科学版)》2021,(12)
水下极低频电磁信号探测在深远海探测领域应用前景广阔,然而极低频电磁信号易受海流运动感应电磁噪声影响,为了拓展极低频电磁探测在海洋中的应用,需要通过信号处理的方式压制噪声,提高信号信噪比。小波分析具有多分辨率分析及时频局部化的优势,适于处理海水运动感应电磁噪声等非平稳信号,本文将海流感应电磁场特征引入小波分析中,基于正演模拟及小波系数分析确定阈值的小波阈值去噪方法压制噪声,利用合成的包含大地电磁信号、海流感应电磁噪声的极低频信号进行实验,并利用南黄海海域电磁信号探测试验数据进行验证。研究结果表明,噪声压制后的极低频信号信噪比明显提高,信号在时频域识别能力得到改善。 相似文献
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通过对单点系泊潜标进行动力学分析,利用集中质量法建立系统的动力学模型,编制Matlab仿真程序,通过数值模拟获取不同海流条件下,分析潜标系统的姿态。文中将一个典型潜标系统的浮球、锚链、重力锚等部件,利用集中质量法简化为节点,通过模拟真实环境中的海流流速在垂直方向的分布,设计1～5 m/s的流速剖面分布。将节点的物理参数和海流参数代入仿真程序中,得到各节点的姿态、节点间张力以及重力锚与海底摩擦力等数据。分析仿真结果,海流流速与节点的偏降、节点间的张力以及重力锚与海底的摩擦力成正比,仿真结果与实际情况相符。文中设计的建模方法和仿真程序可以为单点系泊潜标的总体设计、姿态估算、锚系配置提供工程参考。 相似文献
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2008年8—9月份吕宋海峡西南侧锚定ADCP的斜压海流观测 总被引：1，自引：0，他引：1
2008年8月份在吕宋海峡西侧大约4000m深的海域首次布放了两套深水潜标，对上层海洋进行了为期一个月的高频率ADCP海流采样。对数据进行了多重处理，分解得到平均流和逐层潮流，并分析了具有斜压性海流的频率分布。调和分析结果表明，该海域以不规则日周期潮流为主，潮流椭圆随深度旋转。海流的功率谱分析表明，内潮能量在温跃层附近最大，并随深度减小，同时海流有明显的近惯性振荡信号。该观测为了解吕宋海峡深层海洋的潮流、近惯性运动以及海峡水交换提供了第一手的资料。 相似文献
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The Wigner-Ville distribution (WVD) function was originally proposed by Wigner in quantum mechanics and Ville applied it for signal analysis. This method has made it possible to represent a signal's power density spectrum in the time-frequency domain as a natural extension of the Fourier transform method (FTM). Recently, it has attracted great interest for its validity to analyze time-varying signals accomplished by the development of high-speed digital signal processing, and it is used for analyzing nonstationary signals. Conventionally, a sonar beamformer is constructed using delay lines, but the development of the high-speed processor has made it possible to apply the FTM for sonar beamforming. However, the bearing resolution of the beamformer is not enough for discriminating small underwater objects on the sea bottom by this method. To solve this problem, we aim to apply the WVD method, which can represent finer structure of signals as a natural extension of the FTM, for sonar beamforming to obtain sharper beam patterns than those of the beamforming method by FTM. Simulation results by computational calculations to clarify the resolution by the WVD method, which is presented in this paper, becomes approximately twice as high as by conventional FTM. The results of an experiment at sea also show the performance of this method 相似文献
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《中国海洋工程》2021,(4)
Inductively coupled channels are based on the electromagnetic induction principle and realize long-distance current signal transmission through seawater. Due to a few difficulties in performing actual experiments, it is unclear how the seawater medium affects the frequency selectivity of the current signal. In this paper, a dual dipole model of the inductively coupled seawater transmission channel is established for the traditional short-distance current field transmission mode. The transmission characteristics of electrical signals in seawater are theoretically derived. A platform is used to measure the amplitude-frequency and phase-frequency characteristics of the current signal transmission in seawater with transmission frequencies ranging from 30 kHz to 1 MHz, and transmission distances in the vertical range of 4 m. The COMSOL Multiphysics simulation and practical test analysis are carried out to analyze the frequency selectivity of seawater conductivity. It is proved that the seawater resistance increases as the frequency increases, which is the key problem that affects the current signal. This study provides an important theoretical support and experimental evidence for improving the transmission performance of long-distance underwater current signals. 相似文献
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A method based on empirical mode decomposition (EMD) and time-varying autoregressive (TVAR) model is proposed here to identify the modal parameters of time-varying systems, such as the Floating Production Storage and Offloading (FPSO) single point mooring system. For the EMD–TVAR method, the original signal is decomposed into a finite number of ‘intrinsic mode functions’ (IMFs) by the EMD. Each IMF can be represented as a TVAR model. Then, the time-varying modal parameters i.e., instantaneous frequency (IF) and modal dumping, can be obtained by the basis functions expansion method. The proposed EMD–TVAR method has good results in two experiments compared with the Huang–Hilbert transformation and Short Time Fourier Transform method, and it has been used to analysis the modal parameters of FPSO single point mooring system successfully. The system's time-varying characteristic and its frequency distribution can be known from the modal analysis results. 相似文献
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A design procedure for an amplitude-modulated and nonlinear frequency-modulated (AM-NLFM) signal is introduced. The designed signal can drive a given transducer to its peak power to produce a sound pressure waveform into the water with a desired power spectrum and maximum possible energy. The signal can be formed either in the time domain or in the frequency domain. The frequency domain approach gives an output power spectrum precisely identical to a preferred shape. Therefore, the sidelobe levels after matched filtering are not raised by unwanted spectral magnitude ripples which exist when a time domain method is adopted. The absence of spectral ripples is desirable for applications requiring long range transmission and good multipath discrimination capability. An acceptable tradeoff between time resolution and sidelobe levels is achieved by properly choosing the desired power spectral shape. As the time resolution is usually the most critical specification for precision travel-time measurements, it is shown that by sacrificing some of the transducer's output power capability, a waveform with a considerably wider bandwidth can be transmitted, resulting in a significantly enhanced time resolution. A quasi-steady-state (QSS) approximation is used in the signal design, leading to a manageable and intuitive design procedure 相似文献
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Nelson E. Martins Srgio M. Jesus 《Oceanic Engineering, IEEE Journal of》2006,31(3):646-656
A blind estimator of the ocean acoustic channel impulse response envelope is presented. The signal model is characterized by a deterministic multipath channel excited by a highly nonstationary deterministic source signal. The time-frequency (TF) representation of the received signal allows for the separation between the channel and the source signal. The proposed estimator proceeds in two steps: First, the unstable initial arrivals allow for the estimation of the source signal instantaneous frequency (IF) by maximization of the radially Gaussian kernel distribution; then, the Wigner-Ville distribution (WV) is sequentially windowed and integrated, where the window is defined by the previously estimated IF. The integral gives the channel impulse response envelope, which turns to be an approximation to the blind conventional matched filter (MF). The blind channel estimator (CE) is applicable upon the following conditions: that the multipath channel contains at least one dominant arrival well separated from the others, and that the IF of the source signal is a one-to-one function. Results obtained on real data from the INternal TIde Measurements with Acoustic Tomography Experiments (INTIMATE'96), where the acoustic channel was driven by an linear frequency modulation signal, show that the channel's envelope detailed structure could be accurately and consistently recovered, with the correlation of the estimates ranging from 0.796 to 0.973, as compared to the MF result 相似文献
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B.J. Huang 《Ocean Engineering》1980,7(2):207-228
A simple theoretical model is developed to assess the technical feasibility of pumping cold deep nutrient-rich seawater for mariculture and nuclear power plant cooling. Simulations for two locations in eastern Taiwan have shown that replacing warm surface seawater by cold deep seawater as nuclear power plant coolant is technically feasible for pipes with diameters larger than 150 cm and intake levels deeper than 400 m. The gravitational force-induced flow system is also examined and shown not to be suitable for the nuclear power plant cooling application. However, it is shown than both systems are suitable for mariculture applications. The required pumping powers at different pipeline conditions are also presented in the paper, for design purposes. 相似文献
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Continuous flow analysis of dissolved total phosphorus in seawater by UV-K2S2O8 online digestion method 总被引：1，自引：0，他引：1
Several methods for analysis of dissolved total phosphorus in seawater were reviewed. Discussions were focused on UV irradiation and persulphate oxidation methods which are the most popular dissolved organic phosphorus determination methods presently. The compounds used for the phosphorus recovery test were categorized into three groups according to their chemical structure. It was found that low power UV irradiation can decompose P-O-C or P-C bonds efficiently but may be inefficient for P-O-P bonds. Heating-bath in acid condition is useful for decomposing P-O-P bonds. Using the continuous flow analysis system (Auto-analyzer Ⅱ), UV digestion and heating-bath, series experiments were carried out based on the above analysis. Eleven model compounds were employed for the phosphorus recovery test and the factors influencing the decomposition efficiency of dissolved compounds containing phosphorus were clarified. Finally, the optimal design for determination of dissolved total phosphorus in seawater based on the routine continuous flow analysis system was presented. For the organic mono-phosphate, the recovery is more than 90% and a recovery of 33%～51% was obtained for inorganic or organic polyphosphates. Up to now, this is the highest decomposition efficiency for dissolved phosphorus based on the continuous flow analysis system. 相似文献
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The characteristics of directional spread parameters at intermediate water depth are investigated based on a cosine power ‘2s' directional spreading model. This is based on wave measurements carried out using a Datawell directional waverider buoy in 23 m water depth. An empirical equation for the frequency dependent directional spreading parameter is presented. Directional spreading function estimated based on the Maximum Entropy Method is compared with those obtained using a cosine power ‘2s' parameter model. A set of empirical equations relating the directional spreading parameter corresponding to the peak of wave spectrum to other wave parameters like significant wave height and period are obtained. It shows that the wave directional spreading at peak wave frequency can be related to the non-linearity parameter, which allows estimation of directional spreading without reference to wind information. 相似文献
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近海海上风电场水下噪声传播模型适用性研究 总被引：1，自引：0，他引：1
通过现场采集近海海上风电场工程区运营期风机水下噪声和背景噪声数据,计算了噪声信号的倍频带声压级,功率谱级和峰值声压级,确定了海上风电场水下噪声总声源级为148.3 d B,以此开展近海海上风电工程风机水下噪声频域特性、功率密度谱特性等研究。在此基础上使用Kraken简正波模型和Bellhop射线模型对风电场运营期风机水下噪声在水平与垂直方向上的传播进行模拟,模拟了噪声在不同频带内的衰减程度,结果显示模型模拟结果在不同频率下的衰减趋势有着很大差异,产生了明显的多途干涉现象,通过实测数据对建立的噪声传播模型进行验证,发现Kraken简正波模型在500 Hz以下,Bellhop射线模型在500 Hz以上适合模拟实际水下噪声传播情形,同时海区本身背景噪声的存在会对预测的准确性产生影响。这些结论可用于进一步对近海海上风电场水下噪声传播的研究。 相似文献