浅海声信道“时域衰落”特性研究

研究了浅海声信道“时域衰落”与海况，波高，中心频率和作用距离的关系，结果表明，在相同海况下，波浪愈高，振幅起伏愈明显，在同样作用距离，深度下，中心频率２０ｋＨｚ是信道较好的传播频率，其振幅起伏小，自相关时间较长，在该声道中，作用距离在２０００－２５００ｍ处振幅起伏最大。     

指数剪切流作用下海洋立管振动特性研究

细长海洋立管在复杂来流作用下的涡激振动（VIV）是海洋工程领域备受关注的热点问题。采用双向流—固耦合方法,对立管在指数剪切来流作用下的涡激振动实施了数值仿真研究。基于均方根振幅包络图的显性模态分析发现,与线性剪切流工况相比,当流速较小时,立管模型在指数剪切流作用下振幅最大值相对较小;流速较大时,指数剪切流的非线性分布致使立管的振动响应增强。通过对立管模型均方根振幅包络峰、谷处的振动响应频率分析发现,在波峰位置处振动频率单一且较为稳定,而在相邻波谷位置处多频共存现象显著,两者有显著差异。沿管的轴向波形主要表现为驻波主导和驻波—行波混合模式,行波一般间歇发生,流速越大发生频次愈高,其在横流向的传播方向通常由高流速区段传递至低流速区段。指数剪切流作用下,沿管体轴向各截面位置处的旋涡脱落模式差异显著,高流速段尾流区的旋涡发放具有较强的周期性,涡管完整且与管轴存在一定偏离角度,低流速段蜂窝状离散涡旋较多。     

海洋声速起伏对声传播及定位精度影响分析

海洋中声速起伏导致水声信道发生变化,进而引起声线到达结构的变化,对水声传播及定位精度产生一定影响。为讨论这一效应,基于TDOA体制建立了考虑声线弯曲的水下目标无源定位模型,分析了声速起伏对水下声传播路径及传播时间的影响,进而研究了声速起伏对水下无源定位测量精度影响程度。结果表明:当水平传播距离较大时,声速剖面起伏对声传播路径及传播时间的影响更为显著;以典型四元阵为例,若基线长度为20 km,接收阵位于水下5 km处,在不考虑其它随机误差影响下,海洋声速起伏造成的声源定位误差量级在0.5 m以内。分析结果有助于更好地利用环境特征优化无源定位测量方案,可为高精度水下无源定位系统设计及精度评估提供依据。     

海洋老地震资料匹配处理技术

根据海洋老地震资料联片处理中存在的振幅、频率等不一致性问题,详细论述了解决振幅、频率匹配的方法技术,提出了消除区块间多种不一致性的实用有效的处理流程.通过采用测井约束下的振幅一致性处理技术和匹配滤波统一子波处理技术,较好地解决了相邻区块不同年度地震资料联片处理中振幅和频率的匹配问题,并且在实际应用中取得了明显的效果.     

浅海声信道文本信息传输研究

论文首先介绍浅海声信道声传输的基本特性,指出在此类信道中进行多媒体信息传输的特殊困难.随后论述了文本信息传输所具有的抗噪声、抗起伏等优良性能,较能适应于水声信道的复杂性和多变性,但属于较高速率的文本信息传输,多途干扰仍然是文本信息正确检测的根本障碍.文中分析了频率跳变技术克服时域扩散较短的浅海多途的可行性和需解决的关键技术.海上获得的初步实验结果说明了文本信息传输所具有的优越性,值得今后继续深入的研究.     

干涉谱法测量水下竖直运动目标轨迹

以被动测量竖直运动目标轨迹为目的,通过分析多途信号声压场模型,讨论了经过相干多途信道的目标辐射噪声在接收点产生相干干涉的现象。在目标水平距离已知的情形下,给出了干涉频率周期与目标深度的关系,提出了利用多途信道的相干干涉信息来解算目标深度轨迹的方法。时频分析可以得到干涉条纹、条纹粗细变化的规律与目标深度变化有关。通过对实测数据的分析,说明本方法的有效性。     

潮汐作用下渤海温跃层波动与起伏的数值研究

研究了潮汐(四大分潮 M₂,S₂,K₁,O₁同时输入)作用下渤海温跃层起伏与波动的三维数值模型(将海洋分为3层,即上混和层、跃层和下混和层),揭示了整个海区温跃层上界面处跃层起伏(在文中指每个时刻跃层波高的周期平均值)的地理分布及叠加在起伏之上的潮周期波动的时空变化,模拟出跃层波动与实测基本一致.结果发现大振幅的跃层波动均发生在海峡及近海地形突变之处.一般界面波动的波高大于甚至远远大于同一位置的表层潮波.从位相以及周期来看,潮波和跃层上下界面波动相互之间,有些海域一致,有些地方则相差甚远.跃层上界深度及厚度的梯度,对跃层起伏分布有一定的影响.跃层起伏还可能与海岸海底摩擦有关.     

不规则波下船舶运动响应特征数值分析

针对不规则波浪作用下Wigley型船的运动响应问题进行了系统的研究,采用统计学方法深入探讨了船舶不规则运动幅值和响应周期的分布规律,并通过傅里叶变换对船舶运动响应进行了频谱特征分析。结果表明,船舶横摇方向与升沉和纵摇方向随机运动的响应特征有显著差异。在升沉与纵摇方向,波浪谱峰频率远离自振频率,前十分之一大振幅运动对应周期离散性较小,基本稳定在波浪谱峰周期附近,但小振幅运动周期分布离散性较大,频谱分析指出船舶升沉与纵摇运动响应频谱在波浪谱峰频率附近出现明显峰值。而在横摇方向,波浪谱峰频率与自振频率相耦合,不同振幅的横摇运动响应周期均稳定在自振周期附近,且周期离散性较小,频谱分析也表明横摇运动响应频谱主要集中于船舶运动自振频率附近。     

海洋内波发展演变数值试验

利用二维非静力数值模式,对海洋跃层中剪切背景流下内波做了线性和非线性模式的数值试验,结果表明:在本文所取的初始场和环境参数下,海洋内波密度扰动大值中心始终出现在密度跃层附近,被跃层所俘获,这与实际观测相一致;线性情况下,流函数扰动中心与密度扰动中心有很好地配合,密度扰动正、负中心附近分别存在流函数扰动的正、负单圈环流中心,且单圈环流垂直贯穿整个水体。线性模式中内波扰动强度在调整适应期后一直呈指数增长发展,剪切背景流是不稳定的;而非线性情况则呈准线性增长发展,以后增长停止而进入稳定期,且后者的增长也比前者要慢;非线性效应抑制了内波增长而使之趋于稳定,具有维稳作用。随着积分时间增加,线性模型中内波波包波形基本不变。波包的负、正振幅大体相同,整个波包宽度也基本不变,仅振幅有所增长;内波波包传播速度与其振幅无关,且传播速度基本不变。非线性情况下内波波包的形状则发生改变,其负、正振幅之比越来越大,波峰前的陡降也越来越强;并且内波传播速度与其振幅和波形有关,通常非线性效应越强,内波传播越慢。从本文中海洋内波的波长、周期和传播速度来看,该内波属于高频内波(短周期内波),性质为内重力惯性波。     

潮汐作用影响下滨海地区地下水位的变化频率和滞后性

为了解潮汐对滨海地区地下水位的影响,为相关水利工程建设和水资源开发利用提供依据,文章根据莱州湾南岸的水文观测井和潮位站的观测数据,采用时间序列分析方法和互相关分析方法,研究地下水位相对于潮位的变化频率和滞后性。研究结果表明:由于受地下卤水开采的人为干扰,与潮位相比,地下水位在较低频率时振幅较高;去长期趋势处理对于排除人为干扰效果明显,地下水位的频率振幅与潮位波动规律一致,即均存在明显的12h和24h的周期,但大部分观测井的地下水位受24h周期影响较大,这与潮位差异显著;互相关分析进一步证实地下水位与潮位具有相同频率,潮汐向陆地侵入对地下水位的影响显著,地下水位相对于潮位的滞后时间随其距平均高潮线水平距离的增加迅速上升,二者呈抛物线关系。     

应用于海洋物性监测仪的小型浮标水动力分析

为验证研制的国产气象水文传感器的性能,设计了直径3 m的圆盘式小型浮标,拟搭载所有传感器在2000 m以深海域开展应用试验。统计了试验海域的有义波高、波浪周期、最大风速和表层流速等环境参数,计算了浮标的质量、重心、吃水、受力投影面积和惯性矩等设计参数。以三维势流理论和波浪辐射-绕射理论为基础,使用AQWA软件在频域内对浮标进行水动力计算,仿真了浮标工作时所受的一阶波浪力、附加质量、附加阻尼和运动幅值响应因子,通过仿真结果对浮标设计方案进行优化,优化后的浮标计算结果随波浪频率变化平缓,幅值合理且无激增点,表明该浮标与工作海域内的波浪没有发生明显谐振,对环境载荷有足够的承受能力,方案可用于浮标研制。     

浙江近海波浪能资源的初步研究

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。     

复杂海况下水下拖缆数值分析研究

为了解复杂海况下风浪对水下拖缆的影响进行了动力学仿真研究。以P-M谱作为海浪谱密度公式,计算一组不同频率的组成波,采用线性波浪叠加法,计算由波浪引起的空间流场速度及加速度,推导叠加波的干扰力,将不规则波引入水下拖缆动力学模型中,采用四阶龙格库塔积分法对动力学模型进行数值求解,讨论拖缆在不同海况下的动力特性。结果表明,随着海况等级的提高,叠加波的频率范围逐渐降低,拖缆的垂向偏移量以及拖点处张力均会产生明显增加及波动,且其波动周期接近叠加波的平均周期;在不同浪向角的作用下,拖缆波动规律与规则波作用下类似,但也具有一些不规则波的特点。     

海浪外观统计与内部结构关系的数值研究

基于数值模拟的三维随机海面,从时间域和空间域两个角度对海浪的外观特征和内部结构间的关系进行了研究,得出了一些在工程应用上有参考价值的结果.文章认为,将波面看作是若干随机正弦波叠加的海浪结构模型,用于描述平稳、均匀和窄谱的海况时,其频谱分析结果与外观统计结果基本上是一致的;但是,当海面处于谱宽度较大的风浪或混合浪状态时,一些频谱分析结果必须经过修正才能应用.同一海况下,海面波动的时间过程和沿主波向的空间分布是明显不同的.当海面有突发性大浪时,外频谱能够很好地反映不平稳海况,而频谱则把这种不平稳性平滑了.文章着重讨论了波长和波陡的计算、频谱和外频谱的差异.     

Experimental Study on Crescent Waves Diffracted by A Circular Cylinder

Crescent waves often observed on the sea surface are unusual wave pattern induced by the instability of Stokes wave. The paper presents the experimental results of the wave field around a circular cylinder generated by the diffraction of crescent wave in order to examine the difference of diffracted crescent waves from the commonly-used diffracted Stokes waves. The results show that with the existence of the cylinder, the crescent wave pattern can still get fully developed, and with the presence of this type of wave pattern, the symmetry breaking of the wave amplitude distribution occurs and there are extra wave components at the frequencies of 0.5ω₀, 1.5ω₀ and 2.5ω₀ (ω₀ is the frequency of Stokes waves) appearing in the wave amplitude spectrum.     

Ocean surface current retrieval and imaging with a new shore-based X-band radar based on time-shifted up-and-down linear frequency modulated signal

This paper proposes a multifunction radar that can not only measure sea currents but also perform sea-surface imaging. The fundamental aspect of the proposed radar comprises transmitting time-shifted up-and-down continuous wave linear frequency modulated signals that allow for the offset of two one-dimensional range images of the sea surface that respectively correspond to the upward linear frequency modulated(LFM) signal and the downward LFM signal. Owing to the Doppler frequency shift from the sea surface, a range offset, which is proportional to the radial velocity of the sea surface, occurs between the upward and downward LFM signals. By using the least-squares linear fitting method in the transformed domain, the range offset can be measured and the current velocity can be retrieved. Finally, we verify the accuracy of current measurement with simulation results.     

An experimental study of a pulse compression scatterometer system

The ideal scatterometer, operating from either an aircraft or a satellite platform, should be capable of making rapid, accurate estimates of the sea backscatter cross sectionsigmaover as wide a range of grazing anglespsias possible. Efficient operation over a large range of grazing angles is desirable because 1)sigmabehavior for90deg geq psi geq 70degyields rms gravity wave slope information and is an indicator of sea state and 2)sigmabehavior for70deg geq psi geq 0degyields data on surface wind magnitude and direction as well as information about the power spectrum of the sea. A "hybrid" estimation procedure has been developed for pulse compression radars which uses both frequency and spatially decorrelated samples ofsigmato provide an unbiased estimate ofsigmahaving minimum variance over the entire range of grazing angles for which radar reception is not noise-limited.     

A new formula for the sea state and structural parameters influencing the stability of homogeneous reshaping berm breakwaters

The berm recession of a reshaping berm breakwater has a very important role for the stability of this kind of structure. Based on a 2D experimental modeling method in a wave flume, the recession of the berm due to sea state and structural parameters has been studied. Irregular waves with a JONSWAP spectrum were used. A total of 215 tests have been performed to cover the impact of sea state conditions such as wave height, wave period, storm duration and water depth at the toe of the structure, and structural parameters such as berm elevation from still water level, berm width and stone diameter on berm recession. In this paper, first a new dimensionless parameter is introduced to evaluate the combined effect of wave height and wave period on berm recession using results of the experimental work. Then, a formula that includes some sea state and structural parameters is derived using the new dimensionless parameter for estimating the berm recession. A comparison is made between the estimated berm recessions by this new formula and formulae given by other researchers to show the preference of using the new dimensionless parameter. The comparison shows that the recession estimated by the new formula has not only a better correlation with the present experimental data, but also has an improved correlation with other experimental results within the range of parameters tested. Outside the range of parameters tested the Lykke Andersen (2006) formula performed best.     

Radiophysical investigations of sea roughness (radiooceanography) at the Ukrainian Academy of Sciences

Experiments on the scattering of radio waves in the range 200 m to 3 cm from a rough sea surface are described. Amplitude, frequency, and space-time characteristics of scattered radio signals at different states of the sea surface are presented. It is shown that the problem of the short and medium wave scattering from the sea can be solved by the perturbance method. In this case the mechanism of scattering is of "resonant" character. The intensity of the backscatter signals is proportional to the density of the spatial spectrum on the half-length of the radio waves. The high frequency radio wave scattering is well described by a two-scale model of the scattering surface, "ripple on the large wave." The intensity of scattered radio signals is also proportional to the spectrum density of "ripples" whose length is approximately equal to half a radio wave. The effect of the large waves is to modulate the amplitude of a scattered radio signal and to broaden its frequency spectrum. Methods of solution of the reverse problem were considered. This allowed determination of parameters of sea roughness by characteristics of scattered radio signals. The principles of design of the corresponding equipment are described.     

The propagation of high frequency internal waves in the Celtic Sea

The continental slope to the south of the Celtic Sea is an area of extremely rough topography and tidal currents of the order of 50cm/s (with components both along and across the slope). This is a region of intense and complicated internal wave and internal tide activity. Historical current meter data from moorings close to the shelf-break show bursts of high frequency, large amplitude internal waves occurring, on average, at either once or twice per M2 tidal cycle. Wave packets at 9 moorings along the shelf-break and further on-shelf are identified using conditional sampling. The paths travelled by these wave packets are calculated using their fluctuation orientation, linear wave theory and the low frequency current. The records are up to 60 days long, allowing the ensemble statistics of propagation direction and wave characteristics to be calculated for a large number of wave packets. This analysis shows that only a fraction of the observed wave packets have orientations consistent with generation by the across-slope barotropic tide. This mechanism accounts for 20% of the wave packets in the north-west Celtic Sea and 29% in the southeast Celtic sea. A similar fraction of the wave packets (23% in the north west and 27% in the south east) have orientations clearly consistent with generation by an along-slope flow over the rough topography on the slope. The remaining wave packets are attributed to generation by tidal flow over topography close to the moorings and possibly internal wave resonance within canyons.