采用混合模型数值模拟从深海到浅海内波的传播

在南海东沙岛附近, 从MODIS遥感图像发现内波传播是从深海经陆架坡再到浅海, 由于深海和浅海环境条件的差异以及传播模型的适用条件不同, 因此 不能采用同一模型模拟内波的传播, 需用两种模型来分别模拟内波在深海和浅海中的传播. 采用差分法, 首先用非线性薛定谔方程模拟了深海内波的传播, 然后用EKdV方程模拟了内波在浅海中的继续传播. 模拟结果与实际的MODIS遥感内波图像相符合, 并与应用单一模型模拟结果相比, 混合模型模拟该海区的内波传播更接近遥感实测, 表明了混合模型的合理性.     

应用非线性薛定谔方程模拟深海内波的传播

本文选取东沙岛以东深海区域,应用描述深海内波的非线性薛定谔方程,采用啁啾的思想,研究了频散和非线性效应之间的关系,模拟了深海内波的传播.数值模拟内波演变趋势与MODIS影像拍摄到的内波演变趋势基本符合,从而验证了应用非线性薛定谔方程模拟深海弱非线性内波传播的合理性. 关键词： 深海内波 啁啾 非线性薛定谔方程 频散和非线性     

基于光学遥感图像的南海北部内孤立波振幅反演模型与应用

收集2015年1月至2018年12月共计4年的MODIS光学遥感图像,利用神经网络算法建立适用于南海北部深海与浅海的内孤立波振幅反演模型。模型的计算结果与现有的南海实测数据对比,无论对于浅海(文昌海域)还是深海(东沙岛附近海域和深海盆地),反演精度都高于15%。利用模型得到了南海北部内孤立波的振幅分布,发现振幅分布与水深呈正相关,相关系数为0.81。计算了南海北部文昌海域、东沙岛附近海域以及深海盆地的内孤立波振幅分布,发现同一条内孤立波,振幅不同。南海北部文昌海域、东沙岛附近海域以及深海盆地的单条内孤立波振幅与水深呈正相关,相关系数分别为0.79,0.91,0.81。本文为南海北部海域深海与浅海的内孤立波振幅反演提供借鉴。     

南海北部海域内波环境下声传播损失起伏统计特性

浅海内波会引起声传播能量随时间的起伏变化,进而影响水声设备的工作性能.本文利用2015年南海北部一次浅海声场起伏实验数据,对比分析了浅海线性内波和孤立子内波条件下的声传播损失统计特性.在孤立子内波条件下,声传播损失起伏明显加剧,可达11 dB,且分布明显展宽,相对于线性内波的环境,声传播损失起伏可增加5 dB.从简正波...     

颗粒Reynolds正应力轨道模型及后台阶两相流动数值模拟

用随机过程理论建立气固两相耦合脉动量Lagrange方程,并建立了气固两相流随机颗粒轨道模型中颗粒Reynolds正应力的Lagrange方程.将新的模型运用于各向同性的湍流衰减的流场中,模拟颗粒的湍流扩散特性,与文[1]中的模型和实验结果作比较,并使用新模型模拟了后台阶两相流动.     

深远海线性内波条件下声场时间相关性的空间分布

深海声场特定的干涉结构导致其时间相关性的空间起伏,研究这种空间特性可以为水声信号的探测与处理提供重要参考。利用抛物方程声场仿真模型,联合Monte-Carlo数值方法计算分析了深远海线性内波条件下声场时间相关性的空间分布特性。与现有的研究相比,给出了时间相关性的距离和深度起伏特征。结果表明,当接收达到一定距离,声场时间相关性的空间分布具有与声场干涉条纹类似的结构,声场干涉越强,时间相关性越好。此外,声源频率和声速标准差的变化会引起时间相关性空间分布规律的改变,且会聚区传播模式下的改变强于深海声道传播模式。      

深海内波非线性薛定谔方程的研究

考虑以跃层为界的两层分层流体,在弱非线性条件下,从分层流体的动力学基本方程组出发,应用多重尺度方法推导出描述深海内波的非线性薛定谔方程,分析方程中频散和非线性系数,求出非线性薛定谔方程的孤子解,并通过数值实验验证了理论的正确性.     

浅海孤子内波对水平纵向相关性的影响*

基于理论和模型仿真对孤子内波存在下的水平纵向相关系数的变化进行了研究。理论上推导出浅海孤子内波存在情况下的声场水平纵向相关系数随时间的变化规律,并且运用二维抛物方程模型（Ram）验证理论推导结果。结果表明,孤子内波会导致水声纵向相关系数的变化,并且水平纵向相关系数的变化是由于简正波的干涉所导致。我们给出特定环境下的水平纵向相关系数随时间变化的结果,当环境中第一、二号简正波占主导地位时,水平纵向相关系数会呈现出周期变化的结果。     

南中国海海域存在孤立子内波条件下的 声场统计特性*

利用南海浅海海域低频声传播起伏实验中获取的水文数据,结合二维平流模型重构出声传播路径上的动态声速场,使用蒙特卡洛方法研究了有、无孤立子内波经过声传播路径条件下的声传播损失统计特性,并与实验结果进行了对比分析。仿真和实验结果表明：当孤立子内波经过声传播路径时,声传播损失起伏剧烈;与“下发上收”相比,“下发下收”情况下传播损失的概率分布更加分散。     

锯齿振荡的湍流模型数值模拟

用Dubois & Samain与Lichtenberg等人提出的解释托卡马克中锯齿振荡的湍流模型,通过求解耦合了锯齿触发条件的一维输运方程,模拟了欧姆加热情形T-10托卡马克中锯齿振荡行为,同时对得到的数值结果作了分析.与采用磁重联模型模拟得到的结果比较表明,采用湍流模型得到的数值结果与实验符合较好,而用磁重联模型则难以得到满意的结果.     

Freak waves in random oceanic sea states.

Freak waves are very large, rare events in a random ocean wave train. Here we study their generation in a random sea state characterized by the Joint North Sea Wave Project spectrum. We assume, to cubic order in nonlinearity, that the wave dynamics are governed by the nonlinear Schr?dinger (NLS) equation. We show from extensive numerical simulations of the NLS equation how freak waves in a random sea state are more likely to occur for large values of the Phillips parameter alpha and the enhancement coefficient gamma. Comparison with linear simulations is also reported.     

Observationally constrained modeling of sound in curved ocean internal waves: examination of deep ducting and surface ducting at short range

A study of 400 Hz sound focusing and ducting effects in a packet of curved nonlinear internal waves in shallow water is presented. Sound propagation roughly along the crests of the waves is simulated with a three-dimensional parabolic equation computational code, and the results are compared to measured propagation along fixed 3 and 6 km source/receiver paths. The measurements were made on the shelf of the South China Sea northeast of Tung-Sha Island. Construction of the time-varying three-dimensional sound-speed fields used in the modeling simulations was guided by environmental data collected concurrently with the acoustic data. Computed three-dimensional propagation results compare well with field observations. The simulations allow identification of time-dependent sound forward scattering and ducting processes within the curved internal gravity waves. Strong acoustic intensity enhancement was observed during passage of high-amplitude nonlinear waves over the source/receiver paths, and is replicated in the model. The waves were typical of the region (35 m vertical displacement). Two types of ducting are found in the model, which occur asynchronously. One type is three-dimensional modal trapping in deep ducts within the wave crests (shallow thermocline zones). The second type is surface ducting within the wave troughs (deep thermocline zones).     

Low-frequency horizontal acoustic refraction caused by internal wave solitons in a shallow sea

The effect of internal wave solitons on the sound field generated by a point source in a shallow sea is considered. In the framework of the theory of “horizontal rays and vertical modes,” the sound field pattern governed by the aforementioned hydrodynamic effect is investigated. It is shown that solitons can induce time-periodic focusing and defocusing of horizontal rays propagating at shallow angles to the internal wave front. This may result in the formation of “dynamical” horizontal sound channels, which, in its turn, results in considerable temporal fluctuations of the field along the acoustic track oriented along the internal wave front. For the sound field calculations, an approach is developed on the basis of the parabolic approximation in the horizontal plane and the mode representation in the vertical direction. The results obtained can be used for remote monitoring of internal wave packets in a shallow sea.     

Extreme wave formation in unidirectional sea due to stochastic wave phase dynamics

The authors consider a stochastic model based on the interaction and phase coupling amongst wave components that are modified envelope soliton solutions to the nonlinear Schrödinger equation. A probabilistic study is carried out and the resulting findings are compared with ocean wave field observations and laboratory experimental results. The wave height probability distribution obtained from the model is found to match well with prior data in the large wave height region. From the eigenvalue spectrum obtained through the Inverse Scattering Transform, it is revealed that the deep-water wave groups move at a speed different from the linear group speed, which justifies the inclusion of phase correction to the envelope solitary wave components. It is determined that phase synchronization amongst elementary solitary wave components can be critical for the formation of extreme waves in unidirectional sea states.     

Doppler spectrum of polarimetric scattering field from two-dimensional time-varying nonlinear sea surfaces

Polarimetric scattering model of second-order small-slope approxi-mation combined with “choppy wave" model (CWM) for describing nonlinear hydrodynamic interactions between ocean waves is utilized in this paper to investigate the influence of sea surface nonlinearities on backscattering coefficient as well as Doppler spectrum signatures including Doppler shift and spectral bandwidth. Simulation results show that at moderate to large incidence angles the Doppler shift and spectral bandwidth of the CWM nonlinear sea surfaces are significantly larger than those of linear sea surfaces, in particular at low grazing angles. In addition, Doppler signatures show distinct polarization dependence, and most importantly the cross-polarized Doppler signatures significantly differ from the co-polarized ones. It is also indicated that co-polarized Doppler shift increases obviously with wind speed increasing, whereas the cross-polarized Doppler shift looks less sensitive to wind speed variations. The difference of Doppler signatures between co- and cross-polarization is potentially valuable for ocean remote sensing applications, especially for observing very high winds.     

Solitary waves and rogue waves in a plasma with nonthermal electrons featuring Tsallis distribution

In this Letter, we discuss the electron acoustic (EA) waves in plasmas, which consist of nonthermal hot electrons featuring the Tsallis distribution, and obtain the corresponding governing equation, that is, a nonlinear Schrödinger (NLS) equation. By means of Modulation Instability (MI) analysis of the EA waves, it is found that both electron acoustic solitary wave and rogue wave can exist in such plasmas. Basing on the Darboux transformation method, we derive the analytical expressions of nonlinear solutions of NLS equations, such as single/double solitary wave solutions and single/double rogue wave solutions. The existential regions and amplitude of solitary wave solutions and the rogue wave solutions are influenced by the nonextensive parameter q and nonthermal parameter α. Moreover, the interaction of solitary wave and how to postpone the excitation of rogue wave are also studied.     

ETM图像大气邻近效应校正

准确地利用所获遥感图像反演地物的反射率,是提高遥感图像后续产品的质量和遥感定最化应用的前提.然而,由于大气对电磁波的散射作用,星载传感器所接收到的被观测目标的电磁辐射信号包含了来自背景地物的贡献,产生了邻近效应,使得所获遥感图像模糊不清,对比度降低,反演的地物反射率精度下降,从而降低了遥感图像后续产品的质量和定量遥感的精度.以ETM图像为实验数据,根据大气辐射传输模型,采用点扩散率函数法进行了ETM图像大气邻近效应校正实验.实验结果表明,经过大气邻近效应校正后,ETM图像的对比度明显增大,图像清晰,可识别细节增加,反演的地物反射率更加准确.