水下对空成像中的波面估计

水下相机向上对海洋表面场景成像时会形成扭曲的图像,为了矫正图像并重构场景,提出了一种基于水下对空成像的波面计算方法,通过分析光线在空一水交界面的传输特性,建立了前向散射和消光边界模型,并在消光边界处估计出表面的径向斜率。从而为海浪高度的计算奠定了基础。     

零视距地物长波红外特征场景仿真研究

为仿真地物长波红外场景图像,根据地表温度随时间变化的规律,并结合气象状况、背景材质、热特性参量、热状态等参数,在对太阳辐射、大气长波辐射、大气温度和地表热传导等影响地表温度变化的因素进行分析的基础上,建立了基于热平衡理论和热传导过程的方程。解算出多种常见地表一日之中的温度变化情况,并将其应用于由相同景物可见光纹理图像反演出的相应红外纹理图像中。在考虑景物表面自身发射、反射的辐射计算模型的前提下,生成了具有相似红外纹理细节的地表红外场景。结果表明,该方法可生成接近真实感的红外场景,有效地模拟仿真地物的长波红外特征。     

The surface mode supported by an asymmetrical waveguide containing left-handed materials

The propagating characteristics of surface mode supported by an asymmetrical three-layer waveguide containing left-hand materials are studied in detail. The forward and the backward surface modes have been identified respectively through the slope of dispersion curve and the total energy flux of the structure. Research shows that anti-symmetry mode corresponds to forward surface wave. Furthermore, to symmetry mode, the type of surface mode depends on the distribution of the electric field in the left-handed layer. It is found that, the conversion between backward surface wave and forward surface wave is realizable by varying the thickness of the left-handed layer and the permittivity of the substrate.     

Nonlinear dynamics of magnetohydrodynamic flows of a heavy fluid on slope in the shallow water approximation

Magnetohydrodynamic equations for a heavy fluid over an arbitrary surface are studied in the shallow water approximation. While solutions to the shallow water equations for a neutral fluid are well known, shallow water magnetohydrodynamic (SMHD) equations over a nonflat boundary have an additional dependence on the magnetic field, and the number of equations in the magnetic case exceeds that in the neutral case. As a consequence, the number of Riemann invariants defining SMHD equations is also greater. The classical simple wave solutions do not exist for hyperbolic SMHD equations over an arbitrary surface due to the appearance of a source term. In this paper, we suggest a more general definition of simple wave solutions that reduce to the classical ones in the case of zero source term. We show that simple wave solutions exist only for underlying surfaces that are slopes of constant inclination. All self-similar discontinuous and continuous solutions are found. Exact explicit solutions of the initial discontinuity decay problem over a slope are found. It is shown that the initial discontinuity decay solution is represented by one of four possible wave configurations. For each configuration, the necessary and sufficient conditions for its realization are found. The change of dependent and independent variables transforming the initial equations over a slope to those over a flat plane is found.     

Emergence of seismic metamaterials: Current state and future perspectives

Following the advent of electromagnetic metamaterials, researchers working in wave physics have translated concepts of engineered media to acoustics, elastodynamics and diffusion processes. In elastodynamics, seismic metamaterials have emerged in the last decade for soft soils structured at the meter scale, and have been tested with full-scale experiments on holey soils. Born in the soil, seismic metamaterials have emerged from the field of tuned-resonators buried in the soil, around building's foundations or near the soil-structure interface as local seismic isolators. Forests of trees have been interpreted as above-surface resonators, and coined natural seismic metamaterials. We first review some advances made in seismic metamaterials and dress an inventory of which material parameters can be achieved and which cannot, from the effective medium theory perspective. We envision future developments of large scale auxetic metamaterials for building's foundations, above surface resonators for seismic protection and metamaterial-like transformed urbanism at the city scale.     

HREELS study of the oxidation of AL(111) between 300 and 20 K

The oxidation of Al(111) was studied by HREELS for sample temperatures of 300, 120 and 20 K and for oxygen coverages below and above the monolayer coverage. The experimental results are explained within the following model: At 300 K oxygen is chemisorbed both above and below the first Al layer, giving rise to losses between 65 and 80 meV. This oxygen double-layer grows in islands and stabilises the oxygen against diffusion into the bulk. At 120 K chemisorbed oxygen above the Al surface and a distribution of subsurface oxygen including oxidic species with losses between 105 and 120 meV are found. At 20 K above-surface oxygen is no longer stable enough to be measured by HREELS; sticking coefficient and rate of oxidation are increased.     

Drift velocity in an amplification zone of wind waves

Drift wind flow in the amplification zone of a wind wave is investigated experimentally. It is found that the wind drift diminishes in proportion to the wave steepness on the forward slope of a wave in the region of a water surface deformed by vortices in a viscous layer of air. Vortices delaying the drift are found in the viscous layer on the back slope of a wave where the shift velocity of water diminishes.     

Transmission scatter cross sections across two-dimensional random rough surfaces – full wave solutions and comparison with numerical results

Abstract

In this paper, the full wave expressions for the bistatic transmission scattering cross sections across two-dimensional random rough surfaces are obtained. The full wave analysis accounts for the surface height/slope correlations. Analytical and numerical comparisons of the full wave solutions with the small perturbation and physical optics solutions are made for isotropic random rough surfaces. The full wave results are also compared with the numerical results based on Monte Carlo simulations of one-dimensional random rough surfaces. Detailed consideration is given to illustrating the relationship between these full wave solutions and the original full wave solutions including the impact of accounting for the height/slope correlations in this analysis.     

Specially profiled reflector for spherical front beams focusing

Focusing the power emitted from a source with spherical wave front by a specially profiled reflecting surface has been attempted using the simple ray tracing method. The reflector (mirror) surface profile that achieves such power redistribution is obtained from solving an ordinary first order differential equation using common numerical techniques. Furthermore, the proposed methodology can be useful to design complex reflectors for the electomagnatic and the acoustic systems, which finds broad engineering, industrial and biomedical applications.     

On the scattering of cascade radio emission by rough lunar surface during radiation emergence into vacuum

The scattering of the radio emission of an almost horizontal cascade by the rough lunar surface during radiation emergence into vacuum is considered in the Born approximation. The point approximation for the shower disk size is considered taking into account its evolution along the track. The lunar surface was set either as a sinusoidal one in the directions perpendicular to and coinciding with the shower disk velocity or as having an arbitrary shape at the point of particle incidence. Scattering associated with wave phase increment and with variations of slope of the normal to the rough surface for various rays forming the total field of the scattered wave is considered.     

Ray tracing/correlation approach to estimation of surface-based duct parameters from radar clutter

This paper describes a technique to estimate surface-based duct parameters by using a simple ray tracing/correlation method. The approach is novel in that it incorporates the Spearman rank-order correlation scheme between the observed surface clutter and the surface ray density for a given propagation path. The simulation results and the real data results both demonstrate the ability of this method to estimate surface-based duct parameters. Compared with the results obtained by a modified genetic algorithm combined with the parabolic wave equation, the results retrieved from the ray tracing/correlation scheme show a minor reduction in accuracy but a great improvement on computation time. Therefore the ray tracing/correlation method might be used as a precursor to more sophisticated and slower techniques, such as genetic algorithm and particle filters, by narrowing the parameter search space and providing a comprehensive and more efficient estimation algorithm.     

具有不规则自由表面的粘弹性介质中声波模拟的有限差分算法

声波在具有不规则自由表面的粘弹性介质传播的数值模拟工作在地震勘探,地震预测中非常重要,特别是当模型有起伏的自由界面和较强的衰减特性时更是如此。基于镜像方法和直接方法,本文发展了一种二维有限差分算法,可以模拟不规则自由表面引起的声波散射问题。该方法将自由表面条件与速度应力方程结合求解粘弹性波动方程,在垂直和水平自由段及拐点处设置相应的边界条件。该方法假设自由表面穿过剪切应力和相应参数的网格点。为了提高计算精度,分别计算了水平和垂直方向上应力镜像值,而对质点速度,采用先水平方向后垂直方向分析进行镜像计算和更新。将粘弹性水平自由表面镜像方法和不同倾斜度平滑自由表面的改进方法的数值结果进行对比,并验证了算法的精度。     

Resonant generation of internal waves on a model continental slope

We study internal wave generation in a laboratory model of oscillating tidal flow on a continental margin. Waves are found to be generated only in a near-critical region where the slope of the bottom topography matches that of internal waves. Fluid motion with a velocity an order of magnitude larger than that of the forcing occurs within a thin boundary layer above the bottom surface. The resonant wave is unstable because of strong shear; Kelvin-Helmholtz billows precede wave breaking. This work provides a new explanation for the intense boundary flows on continental slopes.     

On the surface cascade radio emission escape into vacuum through the rough lunar surface

The radio emission escape of an almost horizontal cascade through the rough lunar surface into vacuum is considered. The point approximation is accepted for the shower disk size, taking into account its evolution along the cascade track. The lunar surface is assumed to be flat, but having a single perturbation such as a cylindrical “hill” perpendicular to the shower disk motion direction. The perturbation is not assumed to be small. Diffraction associated with the wave phase progression and variations of the slope of the normal to the rough surface for various rays forming the total wave field in vacuum is taken into account.     

Observation of intermittency in wave turbulence

We report the observation of intermittency in gravity-capillary wave turbulence on the surface of mercury. We measure the temporal fluctuations of surface wave amplitude at a given location. We show that the shape of the probability density function of the local slope increments of the surface waves strongly changes across the time scales. The related structure functions and the flatness are found to be power laws of the time scale on more than one decade. The exponents of these power laws increase nonlinearly with the order of the structure function. All these observations show the intermittent nature of the increments of the local slope in wave turbulence. We discuss the possible origin of this intermittency.     

Study on the shadowing effect for optical wave scattering from randomly rough surface

Based on the Kirchhoff approximation for rough surface scattering and by calculating the shadowing function of the rough surface, the formula of the scattering cross section of the dielectric rough surface is presented with consideration of the shadowing effect for the optical wave incidence. It is obtained that in comparison with the conventional Kirchhoff solution, the shadowing effect should not be neglected for the optical wave scattering from the rough surface. The influence of the shadowing effect for different incidence angle, surface root mean square slope, and surface roughness on the scattering cross section is discussed in detail.     

海面波浪谱对深海低频环境噪声的影响

提出了一种适用于深海低频环境噪声的波浪谱,通过声压谱和波浪谱的理论关系,分析了深海低频噪声在百赫兹以下的谱特征,解释了不同频段噪声谱的主要产生机理。将深海传播条件下海面波浪谱与海面风速相结合,利用波浪发声理论得到一种低频海洋环境噪声理论表示方法。仿真结果表明,波浪谱决定着辐射噪声谱的强度和斜率,本模型得到的理论噪声谱可以对低频海洋环境噪声进行预报。2016年的深海实验观测数据分析显示,统计的环境噪声谱级在1 Hz至100 Hz频段范围内大于70 dB,并且噪声谱在低频段呈倒“N”型,在3⁴ Hz处为噪声谱的谷值,噪声级为70 dB,在50 Hz处为噪声谱的峰值,噪声级为92 dB,通过理论计算和实验对比,相关系数为0.95,理论结果和实验测量对比结果符合较好。     

Simulating regular wave propagation over a submerged bar by boundary element method model and Boussinesq equation model

Numerical models based on the boundary element method and Boussinesq equation are used to simulate the wave transform over a submerged bar for regular waves.In the boundary-element-method model the linear element is used,and the integrals are computed by analytical formulas.The Boussinesq-equation model is the well-known FUNWAVE from the University of Delaware.We compare the numerical free surface displacements with the laboratory data on both gentle slope and steep slope,and find that both the models simulate the wave transform well.We further compute the agreement indexes between the numerical result and laboratory data,and the results support that the boundary-element-method model has a stable good performance,which is due to the fact that its governing equation has no restriction on nonlinearity and dispersion as compared with Boussinesq equation.     

射线追踪、辛几何算法与波场的数值模拟

论述了射线追踪、辛几何算法与波场的数值模拟之间的关系,说明了辛几何算法长时间守恒性质及运用辛几何算法进行射线追踪的必要性.对线性层状模型,利用辛几何算法和Maslov渐近理论对波场进行了数值模拟,并与解析解进行了比较.