海底密度梯度对底回波信号空间相关特性的影响研究

研究了海底表层沉积物的垂直密度梯度对底回波空间相关特性强度的影响。底回波散射截面与空间相关特性间的关系为:随入射角的增大,回波散射截面下降得越快,底回波空间相关特性越强。因此,将密度-深度模型引入海底高频回波模型进行回波仿真,计算回波的空间相关函数,通过分析密度梯度对底回波散射截面大小的影响,考察其对底回波空间相关特性强度的影响。从仿真分析结果可以看出,海底表层沉积物密度梯度对空间相关特性的影响较为显著,随着密度梯度的增大,回波散射截面变大且随入射角的增大下降趋势变快,空间相关半径变长,空间相关特性变强。      

海底回波空间相关特性研究

阐述了海底回波空间相关特性的基本原理,引入了Kirchhoff模型进行回波仿真并计算空间相关函数,分析了密度比、声速比、衰减系数、海底表面不平整性空间谱的谱系数和谱指数等底质参数对回波空间相关特性的影响。通过建立上述底质特性参数与沉积物平均粒径的φ值之间的表达式,得到了回波空间相关特性随沉积物平均粒径的变化趋势:随着φ值的增大,空间相关函数的主瓣宽度变窄。将仿真结果与2007年10月份采集的海试数据进行比较,发现在海底底质相同的情况下,二者的空间相关函数波形符合得较好。      

Spatial correlation analysis of sea-bottom backscattering

##正##The principle of the spatial correlation characteristics of sea-bottom backscattering was expounded.The Kirchhoff model is introduced to simulate sea-bottom backscattering and the spatial correlation is calculated.Some sediment characteristic parameters,such as the ratio of sediment mass density to water mass density,the ratio of sediment sound speed to water sound speed,the attenuation coefficient of sound waves,the strength and exponent of seabottom interface roughness spectrum,are considered while simulating and their influences on spatial correlation are analyzed.By establishing the expressions of the sediment characteristic parameters with the logarithmic sediment grain size,the variation tendency of the spatial correlation versus the logarithmic sediment grain size is obtained.The width of the major lobe of the spatial correlation function narrows as the logarithmic sediment grain size increases.The comparisons between simulated data and in-situ data collected in October 2007 have proven that their spatial correlation function waveforms agree well under the same sediment characteristics.     

浅海小掠射角的海底界面声反向散射模型的简化

在浅海,尤其是负梯度声速剖面和海面较为平静的浅海波导,海底界面反向散射是浅海混响的主要来源.经验散射模型只适用于分析浅海混响平均强度衰减特性,而基于物理机理建立的反向散射模型克服了这一缺陷,但同时也引入了其受地声模型约束的问题.本文结合了海底反射系数的三参数模型,对浅海远场海底反向散射模型进行了简化,以减少地声模型的输入参数.理论分析了海底反射系数的相移参数可以描述海底对声场的散射作用,无需任何海底地声参数的先验知识.通过对海底反向散射模型近似简化,结果表明在临界角附近和甚小掠射角范围内的海底粗糙界面反向散射模型的角度特性和强度特性受海底沉积层的影响不同:在临界角附近,海底反向散射的角度特性受海底反射系数的相移参数加权,而其散射系数则近似与相移参数无关;对于甚小掠射角,海底反向散射的角度特性近似与海底反射系数的相移参数无关,其散射系数则近似与相移参数的4次方成正比.     

Study of scattering from turbulence structure generated by propeller with FLUENT

In this article, the turbulence structure generated by a propeller is simulated with the computational fluid dynamics (CFD) software FLUENT. With the method of moments, the backscattering radar cross sections (RCS) of the turbulence structure are calculated. The scattering results can reflect the turbulent intensity of the wave profiles. For the wake turbulence with low rotating speed, the scattering intensity of HH polarization is much smaller than VV polarization at large incident angles. When the turbulence becomes stronger with high rotating speed, the scattering intensity of HH polarization also becomes stronger at large incident angles, which is almost the same with VV polarization. And also, the bistatic scattering of the turbulence structure has the similar situation. These scattering results indicate that the turbulence structure can also give rise to an anomaly compared with traditional sea surface. The study of electromagnetic (EM) scattering from turbulence structure generated by the propeller can help in better understanding of the scattering from different kinds of waves and provide more bases to explain the anomalies of EM scattering from sea surfaces.     

Study of the Electromagnetic Scattering from the Very Rough Fractal Sea Surface

In this paper, based on the fundamental formulae of the first-order and second-order Kirchhoff approx-imation mad with consideration of the shadowing effect, the backscattering enhancement of the one-dimensional very rough fractal sea surface with Pierson-Moskowitz spectrum is studied under the second-order Kirchhoff approximation at microwave frequency. The numerical results are compared with those of the first-order Kirchhoff approximation and integral equation method. The dependencies of the bistatic scattering cross section and the backscattering enhancement on the incident angle, fractal dimension, and windspeed over the sea surface are analyzed in detail.     

Behaviour of scattering from a rough surface at small grazing angles

The particular problem of wave scattering at low grazing angles is of great interest because of its importance for the long-distance propagation of radio waves along the Earth's surface, radar observation of near surface objects, as well as solving many other fundamental and applied problems of remote sensing. One of the main questions is: how do the scattering amplitude and specific cross section behave for extremely small grazing angles? We consider the process of wave scattering by a statistically rough surface with the Neumann boundary condition. This model corresponds to sound scattering from a perfectly 'hard' surface (for example, the interface between air and the sea surface) or 'vertically' polarized electromagnetic waves scattered by a perfectly conducting one-dimensional (i.e. cylindrical) surface when the magnetic field vector is directed along the generating line of this cylindrical surface. We assume that the surface roughness is sufficiently small (in the sense of the Rayleigh parameter) and the surface is rigorously statistically homogeneous and therefore, infinite. We confine ourselves only to the first-order approximation of small perturbation theory and therefore consider every act of wave scattering in the Born approximation when the Bragg scattering process takes place. Only one resonant Fourier component of surface roughness is responsible for the scattering in a given direction. However, we take into account the attenuation of incident and scattered waves due to the multiple scattering processes on the path 'before' and 'after' a scattering event in a given direction. Also we consider every one of these multiple scattering events only in the Born approximation. The main result we have obtained is that for small grazing angles the scattering cross section of the diffuse component decreases as the second power of the grazing angles with respect to the incident and scattered directions, and as the fourth power of the grazing angle for the backscattering (radar) situation. Generalizing our results from plane-wave scattering to finite beams allows us to obtain the criterion on the beamwidth. For sufficiently narrow beams the multiple scattering processes do not play any role because of a short 'interaction path', and only single Bragg scattering determines the scattering amplitude (which does not tend to zero for small grazing angles). However, for sufficiently wide beams the result obtained for infinite plane waves becomes valid: due to the above-mentioned multiple scattering processes, the scattering amplitude tends to zero for small grazing angles. Consequently, the behaviour of the scattering cross section for small grazing angles depends on the radiation pattern width of the transmitting and receiving antennae: for sufficiently wide beams the scattering cross section decreases to zero at small grazing angles, but for narrow beams it tends to the finite non-zero value.     

Multiple scattering of light transmission in a smoke layer

The visibility in a fire scene decreases because of the existence of smoke produced by the flammable materials. With the growth of smoke concentration, the relationship between light and smoke becomes complicated due to the multiple scattering. In this paper, the radiative transfer equation (RTE) that considers the multiple scattering was applied to calculate the light transmission in a smoke layer. As input parameters of RTE, the single scattering albedo, asymmetry parameter and extinction cross section of single smoke agglomerate were calculated by the discrete dipole approximation (DDA) method. The effects of smoke agglomerate diameter, number density of smoke layer, and the incident light wavelength were considered. The results show that the light transmitted flux decreases with the growth of smoke diameter and number density, and increases with the growth of wavelength. The smoke diameter is dominant among the three parameters, and the light transmitted flux tends to be stable when the wavelength reaches a certain value.     

Absorption/Scattering of Scalar Waves by Dilatonic Black Hole

We investigate the scattering and absorption cross sections of scalar waves by Garfinkle-Horowitz-Strominger (GHS) black hole spacetime with partial wave method. We find that the total absorption cross section oscillates around the geometric optical value, and the scattering angle width becomes narrower and the damping oscillation pattern becomes more complex as l increases. With fixed frequency ω, the main glory peak value becomes higher and the glory width becomes wider as the magnetic parameter a increases.     

Polarization dependency of enhanced multipath radar backscattering from an ocean-like surface

Previous analyses of electromagnetic scattering from a two-scale ocean surface assumed the tilts on a large-scale surface to be small. This means that multiple scattering between large-scale roughnesses is insignificant. If the tilts are not small, multipath-enhanced backscattering may occur due to quasi-specular reflections between the opposite slopes of a large-scale surface component. We have considered the simplest situation; this involves one reflection from the large-scale component and one single-scattering from the small-scale component. The coherent addition of this process to the reciprocally reverse one creates multipath-enhanced backscattering. The relative gain in the HH backscattering cross section up to the level of the VV signal was obtained for surface-wave slopes of about 30° and for large incidence angles. This gain occurs because the VV signal experiences an extinction during reflection at incident angles close to the pseudo-Brewster angle. The presented model provides insight into one scattering mechanism that is possibly responsible for the departure of radar sea experimental data from predictions by the conventional two-scale model.     

Backscattering coefficients and mean penetration depths of 1–4 keV electron scattering in solids

In the present study, we have performed Monte Carlo simulation of 1–4 keV electrons impinging on semi-infinite Al and Au to determine the transport cross-section, the backscattering coefficient and the mean penetration depth using a new approximation of the differential elastic scattering cross section. The mean number of the wide angle collisions suffered by the electron before slowing down to rest, and the backscattering coefficient are analytically calculated using Vicanek and Urbassek theory. The analytical results are compared with the numerical ones obtained from Monte Carlo simulation. The present results are found to reveal good agreement with experimental results. PACS 68.37.-d; 68.37.Nq     

Electromagnetic Scattering from One-Dimensional Time-Varying Sea Surface with Gaussian-Like Beam Incidence

A widely used iterative technique named the Method of Ordered Multiple Interactions is given for calculat-ing the Gaussian-like beam scattering from a time-varying sea surface with the Pierson-Moskowitz (P-M) spectrum. Thisis done by solving the magnetic field integral equation for the current induced on an infinite rough surface. Following thediscretization of the integral equation, the unknown currents can be determined more rapidly with the LU decomposition.Numerical results are presented with emphasis on the electromagnetic backscattering at low grazing angle incidence. Itis shown that the backscattering cross section is proportional to the nearly fourth power of the grazing angle for thebackscattering cross section with a different beam waist, surface length, and velocity of the wind are discussed     

溶剂密度对β胡萝卜素拉曼散射截面的影响

测量了含有9个CC共轭双键的链状多烯类生物分子β胡萝卜素在9种溶剂中的紫外—可见吸收和拉曼光谱.结果表明,β胡萝卜素C—C,CC键拉曼散射截面随溶剂体密度增加而线性增加,其机理是β胡萝卜素分子在体密度大的溶剂中受浮动干扰小,分子结构有序性高,π电子离域扩展,产生强的相干弱阻尼CC键振动,而产生大的拉曼散射截面.密度对分子拉曼散射截面影响研究还没见过报道,本文为多烯类链状分子的光散射理论研究提供了实验线索,也对多烯类链状分子性能研究及光电器件研制有参考价值. 关键词： 拉曼散射截面 多烯分子 吸收光谱 分子结构有序     

He同位素原子与HBr分子碰撞的微分截面

基于作者构造的He-HBr体系的各向异性势,采用密耦方法计算了³He,⁴He,⁶He和⁷He与HBr分子在碰撞能量分别为40和75meV时的微分截面,详细讨论了入射氦同位素对微分截面的影响.结果表明：在相同碰撞能量时,随着同位素氦原子质量的增加,总微分截面在0° 时的角分布逐渐增大,同一级衍射振荡极小值位置逐渐向小散射角方向移动;弹性与总非弹性截面交界角逐渐减小,总非弹性截面逐渐增加.碰撞能量越低,入射同位素He原子的 关键词： 同位素效应 微分截面 各向异性势 He-HBr体系     

Electromagnetic Scattering from One-Dimensional Time-Varying Sea Surface with Gaussian-Like Beam Incidence

A widely used iterative technique named the Method of Ordered Multiple Interactions is given for calculating the Gaussian-like beam scattering from a time-varying sea surface with the Pierson-Moskowitz (P-M) spectrum. This is done by solving the magnetic field integral equation for the current induced on an infinite rough surface. Following the discretization of the integral equation, the unknown currents can be determined more rapidly with the LU decomposition. Numerical results are presented with emphasis on the electromagnetic backscattering at low grazing angle incidence. It is shown that the backscattering cross section is proportional to the nearly fourth power of the grazing angle for the plane and beam incidence. This is consistent with the result given in some references. The angular distributions of the backscattering cross section with a different beam waist, surface length, and velocity of the wind are discussed.     

Scattering of an electromagnetic wave from a slightly random dielectric surface: Yoneda peak and Brewster angle in incoherent scattering

Abstract

The scattering of an electromagnetic wave from a two-dimensional, slightly rough dielectric surface is studied based on the stochastic functional approach. It is shown that in the case of TM(p)-polarized incidence there exists a zero in the incoherent scattering at the angle we call the ‘Brewster scattering angle’, which depends on the incident angle in contrast to the Brewster angle of coherent reflection which is independent of the incident angle, that a ‘quasi-anomalous scattering’ can generally occur in the optically denser medium at the critical angle of total reflection in both TE(s)- and TM(p)-polarized incidence, regardless of which side of the random surface is illuminated, and that the Yoneda peak in the x-ray scattering can be interpreted as a special case of the quasi-anomalous scattering which becomes sharper when the relative refractive index becomes closer to unity as in the x-ray region. Cross-polarized scattering and enhanced backscattering due to the second-order effect are also calculated.     

基于Mie散射理论的C@H_2O复合粒子散射特性研究

大气中大量存在的复合粒子会对激光传输效率产生很大影响。由于空气中水蒸气含量较高,以C作为凝结核外层包裹以水的核壳结构微粒对光传输具有明显的散射效应。本文应用Mie散射理论对C@H_2O核壳结构微粒的散射特性进行了理论分析和数值计算,首先给出了不同入射波长、核粒子半径以及水膜厚度条件下散射强度分布变化曲线;其次给出了不同入射波长、核粒子半径以及水膜厚度条件下偏振变化情况;最后讨论了光学截面与粒子半径之间的关系。结果表明各参数对前向散射强度影响较大,入射波长越大散射强度越弱,C核半径增大粒子的前向散射增强,水膜厚度增大粒子的前向散射增强,而后向散射无明显影响;入射波长较大时,粒子在多个角度出现线偏振光,入射波长增大、碳核半径变大、水膜厚度增大,偏振度峰值都会增多;随着入射波长的增大,散射截面最大峰值位置向着半径增大的方向移动,并伴随一定的振荡现象,散射和消光截面在碳核半径为0.1μm左右达到最大值。     

相对论离子与氦二聚体碰撞电离的理论研究

基于对称程函近似,从理论角度研究了相对论离子与氦二聚体碰撞双电离的直接碎裂通道:由入射离子与两个原子的作用造成的双电离。从不同的近似角度出发,建立了两个理论模型,第一个模型将氦二聚体的两个原子看做一个整体,而另外一个模型完全忽略了两个氦原子之间的相互作用。模型的数值计算结果表明,总截面与氦二聚体的空间取向有很强的依赖关系,当氦二聚体与入射离子束流平行时,总截面达到最大值,在垂直时为最小值。进一步研究发现,模型一的结果在氦二聚体核间距较小时更为合理,核间距较大时,模型二更为精确。     

双尺度动态分形粗糙海面的电磁散射及多普勒谱研究

利用更为符合海面实际散射场计算的双尺度法分析并计算了一维动态分形粗糙海面的后向散射截面，同时与微扰法有关结果进行了比较.研究了动态分形海面后向散射信号的多普勒谱分形特征，分析了不同极化状态下后向散射截面和多普勒谱谱宽随入射角及分维的变化规律，最后详细讨论了多普勒谱中心频率与海面Bragg谐振频率间的关系. 关键词： 双尺度模型 分形海面 电磁散射 多普勒谱     

Scattering from an elliptical cylindrical plasma for electromagnetic waves with wavelength much greater than the dimensions of the plasma cross-section

The distribution of the electric potential inside and outside a cold plasma column with elliptical cross-section when electromagnetic waves scatter from it are obtained. It is assumed that the wavelength of incident wave is much greater than the dimensions of the cross section of the plasma column. The electrical potential inside and outside a magnetized elliptical plasma column for an anisotropic scattering process are obtained. The graphs of the ratio of the electric field inside the plasma and the incident electric field versus the incident angle, the geometrical dimension of the elliptical plasma column and cyclotron frequency are presented separately. Finally, scattering of the same waves from an elliptical cylindrical inhomogeneous isotropic cold plasma is also investigated.