光束在尾流气泡中传输的复散射效应

为了了解光束在尾流气泡中的传输特性,为前向光尾流的探测提供理论依据,研究了光束在尾流中传输时传播方向上和横截面方向上的辐射强度分布特性.基于辐射传输方程的小角度近似解,得到了探测截面上的约化强度和漫射强度的表达式,其中漫射强度表征了复散射的强弱;针对典型的尾流气泡分布,通过数值计算分析了光束传输方向上的约化强度和漫射强度与接收视场角、光学厚度和光束大小的关系,也计算分析了光束横截面方向上的辐射强度随光束大小和横向距离的变化关系.结果表明,光束在尾流气泡中传输时复散射效应明显,且复散射的强弱与接收视场角、光束直径、光学厚度和横向距离密切相关.     

光束在尾流气泡中传输的复散射效应

为了了解光束在尾流气泡中的传输特性,为前向光尾流的探测提供理论依据,研究了光束在尾流中传输时传播方向上和横截面方向上的辐射强度分布特性.基于辐射传输方程的小角度近似解,得到了探测截面上的约化强度和漫射强度的表达式,其中漫射强度表征了复散射的强弱|针对典型的尾流气泡分布,通过数值计算分析了光束传输方向上的约化强度和漫射强度与接收视场角、光学厚度和光束大小的关系,也计算分析了光束横截面方向上的辐射强度随光束大小和横向距离的变化关系.结果表明,光束在尾流气泡中传输时复散射效应明显,且复散射的强弱与接收视场角、光束直径、光学厚度和横向距离密切相关.     

尾流气泡幕光学特性的数理模型

对实验室模拟的尾流以及真实尾流气泡幕中气泡的运动规律进行了分析和计算,并提出了气泡幕中气泡随半径分布规律的数学物理模型,引入了最可几半径的概念从单个气泡对激光散射的影响推导出了气泡幕对激光的散射效应的数学表达式,并利用所得到的表达方式在实验室条件下进行了相关计算所建立的理论模型对于气泡分布的计算结果同实验结果相比,一致性相当好在气泡幕对激光散射的影响中,首次对激光偏振化特性的影响进行了区别计算计算结果表明:激光的两种偏振化状态对气泡幕散射的影响从粗糙结构来看没有明显差异,但其精细结构有所不同对不扩束和扩束两个条件下的计算明确显示出,扩束条件下散射光的强度是不扩束条件下的二倍以上,但二者的精细结构并没有可见的差别.     

尾流后向散射光功率衰减特性研究

依据Mie理论光散射在0°和180°有明显增强的结论,应用自行设计的模拟尾流后向散射光功率实验系统在632.8 nm He-Ne激光器直接照明和扩束后照明两种条件下,对模拟尾流后向散射光功率衰减特性进行了研究.结果表明,用后向散射光功率进行探测时,考虑到散射光在水中传播的衰减,对近距离分布的气泡,使用扩束光照明效果较好;对较远距离分布的气泡,使用细光束照明效果较好,其可探测的距离要比扩束光远.     

采用脉冲激光探测模拟的舰船气泡尾流

分析了基于后向散射的脉冲激光舰船气泡尾流探测方法,设计并研制了基于该探测方法的实验装置,并在大型水池中对模拟的气泡尾流进行了探测,实验结果表明,该实验装置可以较好地探测到模拟的舰船气泡尾流.     

尾流中气泡对光传播的影响

从几何光学的角度出发,针对气泡直径与光束直径之间的大小关系,分两种情况,推得了单个气泡的散射光强变化情况,以及气泡速度与散射光强之间的关系.得出散射光强的变化频率与气泡半径和速度存在一定的关系,光束的直径与气泡直径的大小关系决定光强变化的强弱.在此理论基础上,模拟了光强的变化,并与实验所得曲线相比较,验证了实际与理论的相符性.同时用短曝光、连续拍摄的数码相机对气泡进行拍摄,进一步验证了理论推导的正确性.     

尾流气泡幕散射光的性质

用波长为630 nm的激光器作为光源,硅二极管直接接收气泡幕的散射光.对不同气泡幕的散射光作不同点的均值,同时用Matlab语言程序计算散射光不同点自相关,充分证明了散射光的随机性.从散射光的统计性质来看,它的均值和相关性与气泡幕参数之间存在一定的规律,使光探测尾流具有可实现性.     

舰船远场尾流气泡分布特性的数值模拟

为了了解舰船尾流中气泡的分布特征,为激光探测舰船尾流提供依据,建立了计算热分层环境中舰船远场尾流中气泡数密度分布的数学模型,该模型由抛物化的RaNS方程及气泡输运方程组成,采用k－ε两方程湍流模式.利用该模型数值模拟了舰船尾流中的气泡数密度分布特征、气泡尾流宽度扩展规律,气泡数密度衰减规律,并与实测结果进行了比较,二者定性符合得较好.数值模拟表明舰船尾流中半径为10~30 μm的气泡存活时间最长, 为了确保能在距船尾5 km处还能探测到舰船气泡尾流,其探测的对象应以半径10~30 μm的气泡为主,并且探测准确度至少应能达到识别出初始截面上气泡数密度的3%.     

舰船尾流气泡后向光学检测方法研究

为探索舰船尾流后向光学检测方法,研究了尾流气泡对水中激光脉冲后向散射特性的影响。首先基于Fournier Forand 体积散射函数,利用蒙特卡罗(Monte-Carlo)方法理论分析了近距离尾流气泡对激光脉冲后向散射特性的影响。然后,采用蓝绿激光脉冲作为光源,实验研究了模拟尾流气泡对激光脉冲后向散射信号的影响。研究表明,尾流气泡的存在会使得激光脉冲后向散射信号前沿位置在时域左移,后沿位置在时域右移,信号时域宽度增加,能量增强,峰值增大且位置在时域左移。最后根据研究结果提出了一种基于激光脉冲后向散射信号特征变化的舰船尾流气泡后向检测方法。     

基于图像处理的模拟尾流气泡幕研究

利用图像处理技术,对模拟的尾流气泡幕进行了定性和定量分析.压强较小时,位于不同层面上气泡的重叠少,通过二值图像处理可清楚地观察气泡的尺度大小及密度分布;而当压强较大时,不同层面气泡重叠幅度大,大部分气泡的边界难以判断,不能够得到气泡尺度参量及密度分布等信息.利用区域分割技术对获取的气泡幕灰度图像进行伪彩色显示,定性分析了气泡幕中气泡尺度、分布等信息.基于形状分析的等效直径、气泡幕剩余面积和剩余面积变化显示,不同压强下气泡的等效直径分布均在5～100像素范围,平均等效直径为55像素,压强较小条件下小气泡比例较大.     

Multiple-scattering in radar systems: A review

Although extensively studied within the lidar community, the multiple scattering phenomenon has always been considered a rare curiosity by radar meteorologists. Up to few years ago its appearance has only been associated with two- or three-body-scattering features (e.g. hail flares and mirror images) involving highly reflective surfaces.Recent atmospheric research aimed at better understanding of the water cycle and the role played by clouds and precipitation in affecting the Earth's climate has driven the deployment of high frequency radars in space. Examples are the TRMM 13.5 GHz, the CloudSat 94 GHz, the upcoming EarthCARE 94 GHz, and the GPM dual 13-35 GHz radars. These systems are able to detect the vertical distribution of hydrometeors and thus provide crucial feedbacks for radiation and climate studies. The shift towards higher frequencies increases the sensitivity to hydrometeors, improves the spatial resolution and reduces the size and weight of the radar systems. On the other hand, higher frequency radars are affected by stronger extinction, especially in the presence of large precipitating particles (e.g. raindrops or hail particles), which may eventually drive the signal below the minimum detection threshold. In such circumstances the interpretation of the radar equation via the single scattering approximation may be problematic. Errors will be large when the radiation emitted from the radar after interacting more than once with the medium still contributes substantially to the received power. This is the case if the transport mean-free-path becomes comparable with the instrument footprint (determined by the antenna beam-width and the platform altitude).This situation resembles to what has already been experienced in lidar observations, but with a predominance of wide- versus small-angle scattering events. At millimeter wavelengths, hydrometeors diffuse radiation rather isotropically compared to the visible or near infrared region where scattering is predominantly in the forward direction. A complete understanding of radiation transport modeling and data analysis methods under wide-angle multiple scattering conditions is mandatory for a correct interpretation of echoes observed by space-borne millimeter radars.This paper reviews the status of research in this field. Different numerical techniques currently implemented to account for higher order scattering are reviewed and their weaknesses and strengths highlighted. Examples of simulated radar backscattering profiles are provided with particular emphasis given to situations in which the multiple scattering contributions become comparable or overwhelm the single scattering signal. We show evidences of multiple scattering effects from air-borne and from CloudSat observations, i.e. unique signatures which cannot be explained by single scattering theory. Ideas how to identify and tackle the multiple scattering effects are discussed. Finally perspectives and suggestions for future work are outlined.This work represents a reference-guide for studies focused at modeling the radiation transport and at interpreting data from high frequency space-borne radar systems that probe highly opaque scattering media such as thick ice clouds or precipitating clouds.     

Boson peak, flickering noise, backscattering processes and radiative transfer in random media

The method of matrix Green’s functions in the classical theory of electromagnetic waves is stated. This method allows to obtain a closed equation system in the presence of the random media for the calculation both coherent, and incoherent (fluctuating) components of radiation. The density and heterogeneity of scattering media can be arbitrary. The coherent channel is calculated independently. The fluctuating radiation distribution in the medium is developed initially by an interference pattern generated by the coherent channel. The limitations of the processes speed are absent. The theory embraces such phenomena as the boson peak, flickering noise, memory effect, backscattering processes and also conventional radiative transfer equation and Fresnel’s formulae.     

Multiple scattering by particles embedded in an absorbing medium. 2. Radiative transfer equation

This paper continues a systematic theoretical analysis of electromagnetic scattering by a group of arbitrarily sized, shaped, and oriented particles embedded in an absorbing, homogeneous, isotropic, and unbounded medium. The previously developed microphysical approach is used to derive the generalized form of the radiative transfer equation (RTE) applicable to a large group of sparsely, randomly, and uniformly distributed particles. The derivation of the RTE directly from the macroscopic Maxwell equations yields unambiguous and definitive analytical expressions for the participating quantities and thereby fully resolves the lasting controversy caused by the conflicting outcomes of several phenomenological approaches.     

受激布里渊散射对激光在水中衰减特性的影响

测量了不同能量的光束,通过不同长度的水介质在窄线宽与宽线宽两种情况下的衰减系数,研究了受激布里渊散射对激光在水中衰减特性的影响.结果发现,激光光束在水中传播时的衰减系数与激光光强和线宽有关,而不是通常情况下所认为的常数.还从理论上进行了分析. 关键词： 衰减系数 受激布里渊散射     

受激布里渊散射对激光在水中衰减特性的影响

测量了不同能量的光束，通过不同长度的水介质在窄线宽与宽线宽两种情况下的衰减系数，研究了受激布里渊散射对激光在水中衰减特性的影响.结果发现，激光光束在水中传播时的衰减系数与激光光强和线宽有关，而不是通常情况下所认为的常数.还从理论上进行了分析.     

Attenuation and depolarization of millimeter waves due to incoherent scattering in tropical rainfall

The radiative transfer equation has been applied to investigate the effects of multiple scattering on communication at the millimeter wavelength in tropical rain. Horizontal linearly polarized incident waves are assumed. The scattering characteristics are calculated using the Mie theory at frequencies of 16, 34.8, 82, 140 and and at rainfall rates of 12.5, 50 and . Some results of the directional patterns of the incoherent intensities at various points in the rain medium are presented. The frequency dependence of the propagation distance, attenuation coefficient and the variation of incoherent specific intensities in tropical rainfall are investigated. The incoherent power at the receiver relative to the coherent power, which is important in assessing the quality of signal in the receiver, is also investigated. It is observed that the maximum incoherent copolarized received power is about in tropical rainfall at rainfall rate. Also the difference between the copolarized incoherent power in tropical and temperate rainfall is about at frequencies higher than , and about for the cross-polarized incoherent power components. It may therefore be safely said that multiple scattering may not result in serious degradation of the coherent wave component even in the most intense tropical precipitation. It also appears that it is independent of regional rainfall climatology.