内混合强吸收气溶胶粒子光散射的等效性

 以黑碳和水两种成分组成的内混合单分散气溶胶粒子为例,根据其各消光效率因子、吸收效率因子和散射相函数,分析了用等效折射率来描述含有不同成分的内混合气溶胶系统的适用性。结果表明：在瑞利散射区和几何光学区内,内核（碳粒）体积比为0.01,0.1,0.5,0.9时,消光效率在大多数尺度参数下等效性都很好,但在米散射区内相对较差;当体积比大于0.3时,其吸收效率、消光效率等效性较好;除瑞利散射区外,散射相函数在各体积比下的等效性都很差。当考虑内混合气溶胶粒子系统的散射和吸收特性时,一般不难找到等效折射率,但在光散射技术中,应用相函数反演等效折射率的可靠性还有待商榷。     

随机结构多孔介质等效热导率数值计算

本文对二维颗粒弥散多孔介质的辐射导热耦合等效热导率进行了数值计算研究。首先采用随机生成结构方法(Random generate-growth method,RGGM)生成实际多孔材料的复杂结构。在此基础上,采用离散坐标法及有限容积法求解了复杂结构内部辐射导热耦合换热,进而计算得到材料的等效热导率。根据建立的材料随机结构模型及等效热导率数值计算模型,分别研究了衰减系数、弥散相体积分数、温度等因素对材料等效热导率的影响,并将数值计算结果与理论计算结果进行对比,吻合较好,反映出模型用于预测实际多孔材料等效热导率的有效性。     

Anomalous diffraction of light with geometrical path statistics of rays and a Gaussian ray approximation

The anomalous-diffraction theory (ADT) of extinction of light by soft particles is shown to be determined by a statistical distribution of the geometrical paths of individual rays inside the particles. Light extinction depends on the mean and the mean-squared geometrical paths of the rays. Analytical formulas for optical efficiencies from a Gaussian distribution of the geometrical paths of rays are derived. This Gaussian ray approximation reduces to the exact ADT in the intermediate case of light scattering for an arbitrary soft particle and describes well the extinction of light from a system of randomly oriented and (or) polydisperse particles. The implications for probing of the sizes and shapes of particles by light extinction are discussed.     

Kramers-Kronig analysis on the real refractive index of porous media in the terahertz spectral range

We present a terahertz time-domain experimental technique for the detection of scattering from porous media. The method for detection of the scattering enables one to make a decision when Fresnel or Kramers-Kronig (K-K) analysis can be applied for a porous medium. In this study the real refractive index of a tablet is calculated using the conventional K-K dispersion relation and also using a singly subtractive K-K relation, which are applied to the extinction coefficient obtained from the Beer-Lambert law. The advantage of the K-K analysis is that one gets estimates both for absolute refractive index and also dispersion of the porous tablet, whereas Fresnel analysis provides only the absolute value of the index.     

Radiative properties of scattering and absorbing dense media: theory and experimental study

We investigate the validity of the radiative transfer equation to model transmission of light through an absorbing and scattering medium. Assuming that radiative transfer equation is valid, the inverse scattering problem for non-polarized radiative transfer in one-dimensional absorbing and scattering media is solved using a parameter identification method. We discuss how to identify the albedo, phase function and extinction coefficient of the medium. We present experimental data that confirm that this approach is robust and can be used to make reliable predictions of the behavior of scattering absorbing systems.     

Polarization of light and its use in various problems of optics of scattering media

As light passes through scattering media, certain specific features of the polarization of radiation manifest themselves. The paper presents materials on this problem that were obtained at the Institute of Physics of the National Academy of Sciences of Belarus over recent decades. Results of experimental investigations of media that model real objects are described for the case where the dimensionless optical parameters of media and objects coincide. A method for determining the position of a diffuse light source in the atmosphere via predominant oscillations of the light vector of scattered radiation for two directions of observation is proposed. The structure of aerosol formations (smokes, dust and liquid-droplet and crystalline clouds) is interpreted based on the character of depolarization of laser radiation sounding atmosphere. The polarization of laser radiation passing through a turbid medium and reflected from it is studied. Practical applications are proposed. Fundamentals of an applied vector theory of radiation transfer, which made it possible to considerably expand notions of light scattering in strongly turbid media, are given. Studies of light propagation in encapsulated liquid crystals, which are used for solving of a large number of problems, are described. In these objects, ordinary and extraordinary rays that arise in crystals under electric voltage can give rise to a wave that is attenuated to a different degree and whose phase and polarization characteristics are varying.     

Electromagnetic-wave scattering from chiral spheres in chiral media

Summary The scattering of electromagnetic waves in chiral (optically active) media from chiral spheres is studied. Mie-scattering techniques are used to find the exact solution for plane-electromagnetic-wave scattering from a chiral sphere of arbitrary size in an infinitely extended chiral medium of arbitrarily different permeability, permittivity, and chirality, and the scattering and extinction efficiencies for chiral spheres in chiral media are derived. Special cases of achiral exterior medium-chiral sphere and achiral exterior medium-achiral scatterer are considered and in the latter case well-known results of Mie scattering are recovered. Simplified results for small spheres are also found for the limit of Rayleigh scattering.     

Direct measurement of porous media local hydrodynamical permeability using gas MRI.

The concept of hydraulic permeability is at the core of modeling single phase or multi-phase flow in heterogeneous porous media, as it is the spatial distribution of the permeability that primarily governs the behavior of fluid flow in the medium. To date, the modeling of fluid flow in porous media has been hampered by poor estimates of local permeability. Magnetic Resonance Imaging is well known for its ability to measure non-invasively the local density and flow rate of different fluids saturating porous media [1,2]. In this paper we demonstrate the first non-invasive method for the direct measurement of a single projection of the local permeability tensor of a porous medium using gas-phase MRI. The potential for three-dimensional imaging of the medium permeability is also discussed. The limitations of the method are listed and results are presented in a model porous medium as well as in a real oil reservoir rock.     

Optical equivalence of isotropic ensembles of ellipsoidal particles in the Rayleigh-Gans-Debye and anomalous diffraction approximations and its consequences

Light scattering by isotropic ensembles of ellipsoidal particles is considered in the Rayleigh-Gans-Debye approximation. It is proved that randomly oriented ellipsoidal particles are optically equivalent to polydisperse randomly oriented spheroidal particles and polydisperse spherical particles. Density functions of the shape and size distributions for equivalent ensembles of spheroidal and spherical particles are presented. In the anomalous diffraction approximation, equivalent ensembles of particles are shown to also have equal extinction, scattering, and absorption coefficients. Consequences of optical equivalence are considered. The results are illustrated by numerical calculations of the angular dependence of the scattering phase function using the T-matrix method and the Mie theory.     

Tomography-based Monte Carlo determination of radiative properties of reticulate porous ceramics

A 3D digital representation of a reticulate porous ceramic (RPC) sample, generated by computer tomography (CT), is employed to determine its porosity, surface-to-volume ratio, and the minimum size of a representative elementary volume (REV) for continuum domain. Subsequently, the Monte Carlo (MC) ray-tracing technique is applied to calculate the extinction coefficient and scattering phase functions based on the probabilistic distribution functions of the extinction path-length and of the directional cosine of incident radiation. The methodology and governing equations are presented for diffusely and specularly reflecting surfaces. The isotropic assumption is justified by demonstrating that the extinction coefficient is directionally independent.     

Separation of viscothermal losses and scattering in ultrasonic characterization of porous media

This paper presents a method for separating viscothermal and scattering losses in ultrasonic characterization of porous media. This method is based on variations of the static pressure of the saturating fluid. Experimental results were already presented in previous papers and the losses separation was verified experimentally. The aim of this paper is to present an analytic justification of this losses separation in the case of this experimental method and to show that it is possible to estimate acoustic parameters without the knowledge of scattering characteristics. The standard scattering length is used to renormalize speed and transmission through the porous medium, described as an equivalent fluid. Under certain assumptions corresponding to a weak scattering regime, it is shown how viscothermal and scattering losses can be separated easily without knowing scattering characteristics. Application of this model is presented in the case of weak scattering in a polyurethane foam and in the limit case of stronger scattering in a glass beads sample.     

A new numerical method for the analysis of the permeability anisotropy ratio

By combining the lattice Boltzmann method with the three-dimensional computer-simulated images of porous media,a new numerical experimental methodology is used to determine the permeability anisotropy ratio of porous media.The results compare well with the laboratory experimental data.     

二氧化钒纳米点阵红外光学特性研究

针对二氧化钒纳米点阵从半导体到金属的可逆相变,考虑到点阵中各个点之间散射光的交互作用,基于VO2在不同温度和波长下的折射率和消光系数,以及小颗粒的吸收和散射特性,建立了VO2纳米颗粒的数学模型,研究了VO2纳米颗粒的相变光学特性.结果表明,随着波长变化,吸收截面相对散射截面占主导,金属相在980 nm附近出现吸收峰值|随着温度变化,可见光区域的消光系数变化较小,而红外区域较大,其中在近红外区域的消光系数变化最大.在纳米点阵中,消光截面随着颗粒间距变化,当颗粒间距增大时,消光峰值出现红移,且峰值大小也会随之增大|当间距超过一定数值后,峰值反而会逐渐减小.采用多孔氧化铝掩模的方法,通过磁控反应溅射制备VO2纳米点阵,测试结果表明其透过率比薄膜的透过率高.     

米散射理论在新型导光板中的应用

根据米散射理论,提出了新型导光板的设计思路,计算并分析了对于一定波长的入射光,不同粒径的微粒的散射特性。总结了随着微粒粒径的变化,散射效率、消光效率与背向散射效率的变化规律,分析了散射过程中的偏振度随粒子粒径几散射角变化的情况,同时模拟计算了多个微粒对同一波长的入射光经过多次散射后的概率统计结果。     

Reproducibility experiments on measuring acoustical properties of rigid-frame porous media (round-robin tests)

This paper reports the results of reproducibility experiments on the interlaboratory characterization of the acoustical properties of three types of consolidated porous media: granulated porous rubber, reticulated foam, and fiberglass. The measurements are conducted in several independent laboratories in Europe and North America. The studied acoustical characteristics are the surface complex acoustic impedance at normal incidence and plane wave absorption coefficient which are determined using the standard impedance tube method. The paper provides detailed procedures related to sample preparation and installation and it discusses the dispersion in the acoustical material property observed between individual material samples and laboratories. The importance of the boundary conditions, homogeneity of the porous material structure, and stability of the adopted signal processing method are highlighted.     

Contribution of multiple scattering to the dielectric constant of a randomly inhomogeneous medium

Within the statistical theory of multiple scattering of light in random media, the dielectric constant of a suspension is represented as a diagram series in scattering orders and concentration of particles. The contributions of double and triple scattering events are determined. The extinction length and the transport mean free path in highly concentrated suspensions calculated with the use of the optical theorem are in good agreement with the available data. It is shown that the two-particle Born approximation, combined with the Mie form factor and the Percus-Yevick structure factor, is not adequate for systems with a high concentration of scatterers. A contribution to the optical parameters is found that is missing in the above approximation.     

Radiation Transfer in a Binary, Stochastic Medium with Strongly Fluctuating Parameters

The propagation of radiation in a three-dimensional, inhomogeneous, stochastic, scattering medium and in an equivalent homogeneous medium has been investigated by the small-angle iteration method. The statistical structure of the scattering parameters of a three-dimensional medium has been described within the framework of the two-dimensional Poisson process and the binary Markov process. The analytical data obtained point to the fact that the three-dimensional inhomogeneity of a stochastic medium significantly influences the radiation transfer in it.     

An algorithm to determine cirrus properties from analysis of multiple-scattering influence on lidar signals

A Monte Carlo-based evaluation of the multiple-scattering influence on ground-based Raman lidar measurements is presented. The lidar returns from cirrus clouds are analyzed in order to evaluate vertical profiles of the extinction and backscattering coefficients. Results show that for the typical cirrus cloud, the presence of the multiple scattering can lead to an underestimation of the extinction coefficient by as large as 200% whereas the backscattering coefficient is almost unaffected for the Raman lidar technique. An algorithm to select one or a set of phase functions which fit to the lidar data is also presented. It is an iterative procedure based on Monte Carlo scattering simulation. By comparison of the experimental value of the lidar ratio, corrected for the multiple scattering influence, and the phase function used in the Monte Carlo simulation, one can determine a suitable phase function. The validity and sensitivity of the algorithm have been demonstrated by applying it to simulated cases. The application to some real cases indicates that our procedure allows for the establishing of a practical scattering model for the cirrus clouds.     

Estimates of the albedo of powderlike surfaces in the geometrical optics approximation

Results of computer simulation of light scattering by powderlike media composed of large semitransparent particles of different shapes are presented. The geometrical optics approach is used. The cases of particles of spherical, cubic, and random shapes are considered. The one-dimensional geometrical optics model of light scattering by powderlike media, which finds application in investigations of planetary surfaces, meteorites, and lunar samples, is briefly described. The numerical simulation allowed us to estimate the error of this model, which turned out to be within the limits of several percent. The albedo calculated by using the one-dimensional model is shown to be closest to the “three-dimensional” reflectance of the surfaces at a phase angle of about 60°. This albedo also approximates fairly well the integrated reflectance, which in laboratory measurements is determined with the help of an integrating sphere.     

Multiple-scaling methods for Monte Carlo simulations of radiative transfer in cloudy atmosphere

Two multiple-scaling methods for Monte Carlo simulations were derived from integral radiative transfer equation for calculating radiance in cloudy atmosphere accurately and rapidly. The first one is to truncate sharp forward peaks of phase functions for each order of scattering adaptively. The truncated functions for forward peaks are approximated as quadratic functions; only one prescribed parameter is used to set maximum truncation fraction for various phase functions. The second one is to increase extinction coefficients in optically thin regions for each order scattering adaptively, which could enhance the collision chance adaptively in the regions where samples are rare. Several one-dimensional and three-dimensional cloud fields were selected to validate the methods. The numerical results demonstrate that the bias errors were below 0.2% for almost all directions except for glory direction (less than 0.4%) and the higher numerical efficiency could be achieved when quadratic functions were used. The second method could decrease radiance noise to 0.60% for cumulus and accelerate convergence in optically thin regions. In general, the main advantage of the proposed methods is that we could modify the atmospheric optical quantities adaptively for each order of scattering and sample important contribution according to the specific atmospheric conditions.