Analysis of vector radiative transfer through a finite slab of a stochastic precipitation medium

The formulation and solution of the vector radiative transfer equation in a finite slab of a stochastic precipitation medium of binary rain rates are considered. The electromagnetic wave is supposed to encounter alternating layered segments of the two precipitation media, each with a deterministic rain rate. Both the backscattering coefficient and the bistatic coefficient are derived by taking an ensemble average of the iterative solution for the deterministic vector radiative transfer equation. Computer simulations are given to verify the solutions via the Monte Carlo method, to feature the distinctiveness of stochastic precipitation systems, and to illustrate the relationship between the stochastic parameters and the final results. It is also concluded from the computer simulations that a finite slab of a stochastic precipitation medium could be treated as an average rain-rate precipitation layer with an acceptable approximation.     

Noise control of a rectangular cavity using macro perforated poro-elastic materials

The effectiveness of macro perforated porous materials to control noise levels inside a cavity is investigated. This is done using a finite element formulation based on the Biot-Allard theory that accounts for sound propagation in a poro-elastic medium. Earlier investigations have shown that macro-holes in a porous material can enhance low frequency sound absorption. However, this has been demonstrated in free field or waveguide environments. When such an approach is used in a cavity, it is seen that only certain patterns of macro-holes, dictated by cavity mode shapes, enhance noise reduction in the higher frequency ranges. This phenomenon is shown to be independent of porous material properties by considering two different materials. A correlation between the mode shapes and material removal is also established. A detailed convergence study for both cavity and poro-elastic finite element models, establishes the suitability of using higher order interpolation functions for coupled cavity-poro-elastic acoustic analysis.     

基于微结构参数建模的多孔硅绝热层热导率研究

多孔硅由于具有较低的热导率,因而可以将其作为半导体器件中的绝热层.与其他从边界散射等复杂微观热传导机制出发建模研究多孔硅的热导率不同,将多孔硅热导率影响机制更表观地归结到孔洞的存在和分布等结构因素上,把整个多孔硅视为由硅连续材料介质和孔洞连续介质通过串联和并联组合成的复合微结构,给予其低热导率一个更为易于理解和简化的解释.进一步把孔隙率对等效热导率的影响分解为两个不同的部分,即纵向部分和横向部分,半定量地给出不同的孔洞结构和分布下孔隙率与等效热导率的关系.与实验数据进行对比后验证了模型的有效性.继而从结构的角度说明了多孔硅热导率较低的原因.     

Amélioration du transfert thermique par un dépôt poreux sur la paroi d'un échangeur tubulaire

A study of flow regime and heat transfer in an annular heat exchanger partially filled with a porous medium is presented in this work. Constant heat flux and constant wall temperature boundary conditions on the inner cylinder are considered, while the outer cylinder is assumed adiabatic. The study is for both the thermal entry region and the thermally fully developed region. The flow in the porous region is modelled either by the Darcy-Brinkman equation for which an exact solution is developed or by the Darcy-Brinkman-Forchheimer equation in order to take into account inertial effects. For this case a numerical solution based on a control volume method is discussed. The results emphasize the effect of the porous layer attached to the inner cylinder on the thermal development length and heat transfer rate. It is shown that the porous substrate reduces the thermal entry length. When the effective thermal conductivity of the saturated porous medium is of the order of the fluid thermal conductivity, the local Nusselt does not vary monotonically with the thickness of the substrate. However, the use of a porous matrix always leads to an increase in the heat transfer rate provided its thermophysical properties and thickness are well chosen.     

Effect of a pulse entrapping on weak localization of waves in a resonant random medium

We consider theoretically a new physical effect in coherent backscattering enhancement (CBE) of electromagnetic or acoustic non-stationary waves from a discrete random medium under condition of Mie resonant scattering. The effect manifests itself as an angle-cone broadening of a short pulsed signal CBE from the resonant random medium, compared with the case of a non-resonant random medium. The cone broadening is associated with a pulse-entrapping effect when the pulse, while propagating within the resonant random medium, spends most of the time being 'entrapped' inside scatterers. A theory for the predicted effect is based on, first, the well known relation between the contributions of the ladder and cyclical diagrams to the time spectral density of the wave electric field coherence function and, second, a recently derived radiative transfer equation with three Lorentzian kernels of delay describing a pulse entrapping in an ensemble of resonant point-like scatterers. Using the generalized Chandrasekhar H-function, we obtain an exact analytic expression for the non-stationary albedo of the semi-infinite resonant random medium, taking into account the phenomena of a pulse CBE and entrapping. A simple analytic asymptotics is found for the albedo of the later part of the scattered pulse. This asymptotics shows quantitatively how the entrapping affects the peak amplitude and peak line shape of the CBE of a short pulse.     

Extended sources radiation and laplace type integral representation: Application to wave propagation above and within layered media

In has been shown that the sound field reflected by the plane boundary of a layered ground can always be described by a specularly reflected wave, and layer potentials. Despite its generality, this representation is not quite suitable for numerical computation. Dealing with a very simple case, Weyl, and later on Ingard and Thomasson, proposed a representation of the solution in which the layer potential terms are replaced by the sum of surface wave and a Laplace type integral. Such an integral is very convenient for numerical purposes. In this paper, it is shown that this kind of representation can be obtained for a very wide class of sound propagation problems above or within layered media: a half-space bounded by a locally reacting surface, a finite layer of porous medium, a porous medium with depth-varying porosity, a thin elastic plate; wave propagation in shallow water with an impedance bottom. Many other applications could be developed.     

梯度计算的集合变分方案及其在大气Ekman层湍流系数反演中的应用

大气Ekman层湍流系数的准确计算对数值天气预报和污染物扩散计算有着重要的意义. 将集合计算和变分法结合起来, 提出了目标泛函梯度计算的集合变分方案, 并根据正演模式的线性化情况提出了两种计算流程. 利用这种集合变分梯度算法及两种流程对Ekman层湍流系数进行了反演试验, 结果表明这种算法实施简单、方便, 根据观测资料能够比较准确地反演湍流系数值. 关键词： 梯度计算 集合变分 湍流系数 反演     

Refraction of an astigmatic laser beam in a transition layer of a stratified liquid

Refraction of an astigmatic laser beam in a transition layer formed at the boundary between two liquids with different optical characteristics is studied theoretically and experimentally. An algorithm of calculating a ray trajectory in the transition layer of a stratified liquid is considered. The profile of the refractive index in the medium is modeled for three different distributions: linear, sinusoidal, and tangential. Computer modeling based on the above models allowed obtaining two- and three-dimensional images of the laser beam (refractograms) inside and outside of the medium. The influence of the beam parameters and the experimental layout on the shape of refractograms is studied, and optimal experimental conditions are determined. The experimental arrangement for detection of 2D and 3D refractograms by the laser refractography method is described. An experimental technique is developed, and digital registration of the experimental refractograms is carried out. Adaptation of special software for processing 2D refractograms allowed retrieving the refractive index profile in the transition layer of a liquid. Based on the tangential model of the diffusion layer in a saltstratified liquid, the values of the refractive index were obtained that allowed determining the salinity profile in the given layer.     

Realisation of a humidity sensor based on perfect metamaterial absorber

In this paper, we have proposed and investigated a humidity sensor based on perfect metamaterial absorber. The sensor is composed of three layers, which are metallic particle array on the top, porous silicon in the middle layer and metallic film at the bottom. According to the effective medium approximation, the effective permittivity of porous silicon is mainly determined by the filling fraction of water condensation. It is shown that the resonant wavelength displays significant red-shift with the increasing effective permittivity of porous silicon. Furthermore, the simulation results indicate that the refractive index sensitivity of absorber is high to 249 nm/RIU, which makes our structure be an ideal candidate for evaluating the humidity of environment.     

Quantum mechanics as demanded by the special theory of relativity

We present a new approach on the interpretation of the quantum mechanism. The derivation is phenomenological and incorporates an energetic vacuum which interacts with elementary particles. We consider a classical ensemble average for the square of 4-velocities of identical elementary particles with the same initial conditions in Minkowski space. The relativistic extension of a result in Brownian motion allows the variance to be identified with Bohm's quantum potential. A simple relation between 4-velocities and 4-momenta at a specific 4-position with given proper time leads to one of two statistical equations that constitute our quantum theory, the other being the continuity equation. The Klein-Gordon equation is a consequence of these two statistical equations.     

Effect of acoustical damping with a porous absorptive layer in the cavity to reduce the structure-borne sound radiation from a double-leaf structure

Structure-borne sound radiation from a double-leaf structure with a porous absorptive layer in the cavity is studied theoretically as well as experimentally. The study is for establishing a countermeasure to reduce the structure-borne noise radiated from an interior leaf into rooms and for clarifying its reduction effect. The sound field radiated from a double-leaf elastic plate with layers of arbitrary media in the cavity set into vibration by a point force excitation is theoretically analyzed. The effect of the bulk vibration of an absorptive layer is also considered by a simple model into the present theory. Radiation reduction of an inner-layer derived from the theory is experimentally validated. Parametric studies reveal that increasing the ratio of an absorptive layer thickness to the cavity depth is effective to reduce the structure-borne sound radiation but high flow resistivity of the absorbent material is not necessarily required. A practical equation to predict the mass-air-mass resonance frequency for absorbent cavity case is given in a simple form.     

Sheared force networks: anisotropies, yielding, and geometry

A scenario for the yielding of granular matter is presented by considering the ensemble of force networks for a given contact network and applied shear stress tau. As tau is increased, the probability distribution of contact forces becomes highly anisotropic, the difference between average contact forces along minor and major axes grows, and the allowed networks span a shrinking subspace of all force networks. Eventually, contacts start to break, and at the maximal shear stress the packing becomes effectively isostatic. The size of the allowed subspace exhibits simple scaling properties, which lead to a prediction for the yield stress for packings of an arbitrary contact number.     

Experimental study of transient heat transfer characteristics of single-phase natural convection in multidimensional porous bed with volumetric heat generation

Natural convection in an internally heated porous bed of height and diameter of 450 mm and 500 mm, respectively, and superposed with the fluid layer has been experimentally investigated. The onset of natural convection in the bed is indicated by change in the rate of temperature rise within the bed. An empirical model based on local Nusselt number and local Rayleigh number has been developed. A comparison of the present model with the models in literature is made to draw out the differences between the local heat transfer of large multidimensional beds and the average heat transfer of small beds.     

Sound field of a baffled sound source covered by an anisotropic rigid-porous material

A theoretical approach for the sound field of a piston sound source covered by a finite thickness layer of anisotropic rigid-porous material is presented. The formulation is an extension of the method worked out by Amedin et al. [Sound field of a baffled piston source covered by a porous medium layer. J Acoust Soc Am 1995;98(3):1757]. First, in the present study the sound field of a point source is described by cylindrical waves. Then, with the proper boundary conditions, the sound pressure radiated from a piston source covered by a layer of anisotropic porous material can be calculated. The effects of frequency and bulk density of material on the sound propagation in an anisotropic porous material are studied. Finally, the effect of anisotropy is discussed.     

State and parameter estimation in stochastic dynamical models

This paper derives generalized maximum likelihood estimates of state and model parameters of a stochastic dynamical model. In contrast to previous studies, the change in background distribution due to changes in model parameters is taken into account. An ensemble approach to solving the maximum likelihood estimates is proposed. An exact solution for the ensemble update based on a square root Kalman Filter is derived. This solution involves a two step procedure in which an ensemble is first produced by a standard ensemble Kalman Filter, and then “corrected” to account for parameter estimation, thereby allowing a user to take advantage of an existing ensemble filter. The solution is illustrated with simple, low-dimensional stochastic dynamical models and shown to work well and outperform augmentation methods for estimating stochastic parameters.     

Surface layer reflective index changes of Au nanoparticle functionalized porous silicon microcavity for DNA detection

A technique is demonstrated to detect DNA hybridization based on surface layer of Au/porous silicon microcavity (Au/PSM) substrate for very small amount of biomolecules. Simulations show that the increase of effective refractive index for the first layer of PSM will cause a blue shift for its reflectance spectrum, and the blue shift becomes less with the increase of refractive index for one more layers. In experiments, such a blue shift of reflectance spectrum of PSM comes from the increase of refractive index by DNA hybridization on the surface. The detection limit of Au/PSM biosensor is 15.15 nM for 19-base pair DNA, which is comparable to that of reported biosensors based on porous silicon (PS). Therefore such an Au/PSM could be very useful to develop simple, rapid and sensitive optical biosensors when the amount of target is very small.     

Validity of the limp model for porous materials: a criterion based on the Biot theory

The validity of using the limp model for porous materials is addressed in this paper. The limp model is derived from the poroelastic Biot model assuming that the frame has no bulk stiffness. Being an equivalent fluid model accounting for the motion of the frame, it has fewer limitations than the usual equivalent fluid model assuming a rigid frame. A criterion is proposed to identify the porous materials for which the limp model can be used. It relies on a new parameter, the frame stiffness influence (FSI), based on porous material properties. The critical values of FSI under which the limp model can be used are determined using a one-dimensional analytical modeling for two boundary sets: absorption of a porous layer backed by a rigid wall and radiation of a vibrating plate covered by a porous layer. Compared with other criteria, the criterion associated with FSI provides information in a wider frequency range and can be used for configurations that include vibrating plates.