共查询到20条相似文献,搜索用时 31 毫秒
1.
Kienle A 《Physical review letters》2007,98(21):218104
Light propagation in anisotropic random media is studied in the steady-state and time domains. Solutions of the anisotropic diffusion equation are compared to results obtained by the Monte Carlo method. Contrary to what has been reported so far, we find that even in the "diffusive regime" the anisotropic diffusion equation does not describe correctly the light propagation in anisotropic random media. 相似文献
2.
Nonlinear dynamics resulting from the interplay between diffusive and buoyancy-driven Rayleigh-Taylor (RT) instabilities of autocatalytic traveling fronts are analyzed numerically for various values of the relevant parameters. These are the Rayleigh numbers of the reactant A and autocatalytic product B solutions as well as the ratio D=D(B)/D(A) between the diffusion coefficients of the two key chemical species. The interplay between the coarsening dynamics characteristic of the RT instability and the constant short wavelength modulation of the diffusive instability can lead in some regimes to complex dynamics dominated by irregular succession of birth and death of fingers. By using spectral entropy measurements, we characterize the transition between order and spatial disorder in this system. The analysis of the power spectrum and autocorrelation function, moreover, identifies similarities between the various spatial patterns. The contribution of the diffusive instability to the complex dynamics is discussed. 相似文献
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The morphological instability of solidification is analytically studied in the presence of an anisotropic and heterogeneous
phase transition zone with allowance for a liquid flow and convective heat-andmass transfer in this two-phase zone. The mechanism
of breaking the stability of solidification is considered; it consists in a convective heat and impurity transfer during a
liquid flow along channels in the phase transition zone. The morphological instability is subjected to linear analysis with
allowance for a liquid flow in the liquid phase of the system, impurity diffusion in the two-phase zone, and the dependence
of the transfer coefficients on the phase composition. The perturbation evolution parameter is determined for an anisotropic
and heterogeneous two-phase zone, and neutral stability curves of the process are obtained. It is shown that taking into account
impurity diffusion and an increase in the heterogeneity of the phase transition zone broaden the instability region and that
a decrease in the anisotropy narrows this region. A new criterion of convective morphological instability of solidification
with a two-phase zone is found, and it substantially broadens the instability region when the liquid flow velocity increases. 相似文献
5.
Uniqueness,reciprocity theorem,and plane waves in thermoelastic diffusion with a fractional order derivative
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In this work, a theory of thermoelasticity with diffusion is taken into consideration by using the methodology of fractional calculus. The governing equations for particle motion in a homogeneous anisotropic fractional order generalized thermoelastic diffusive medium are presented. Uniqueness and reciprocity theorems are proved. The plane wave propagation in the homogeneous transversely isotropic thermoelastic diffusive medium with fractional order derivative is studied. For the two-dimensional problem, there exist a quasi-longitudinal wave, a quasi-transverse wave, a quasi-mass diffusion wave, and a quasi-thermal wave. From the obtained results, the different characteristics of waves, like phase velocity, attenuation coefficient, specific loss, and penetration depth, are computed numerically and presented graphically. Some special cases are also discussed. 相似文献
6.
We use numerical strategies to examine the linear and nonlinear stability of forward smolder waves in the framework of a simplified thermal–diffusive model, with the hydrodynamic effects completely filtered out. The configuration consists of a horizontal thin solid fuel, over which air blows in the same direction as the smolder front propagation. It is found that, in the absence of convective heat losses, the whole one-dimensional adiabatic solution branch is linearly stable; in contrast, when the convective heat loss effect is taken into account, fingering instability emerges provided the incoming air flow rate is within a narrow range near the one-dimensional extinction limit, a manifestation that is reminiscent of the familiar cellular instability occurring in the context of low-Lewis-number diffusion flames. Accordingly, the fingering instability herein identified in forward smolder combustion is purely thermal–diffusive in nature. Furthermore, a heuristic analysis by drawing an analogy with premixed flame suggests that the occurrence of such fingering instability is the joint consequence of the Lewis number effects and convective heat losses. It is proposed that a Hele–Shaw-type combustion channel may be adopted to experimentally reveal the fingering patterns predicted by current numerical simulations. 相似文献
7.
V. N. Zirakashvili 《Journal of Experimental and Theoretical Physics》1998,87(2):218-222
A variational method is used to obtain estimates of the effective particle transport coefficients in a random static magnetic
field. The particle propagation is described by an anisotropic diffusion equation. The diffusion coefficient parallel to the
local magnetic field is much greater than the transverse diffusion coefficient. For large-scale magnetic-field variations
the diffusion is described by effective coefficients. The variational approach can be used to find the effective parallel
and perpendicular diffusion coefficients. It was shown that the instability growth rate of the magnetic field lines determines
the upper estimate of the effective transverse diffusion coefficient.
Zh. éksp. Teor. Fiz. 114, 398–405 (August 1998) 相似文献
8.
We report on peculiar behaviors due to anisotropic terrace diffusion on step meandering on a vicinal surface. We find that anisotropy triggers tilted ripples. In addition, if the fast diffusion direction is perpendicular to the steps, the instability is moderate and coarsening is absent, while in the opposite case the instability is promoted, and interrupted coarsening may be observed. Strong enough anisotropy restabilizes the step for almost all step orientations. These findings point to the nontrivial effect of anisotropy and open promising lines of inquiries in the design of surface architectures. 相似文献
9.
We study the stochastic dynamics of deposition-evaporation cooperative processes of dimers, trimers, etc., in two- and higher-dimensional lattices. The dimer system in bipartite lattices allows for an exact solution of dynamic correlations and scaling functions by means of a quantum spin equivalence. Autocorrelations exhibit a diffusive asymptotic kinetics and crossovers of different dynamic regimes in highly anisotropic lattices. Monte Carlo simulations combined with finite-size scaling arguments support the validity of the diffusive picture in more general situations. Steady-state coverages and diffusion constants are obtained using mean-field approaches, spin wave calculations, and random walk analyses in nearly jammed configurations. 相似文献
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DETERMINATION OF ELASTIC CONSTANTS OF ANISOTROPIC LAMINATED PLATES USING ELASTIC WAVES AND A PROGRESSIVE NEURAL NETWORK 总被引:1,自引:0,他引:1
In this paper, a procedure is suggested to inversely determine the elastic constants of anisotropic laminated plates using a progressive neural network (NN). The surface displacement responses are used as the inputs for the NN model. The outputs of the NN are the elastic constants of anisotropic laminated plates. The hybrid numerical method (HNM) is used to calculate the displacement responses of laminated plates to an incident wave for given elastic constants. The NN model is trained using the results from the HNM. A modified back-propagation learning algorithm with a dynamically adjusted learning rate and an additional jump factor is developed to tackle the possible saturation of the sigmoid function and to speed up the training process for the NN model. The concept of orthogonal array was adopted to generate the representative combinations of elastic constants, which reduces significantly the number of training data while maintaining its data completeness. Once trained, the NN model can be used for on-line determination of the elastic constants if the dynamic displacement responses on the surface of the laminated plate can be obtained. The determined elastic constants are then used in the HNM to calculate the displacement responses. The NN model would go through a progressive retraining process until the calculated displacement responses using the determined results are sufficiently close to the actual responses. This procedure is examined for an actual glass/epoxy laminated plate. It is found that the present procedure is very robust and efficient for determining the elastic constants of anisotropic laminated plates. 相似文献
12.
Miscible viscous fingering classically occurs when a less viscous fluid displaces a miscible more viscous one in a porous medium. We analyze here how double diffusive effects between a slow diffusing S and a fast diffusing F component, both influencing the viscosity of the fluids at hand, affect such fingering, and, most importantly, can destabilize the classically stable situation of a more viscous fluid displacing a less viscous one. Various instability scenarios are classified in a parameter space spanned by the log-mobility ratios R(s) and R(f) of the slow and fast component, respectively, and parametrized by the ratio of diffusion coefficients δ. Numerical simulations of the full nonlinear problem confirm the existence of the predicted instability scenarios and highlight the influence of differential diffusion effects on the nonlinear fingering dynamics. 相似文献
13.
Annamaria Mazzia Gianmarco Manzini Mario Putti 《Journal of computational physics》2011,230(23):8410-8426
We study the performance of Godunov mixed methods, which combine a mixed-hybrid finite element solver and a Godunov-like shock-capturing solver, for the numerical treatment of the advection–dispersion equation with strong anisotropic tensor coefficients. It turns out that a mesh locking phenomenon may cause ill-conditioning and reduce the accuracy of the numerical approximation especially on coarse meshes. This problem may be partially alleviated by substituting the mixed-hybrid finite element solver used in the discretization of the dispersive (diffusive) term with a linear Galerkin finite element solver, which does not display such a strong ill conditioning. To illustrate the different mechanisms that come into play, we investigate the spectral properties of such numerical discretizations when applied to a strongly anisotropic diffusive term on a small regular mesh. A thorough comparison of the stiffness matrix eigenvalues reveals that the accuracy loss of the Godunov mixed method is a structural feature of the mixed-hybrid method. In fact, the varied response of the two methods is due to the different way the smallest and largest eigenvalues of the dispersion (diffusion) tensor influence the diagonal and off-diagonal terms of the final stiffness matrix. One and two dimensional test cases support our findings. 相似文献
14.
Dobrindt J Rodrigo Teixeira da Silva E Alves C Oliveira CL Nallet F Andreoli de Oliveira E Navailles L 《The European physical journal. E, Soft matter》2012,35(1):3
Using Fluorescence Recovery After Photobleaching, we investigate the Brownian motion of DNA rod-like fragments in two distinct
anisotropic phases with a local nematic symmetry. The height of the measurement volume ensures the averaging of the anisotropy
of the in-plane diffusive motion parallel or perpendicular to the local nematic director in aligned domains. Still, as shown
in using a model specifically designed to handle such a situation and predicting a non-Gaussian shape for the bleached spot
as fluorescence recovery proceeds, the two distinct diffusion coefficients of the DNA particles can be retrieved from data
analysis. In the first system investigated (a ternary DNA-lipid lamellar complex), the magnitude and anisotropy of the diffusion
coefficient of the DNA fragments confined by the lipid bilayers are obtained for the first time. In the second, binary DNA-solvent
system, the magnitude of the diffusion coefficient is found to decrease markedly as DNA concentration is increased from isotropic
to cholesteric phase. In addition, the diffusion coefficient anisotropy measured within cholesteric domains in the phase coexistence
region increases with concentration, and eventually reaches a high value in the cholesteric phase. 相似文献
15.
Gauge theory approach for diffusive and precessional spin dynamics in a two-dimensional electron gas
We develop a gauge theory for diffusive and precessional spin dynamics in a two-dimensional electron gas. Our approach reveals a direct connection between the absence of the equilibrium spin current and a strong anisotropy in the spin relaxation: both effects arise if spin-orbit coupling is reduced to a pure gauge SU(2) field. In this case, the spin-orbit coupling can be removed by a gauge transformation in the form of a local SU(2) spin rotation. The resulting spin dynamics is exactly described in terms of two kinetic coefficients: the spin diffusion and electron mobility. After the inverse transformation, full diffusive and precessional spin density dynamics, including the anisotropic spin relaxation, formation of stable spin structures, and spin precession induced by a macroscopic current are restored. Explicit solutions of the spin evolution equations are found for the initially uniform spin density and for stable, nonuniform structures. Our analysis demonstrates a universal relation between the spin relaxation rate and spin-diffusion coefficient. 相似文献
16.
We study the step bunching kinetic instability in a growing crystal surface characterized by anisotropic diffusion. The instability is due to the interplay between the elastic interactions and the alternation of step parameters. This instability is predicted to occur on a vicinal semiconductor surface Si(001) or Ge(001) during epitaxial growth. The maximal growth rate of the step bunching increases like F4, where F is the deposition flux. Our results are complemented with numerical simulations which reveal a coarsening behavior in the long time evolution for the nonlinear step dynamics. 相似文献
17.
本文提出了一种快速测量液相扩散系数的方法, 该方法以液芯柱透镜作为液相扩散池和成像元件, 利用柱透镜成像过程中特有的折射率空间分辨测量能力, 只需记录一幅瞬态扩散图像, 根据图像的像宽与折射率的对应关系, 基于扩散定律快速计算出液相扩散系数. 实验研究了室温(25℃)下乙二醇和纯水间的扩散过程, 用折射率空间分布法测量了扩散系数, 和其他测量方法得到的结果进行了分析对比, 结果表明:用折射率空间分布法测量液相扩散系数具有数据采集耗时短(~20 ms)、测量速度快(<1 s)、精度高(相对误差<3%)和操作简单的特点, 为快速测定液相扩散系数提供了一种有效的新方法. 相似文献
18.
We study the transport properties of particles draining from a silo using imaging and direct particle tracking. The particle displacements show a universal transition from superdiffusion to normal diffusion, as a function of the distance fallen, independent of the flow speed. In the superdiffusive (but sub-ballistic) regime, which occurs before a particle falls through its diameter, the displacements have fat-tailed and anisotropic distributions. In the diffusive regime, we observe very slow cage breaking and Péclet numbers of order 100, contrary to the only previous microscopic model (based on diffusing voids). Overall, our experiments show that diffusion and mixing are dominated by geometry, consistent with long-lasting contacts but not thermal collisions, as in normal fluids. 相似文献
19.
本文提出了一种抑制合成孔径雷达图像乘性相干斑噪声的各向异性扩散滤波新方法.该方法将受自适应耦合函数控制的平均曲率运动嵌入到传统相干斑抑制的各向异性扩散方程中,形成了一种可有效抑制边缘区域相干斑与同质区块效应现象的各向异性扩散新方程,同时在新建的扩散方程中,引入了由改进Frost滤波与局部方向比率联合构建的一种带方向约束的新扩散函数,进一步削弱了块效应现象且明显改善了抑斑图像的边缘抖动扭曲问题.实验结果表明该方法在有效保护图像边缘的同时,能充分平滑同质区与边缘区域的相干斑,明显削弱块效应现象,有效改善抑斑图像边缘抖动扭曲问题,而抑斑图像无论视觉效果还是参数指标均比多种传统抑斑方法更具优势. 相似文献
20.
We study the dynamics of an overdamped paramagnetic ellipsoidal particle confined above a plane and subjected to an external
rotating magnetic field. Without magnetic forcing, the Brownian ellipsoid exhibits a crossover from a short time anisotropic
diffusion to a long time isotropic one. Application of an external static or rotating magnetic field enables controlling and
varying the crossover time depending on the field frequency and amplitude. We combine analytical results and numerical simulations
in order to explore the diffusive properties of the forced ellipsoid. 相似文献