Modelling a dune field

We present a model to describe the collective motion of barchan dunes in a field. Our model is able to reproduce the observation that a typical dune stays confined within a stripe. We also obtain some of the pattern structures which resemble those observed from aerial photos which we do analyse and compare with the specific field of Laâyounne.     

沙丘粗糙面的二次极化电磁散射

研究了新月形沙丘粗糙面的二次极化电磁散射. 结合射线追踪理论, 由一次散射面元的反射场照射到二次散射面元, 采用基尔霍夫近似推导了二次散射面元的二次极化散射场. 计算结果表明二次极化散射结果在特定的角度和类型范围内有显著影响. 在电磁波射向背风坡时可以发现其同极化散射截面在入射角较大时大于其他入射方向的结果, 入射角在休止角附近时的交叉极化散射截面出现峰值, 以及前后狭长沙丘之间的二次极化散射特别突出. 本文结果可用于反演分析沙漠地区的风场信息. 关键词： 新月形沙丘 二次极化散射 射线追踪 休止角     

Evolution of downsized crescent-shaped dune in wind tunnel experiment

The migration of a downsized crescent-shaped dune was investigated in a wind tunnel experiment.Quantified upwind influx and vertical oscillation of the sand bed were introduced to modulate the saturation level of the sand flux above the dune surface to affect dune evolution.The evolution was recorded by top-view photography and then abstracted as the evolution of self-defined characteristic quantities using a digital image processing algorithm.The results showed that,in contrast to the case for spanwise quantities,the evolution of streamwise quantities corresponds to a linear increase in the modulation magnitude more positively and in a monotonic and convergent manner.In contrast with quantities on the windward face,the changes in quantities with respect to the horns were nonmonotonic with time and almost uncorrelated with the variation in modulation strength,which reveals the distinctiveness of leeside evolution.     

On predicting point mobility plots from measurements of other mobility parameters

Experimental modal analysis techniques are designed to exploit a property of linear vibration theory in order to construct a mathematical model of a structure from the minimum amount of measured mobility data. This property, derived from the orthogonality of normal modes, is that, in principle, all the elements of the full N × N mobility matrix can be derived from measurement of the elements in just one row or column of that matrix. The property is also used in procedural quality “checks”, in which one measures some of the derived mobilities as well as predicting them. The derivation process itself, however, gives rise to an inherent error when the frequency range covered does not include all the natural frequencies of the structure. The source of this error is discussed and some illustrations of its significance are quoted, demonstrating the need for caution when applying modal analysis methods to practical structures.     

Positron mobility in anthracene

We have measured the positron mobility in a sample of scintillation grade anthracene at two temperatures. We obtain at 300 K: =(26.0±0.9±2.6) cm²V^–1s^–1 and at 77 K: =(33.4±1.1±3.3) cm²V^–1s^–1, where the first error estimate is statistical and the second is systematic. We have also made preliminary measurements on a highly purified sample that yields =(130±3±20) cm² V^–1 s^–1 at 300 K. The data are consistent with the hypothesis that the positron is scattered from both impurities and acoustic phonons in the first sample, and predominantly from photons in the second. It appears that positrons in pure anthracene crystals are delocalized and have a mean free path of about 85 Å at room temperature.     

Selection of dune shapes and velocities Part 2: A two-dimensional modelling

We present in this paper a simplification of the dune model proposed by Sauermann et al. which keeps the basic mechanisms but allows analytical and parametric studies. Two kinds of purely propagative two dimensional solutions are exhibited: dunes and domes. The latter, by contrast to the former, do not present a slip face. Their shape and velocity can be predicted as a function of their size. We recover that dune profiles are not scale invariant (small dunes are flatter than the large ones), and that the inverse of the velocity grows almost linearly with the dune size. We furthermore get the existence of a critical mass below which no dune solution exists. It rises the problem of dune nucleation: how can dunes appear if any bump below this minimal mass gets eroded and disappears? The linear stability analysis of a flat sand bed shows that it is unstable at large wavelengths: dune can in fact nucleate from a small sand mass if the proto-dune is sufficiently long. Received 22 December 2001 / Received in final form 31 May 2002 Published online 31 July 2002     

Two-layer exchange flows over a dune: effect of large-scale bottom roughness

Graphical Abstract  

     

结合DEM的红边-近红外植被指数提取城市植被信息

随着生活水平的不断提高,城市植被已成为衡量城市宜居性的重要标准之一,对城市生物多样性评估和保护起到非常重要的作用。因此,合理规划城市植被是解决环境问题和提高生活质量的重要手段。因此,城市植被的提取和监测成为重中之重的任务。目前,城市植被提取一方面受到地域和物种的影响,另一方面也受到地形和建筑物阴影的影响。为解决上述问题,提出了一种结合数字高程模型(DEM)的红边-近红外植被指数模型（RENVI）。首先选取了3景经过辐射定标和大气校正的具有红边波段、且光谱和空间分辨率较高的Worldview-3遥感影像;然后,根据红边波段对于植被具有较高的敏感性,且红边范围内的光谱数据与反映植被生长状况的参数有较好的相关关系原理,采用DEM模型和红边波段光谱差异,有效去除地形和建筑物阴影;最后,在可见光波段范围内建立红边光谱-近红外光谱构建特征空间,构建了红边-近红外植被指数模型,同时与归一化植被指数（NDVI）和增强型植被指数（EVI）进行城市植被提取的定性和定量对比分析。定性分析是利用真实植被影像参考图与模型提取植被影像进行视觉分析;后者是采用用户精度、生产者精度、总体精度和Kappa系数进行量化分析。定性分析表明：NDVI和EVI提取城市植被,由于建筑和道路像元混淆在植被中,产生了错分和漏分的问题。RENVI较好地消除了阴影像元与植被像元混淆问题,能准确的提取城市植被,减少了冗余度,增加了植被指数的信息量。定量分析表明：RENVI模型较NDVI和RVI能够准确提取城市植被,3景影像总体精度分别为89%,81.4%和91.8%,Kappa系数分别为0.852 8,0.791 3和0.905 2。综上所述,该方法有效提高了城市植被提取精度,并取得了较好的提取视觉效果。     

Bifurcation analysis of the transition of dune shapes under a unidirectional wind

A bifurcation analysis of dune shape transition is made. By use of a reduced model of dune morphodynamics, the Dune Skeleton model, we elucidate the transition mechanism between different shapes of dunes under unidirectional wind. It was found that the decrease in the total amount of sand in the system and/or the lateral sand flow shifts the stable state from a straight transverse dune to a wavy transverse dune through a pitchfork bifurcation. A further decrease causes wavy transverse dunes to shift into barchans through a Hopf bifurcation. These bifurcation structures reveal the transition mechanism of dune shapes under unidirectional wind.     

Molecular mobility of nanocellulose hydrogels

The molecular mobility of nanocellulose hydrogels isolated from microcrystalline cellulose is evaluated using the spin probe method, from the correlation time τ (s) and rotational frequency ν = 1/τ(s^–1) of stable nitroxyl radicals introduced into the medium under study. In an aqueous gel medium, the EPR spectrum of the probe features an anisotropic triplet of frozen particles over a temperature range of 77 to 265 K. In an aqueous–ethanolic gel solution, the temperature of onset of rotation of the radical is 85 K lower. The rotational correlation time is determined from the parameters of the EPR spectrum recorded in the temperature range of 180–290 K. The resulting Arrhenius temperature dependence logν = f(1/T) is used to evaluate the activation energy of rotation E of the radical and the preexponential factor ν₀(s^–1), the frequency of rotational vibrations of the particle around the equilibrium position. For the aqueous medium, E = 11.2 kcal/mol; in the presence of ethanol, E = 5.2 kcal/mol; the preexponential factors for the aqueous and aqueous–ethanolic media are ν₀ = 7 × 10¹⁸ and 6 × 10¹⁴ s^–1, respectively. The parameters E and ν₀ measured in the pure solvents and in the samples containing nanocellulose differ little, which is indicative of a high hydrophobicity of the probe molecule (and hydrogel particles) and of their weak interaction with the environment. The high value (~10¹⁸ s^–1) of the preexponential factor is explained in terms of the compensation effect of water.     

Hopping mobility in semiconductor superlattices

We present results on the electrical transport perpendicular to interfaces in GaAs/Al_xGa_1?xAs superlattices. We have measured the current-voltage characteristics on a series of superlattices. This has been simulated numerically, the superlattice being replaced by an effective medium. Using this model we obtain the values of the effective mobility as a function of the superlattice period. Our data are in good agreement with a theory of phonon-assisted hopping transport between localized states, rather than the theory of phonon-limited band transport of Bloch waves.     

Vacancy mobility in polymer crystals

A molecular-dynamics model of the behavior of a vacancy in the chain of an equilibrium polymer crystal (the “collective atom” approximation for polyethylene) is developed for the first time. It is shown that a defect of this type in a polymer crystal has a soliton mobility, as opposed to vacancies in crystals of low-molecular substances. Zh. éksp. Teor. Fiz. 115, 1063–1069 (March 1999)     

Methane mobility in carbon nanotubes

Quasi-elastic neutron scattering has been used to characterize the diffusivity of CH₄ molecules condensed in single-wall carbon nanotubes. It is shown that the two sites of adsorption, previously observed by adsorption volumetry and calorimetry measurements, correspond to a solid-like phase for the more strongly bound site at T<120 K and to a liquid-like component for the more weakly bound site at 70<T<120 K. The diffusion coefficients of the mobile molecules range between 3×10⁻⁷ to 15×10⁻⁷ cm² s⁻¹. The fraction of this viscous liquid diminishes as the temperature is decreased; the adsorbate is fully solidified at 50 K and below.     

Quasiclassical mobility for extrinsic semiconductors

A quasiclassical formulation for mobility in extrinsic semiconductors is presented based on scattering from ionized impurity atoms. Quantum theory enters the otherwise classical Chapman-Enskog expansion of the Boltzmann equation through incorporation of the Thomas-Fermi interaction potential together with the Bom approximation for evaluation of scattering integrals. The following expression results for mobility μ_i, (cgs):

$\text{μ}{}_{\mathrm{i}}\text{3}\text{2}\text{?}{}^2\text{n}{}_{\mathrm{s}}\text{e}{}^3\text{m}{}^1\text{2}\text{2}\text{k}{}_{\mathrm{B}}\text{T}\text{2φ}\text{3}\text{2}\text{1}\text{f(γ)}$

,

$\text{f(γ)=[(1+γ)e}{}^{\mathrm{\gamma }}\text{E}{}_1\text{(γ)?1]}$

, where n_s is impurity concentration, m¹ is effective mass, E₁(γ) is the exponential integral, ? is dielectric constant and γ is dimensionless Thomas-Fermi energy. The structure of the dimensional factor in the preceding expression for μ_i agrees with previous expressions for this parameter.     

Nodal effects in dislocation mobility

We show that, contrary to the prevailing perception, dislocations can become more mobile by zipping together to form junctions. In a series of direct atomistic simulations, the critical stress to move a junction network in a [110] plane of bcc molybdenum is found to be always smaller ( approximately 50%) than that required to move isolated dislocations. Our data support a previously proposed hypothesis about the nature of anomalous slip in bcc transition metals, yet offer a different atomistic mechanism for conservative motion of screw dislocation networks. The same data suggest a hierarchy of motion mechanisms in which lower-dimensional crystal imperfections control the rate of sliding along the low-angle twist boundaries.     

Hole mobility in HgI2

Hole mobility along the layers in HgI₂ has been measured for the first time by the nuclear techniques using alpha particles and by cutting the sample perpendicularly to the layers. The value of the hole drift mobility is in agreement with previous results obtained by Photo-Electro-Magnetic (PEM) effect and it is much higher than hole drift mobility along the c-axis.