不同质量含水量的土壤反射率光谱模拟模型

土壤含水量的时空分布与变化情况对土壤温度变化、陆地—大气间热量平衡以及陆面大气环流产生显著的影响,因此,对大范围内土壤含水量进行实时动态监测,获得某段时间内土壤含水量的连续变化情况具有重要的意义。研究目的是借助高光谱遥感手段,通过构建不同质量含水量的土壤反射率光谱模拟模型,深入了解土壤质量含水量与土壤反射率光谱之间的关系,为监测土壤含水量提供有效手段。利用ASD Field Spectral FR野外光谱仪和加水称重法获得北京市8个采样点的土壤样品不同质量含水量下的土壤反射率光谱实测数据,利用其中2个土壤样品不同质量含水量下的光谱数据构建含水土壤反射率光谱模拟模型,并利用未参与建模的另外6个土壤样品数据对该模型的模拟效果进行了检验。通过数据验证发现,当土壤质量含水量小于田间持水量时,该模型的模拟精度较高;而且对于不同的土壤样品,模型的模拟效果都比较好。最后又利用北京大学校园内三个采样点的实地测量光谱数据对模型进行了验证,光谱的模拟值与实测值之间的均方根误差最小可达0.005 8。因此该模型可实现对质量含水量小于田间持水量的不同类型土壤的反射率光谱进行较高精度的模拟。     

土壤-地雷共振系统模型解析

在研究土壤-地雷共振系统模型的基础上,提出一种共振与反共振相结合的声波探雷模型.根据地雷顺性大及低频声学的特征,把地雷与其上方的土壤分别等效为弹簧和质量块,构成质量-弹簧共振系统,通过对系统频响函数的解析,讨论模型的共振与反共振现象.通过实验发现有、无地雷的地表振动速度比值存在大于1的极大值频率点和小于1的极小值频率点交替出现的现象,与理论预测符合.表明土壤-地雷共振系统同时存在共振与反共振现象,可用于地雷成像的研究. 关键词： 共振模型 反共振 频响函数 声波探雷     

浅海波导中目标散射的边界元方法

提出了一种计算三维散射体在声速剖面随深度变化、距离无关浅海波导中散射声场的数值方法波导边界元方法。当散射体不十分靠近波导界面因而边界多次散射可以忽略时,在边界元计算中可以用自由场格林函数近似波导格林函数。应用镜像法和球波函数加法定理推导了理想波导中球体散射声场的解析解,用来验证波导边界元方法的计算精度,证明该数值方法是准确的。对浅海波导中水下潜器散射声场数值模拟的结果表明,浅海波导海面、海底界面反射、声速剖面等对目标散射声场的幅值和方向性都有很大的影响。      

喷流噪声预测中小尺度湍流声源模型

本文在Tam & Auriault提出的喷流噪声预测方法的基础上,导得计算轴对称喷管伴随格林函数的方程以及声源谱、伴随格林函数与噪声谱密度函数之间的关系,比较了几种典型的声源模型,并且采用了一种随频率变化的长度尺度.验证结果表明,TA预测模型中随频率变化长度尺度与Harper-Bourne提出的声源模型的结合能给出更好的预测结果.     

快速计算地面上车辆目标的电磁散射特性

为快速计算地面上车辆目标的电磁散射特性,实现适用于半空间环境的多层快速多极子方法.对于近区项,利用严格的半空间并矢格林函数进行计算;对于远区项,首先将半空间格林函数分解为直射项和反射项,然后采用实镜像方法计算反射项.方法物理概念清晰,与目前常用的离散复镜像方法相比,具有良好的收敛性和稳定性,大大降低计算时间和内存需求.实例计算并分析地面上汽车模型的电磁散射特性.     

AlGaN/GaN高电子迁移率晶体管中kink效应的半经验模型

初步分析了AlGaN/GaN 器件上的kink效应. 在直流模型的基础上, 建立了AlGaN/GaN 高电子迁移率晶体管中kink效应的半经验模型, 并加入了kink效应发生的漏源偏压与栅源偏压的关系. 该模型得出较为准确的模拟结果, 可用来判断kink效应的发生和电流的变化量. 最后, 我们采用模型仿真结合实验分析的方法, 对kink效应进行了一定的物理研究, 结果表明碰撞电离对kink效应的发生有一定的促进作用.     

分形大地土壤表面与部分埋藏目标复合散射特性

采用土水混合物介电常数的Topp方程模型表示大地土壤的介电特性,应用带限Weierstrass-Mandelbrot分形粗糙面模型和Monte Carlo方法模拟大地土壤表面,运用矩量法研究了带限分形大地土壤表面与部分埋藏矩形截面柱复合模型的电磁散射,得出了复合散射系数的角分布曲线;计算了复合散射系数随带限分形大地土壤表面分维、空间基频、高度起伏均方根、土壤含水率、矩形截面柱几何参数、埋藏深度、倾角、入射波频率等的变化情况,并做了详细分析与讨论;结果表明,土壤表面分维等参数对复合散射系数的影响是非常复杂的,镜像附近的角分布曲线具有明显的分形特征。     

混合自旋反铁磁材料Y2BaNiO5的磁学和热力学性质

在格林函数的理论框架下，采用一维自旋为1的各向异性Heisenberg模型来讨论Y₂BaNiO₅材料的磁学和热力学性质.得到了它的自旋关联函数、低激发谱、基态能(E_g)、比热(C)和静态磁化率(χ)在不同交换各向异性因子下的性质，所得结果与实验和数值模拟结果完全一致. 关键词： Heisenberg模型 格林函数理论 Haldane能隙 反铁磁长程序     

并矢格林函数下的球形超透镜

应用并矢量格林函数法完成了球形超透镜的数值模拟,证明了球形超镜的亚波长成像能力.并矢格林函数法是处理电磁场问题的一种系统理论和有效方法,它弥补了坐标变换法的不足.与平板透镜相比,球形超透镜有几个优点,包括有限的横截面,能成放大或者缩小的像,很高的分辨率,进行二维成像等. 关键词： 球形超透镜 并矢量格林函数法 表面模 高分辨率     

有限温度下Mohapatra-Senjanovic模型的元激发谱

利用实时温度格林函数, 本文给出了有限温度下Mohapatra-Senjanovic模型^[1]的元激发谱. 本文证明, 在实时格林函数的运动方程中, 引入一级正常配对和反常配对作切断近似后给出的结果, 与在DJWB方法^[2]中用松原格林函数用单圈图计算有效势得出的结果一致. 四支元激发谱的质量、凝聚密度和温度的关系如图1、图2所示.     

Mixture model and Markov random field-based remote sensing image unsupervised clustering method

In this paper, a novel method for remote sensing image clustering based on mixture model and Markov random field (MRF) is proposed. A remote sensing image can be considered as Gaussian mixture model. The image clustering result corresponding to the image label field is a MRF. So, the image clustering procedure is transformed to a maximum a posterior (MAP) problem by Bayesian theorem. The intensity difference and the spatial distance between the two pixels in the same clique are introduced into the traditional MRF potential function. The iterative conditional model (ICM) is employed to find the solution of MAP. We use the max entropy criterion to choose the optimal clustering number. In the experiments, the method is compared with the traditional MRF clustering method using ICM and simulated annealing (SA). The results show that this method is better than the traditional MRF model both in noise filtering and miss-classification ratio.     

A method of images for a penetrable acoustic waveguide

In this paper the complex-image approximation to the reflection coefficient for water over a seabed half-space is used to generate an image representation for a bounded acoustic waveguide with an underlying layered seabed. The images are true point sources; they have constant amplitudes which are raypath independent and, in the case of a Pekeris waveguide, frequency-independent. This image representation is ideal for constructing the Green's function kernel of the boundary integral equation method for target scattering in a waveguide. The singular behavior of the Green's function for an infinitesimal source/receiver separation, possibly with the target adjacent to one of the interfaces, is modeled correctly and the image expansion has a simple analytic form which can be analytically differentiated. The method is also accurate for significant source/receiver separations, which means that it can be used in the modeling of scattering from large-sized objects and can also be used as an efficient and accurate short-range propagation model for harmonic and broadband propagation in a penetrable waveguide.     

Electrohydrodynamic flow and particle collection efficiency of a spike-plate type electrostatic precipitator

In this work, the results of electrohydrodynamic (EHD) secondary flow and particle collection efficiency measurements in a spike-plate type electrostatic precipitator (ESP) are presented. The EHD secondary flow was measured using 2- and 3-dimensional particle image velocimetry (PIV) method under the negative DC voltage. The PIV measurements were carried out in several cross-sectional planes along and across the ESP duct. The results show a complex and turbulent flow structure in the ESP. The EHD secondary flow significantly depends on applied voltage and measuring cross-sectional plane position in respect to the spike tip. The partial collection efficiency of the ESP was measured for negative and positive DC voltage. The particle concentration with and without discharge was measured at the ESP exit using an optical aerosol spectrometer.     

高频振动模糊图像仿真与分析

研究了运动模糊图像在空间域和频域上的数学模型,根据运动图像的模糊原理,提出了时间等分运动模糊图像的仿真方法,仿真了高频振动模糊图像.理论分析指出,物体做高频谐波振动时运动模糊成像的频谱会由于Bessel函数的零点的存在而形成系列条纹,对振动模糊图像的频谱做两次Radon变换能检测出振动的方向和振幅,从而得到高频振动模糊...     

实测高光谱和HSI影像的区域土壤盐渍化遥感监测研究

通过典型研究区不同盐渍化土壤光谱反射率数据的变换和分析,选择与土壤含盐量响应敏感波段,建立实测高光谱土壤含盐量反演模型,以校正HSI影像建立的土壤含盐量反演模型。结果表明：实测高光谱土壤含盐量反演模型与HSI影像土壤含盐量反演模型均有较好的精度,模型判定系数(R2)均高于0.57,且模型稳定性较好。校正后的HSI影像土壤含盐量反演模型,模型判定系数有了较大提高,R2从0.571提升至0.681,且通过了0.01的显著性水平,均方根误差(RMSE)值为0.277。模型能够较好地提高区域尺度条件下土壤盐渍化监测精度,运用此方法开展盐渍化土壤定量遥感监测是可行的。     

Full-Quantum Simulation of Graphene Self-Switching Diodes

We present a quantum study on the electrical behavior of the self-switching diode(SSD). Our simulation is based on non-equilibrium Green's function formalism along with an atomistic tight-binding model. Using this method,electrical characteristics of devices, such as turn-on voltage, rectification ratio, and differential resistance, are investigated. Also, the effects of geometrical variations on the electrical parameters of SSDs are simulated. The carrier distribution inside the nano-channel is successfully simulated in a two-dimensional model under zero,reverse, and forward bias conditions. The results indicate that the turn-on voltage, rectification ratio, and differential resistance can be optimized by choosing appropriate geometrical parameters.     

One self-consistent closure for chains of equations for Green's functions. An anisotropic heisenberg model

This article present a self-consistent approach to computation of the correlation function in the method of Green's functions. The basis for the approach is representation of the desired Green's function in the form of a chain of fractions that is subsequently closed. The closure is based on the use of concrete relations imposed on the higher-order correlation functions. General expressions for the correlation functions and conditions for self-consistency of the computations are presented. The method has been tested by computing the magnetization and critical temperature in a Heisenberg model with arbitrary anisotropy parameter. We obtain general expressions for these quantities. The critical temperature obtained is less than the corresponding value given by the molecular-field approximation. The latter approximation also leads to an overestimate of the magnetization values. It is shown that no critical transition is possible for any value of the anisotropy parameter. The corresponding inequalities are obtained. The method is compared with a method for self-consistent computation of correlation functions that was proposed earlier by the author.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 1, pp. 46–52, January, 1996.     

基于自回归模型的光阱中粒子运动模拟

通过分析光阱中颗粒位移信号特性, 建立描述粒子受限布朗运动过程的自回归模型, 进而提出了一种基于自回归模型的光阱中颗粒运动信号模拟的新方法. 对半径为1 μm的粒子处于光阱刚度分别为10, 20, 50 pN/μm 光阱时的位移信号进行了模拟, 得到的模拟位移信号的自相关函数与理论值相一致. 为了进一步阐明自回归模型的有效性, 在相同光阱参数下, 分别采用自回归模型与蒙特卡罗方法模拟光阱中微粒的位移信号, 采用功率谱法分别对两种模拟方法所得的微粒位移标定光阱刚度, 结果表明自回归模型方法能够取得和蒙特卡洛法相同的精度. 因此, 本文为分析光阱中粒子的随机运动提出了一种新的模拟方法, 可以用来对光阱中的噪声及特性进行分析. 关键词： 光阱 布朗运动 信号模拟 自回归模型     

Intrinsic asymmetries in spin-valves: a tight binding model study

We have calculated the I–V curves, dynamical conductance, and tunneling magnetoresistance (TMR) of 1D magnetic tunneling junction through singleband tight binding model calculations based on the non-equilibrium Green's function approach. The difference in density of state of two ferromagnetic leads and the bias dependence of the propagator cause intrinsic asymmetries in TMR and dynamical conductance at finite bias. Besides, we have displayed that large TMR can be obtained even at high bias for half metallic leads.     

Sub-pixel dimensional measurement with Logistic edge model

To separate substandard workpieces from products, sub-pixel edge location with Logistic edge model is proposed for dimensional measurement. In the first step, workpiece rotation is corrected with high precision image rotation to reduce the measurement error and simplify the edge model. Secondly, Logistic edge model is utilized for fitting the discrete edge data to locate the sub-pixel workpiece edge and model parameters are estimated through the objective function in terms of the difference between assumed model and real image data. Finally, an edge pixel compensation method is presented with edge pixel compensation value, which weakens the effect of edge transition zone effectively. Experimental results demonstrate that the proposed method is effective for dimensional measurement with the relative error less than 0.05%.