吸波材料与微波暗室吸波性能研究

主要解决入射波在尖劈吸波材料几何空缺间反射过程的问题以及微波暗室性能是否满足导引仿真要求的问题.对于问题一,首先建立了二维情况下入射波线在尖劈几何空缺间反射过程的数学模型,提出了临界状态的概念,根据光学原理,三角几何原理推出了辐射波在各个反射点处的入射角及反射点距尖劈底面高度表达式,求出了最终反射波的方向、辐射强度.最后根据坐标分解法将其拓展到三维空间.对于问题二,综合考虑辐射波在六个面间的相互影响,运用微元法的思想和微积分的相关知识,推导出静区直接接收到微波信号的公式以及微波信号经整个微波暗室内壁一次反射后被静区接收到的微波功率公式,用数值积分的方法求得了静区得到的反射信号的功率与静区直接得到的微波功率之比,并对部分反射光难以计算的问题提出了数学模型修正的建议和方法.     

尖劈形吸波体和微波暗室中的数学模型

首先讨论电磁波在尖劈形吸波体形成的空缺中的反射问题.通过建立二维空间内两次相邻反射的数学模型,得到尖劈的几何参数和电磁波出射角、更射次数和辐射强度的关系.然后通过仿真计算出数值结果,并且将其推广到三维情况,得到了"在一定条件下,减小尖劈形吸波体顶角的大小,有利于增加反射次数,增强吸波效果"的结论.针对导弹导引试验中的仿真要求,从电磁波辐射角系数的概念出发,提出了一种计算微波暗室中静区的接收到墙面更射的功率与天线辐射的功率的比值的算法.这种算法物理意义清晰,计算过程简单,可较好的应用在导弹的导引仿真试验的分析中.     

尖劈吸波体的研究和微波暗室的模拟

对尖劈形状吸波体的吸波性能进行了研究,并对导弹导引仿真实验用的微波暗室的性能进行分析和仿真.首先对尖劈体的二维反射性能进行了研究,从单条波线反射的原理出发,得到波束平面反射的统计模型.单条波线的反射通过数值模拟得到;波束反射模型则通过对数值模拟的结果进行统计和拟合得到,最终用多项式表示.对于一些简单或特殊的情况,也给出了解析解.通过分析发现,三维反射和二维反射之间有明确的关系.这种关系可以由三维入射角和反射次数决定,而反射次数可以通过二维模型得到.据此将平面反射模型扩展为三维反射模型,从而得到尖劈形状吸波体的三维反射模型.无回波暗室用于模拟没有背景微波辐射的环境,其关键在于选择合适的吸波材料.基于微波反射通量平衡原理,建立了考虑暗室墙面各点之间的相互影响的耦合模型,从而可以求解出在指定的发射源照射之下墙面各点的辐射强度分布.对模型的求解精度和收敛性进行了验证.基于此模型,对一个导弹引导试验进行了数值模拟,推算出了使用两种不同吸波材料时静区接收到的微波信号的信噪比.     

吸波材料与微波暗室问题的数学模型

为分析尖劈形材料的吸波性能,建立了半圆镜像模型将辐射波在尖劈空间的传播等效为射线对等分半圆的切割,确定了反射波线和吸波材料几何参数之间的解析关系.对于微波暗室问题,假设在余弦辐射体上随机选取一个方向的能量波束集中了其所有能量,分别建立了加权能量分布模型和Monte Carlo模型,仿真验证了两模型结论的一致性.     

基于几何光学法和惠更斯原理的微波暗室性能分析

微波暗室在导弹导引仿真中已广泛使用,针对导弹导引仿真试验中用到的微波暗室的静区性能评估问题,分别建立了基于几何光学法和惠更斯原理的评价分析模型.此模型考虑了暗室空间内各个壁面电磁辐射的相互影响,将壁面看作次级余弦辐射体,建立了描述壁面任一点垂直辐射强度的积分微分方程.通过离散方法将问题转化为求解线性方程组的数值问题.计算得到暗室壁面辐射强度分布以及性能指标γ.结果表明,垂直反射率为0.5时不满足要求,而0.05时满足要求.最后考察了模型的收敛性,随着离散度的增高,计算结果趋于稳定.     

尖劈吸波体与导引仿真用微波暗室的性能研究

首先,针对尖劈形状吸波体的性能问题,给出了直接计算法和基于镜像模型的方法,并对其进行了对比计算与仿真.其次,对于微波暗室的性能研究,针对不同的复杂度要求,建立了两种数学模型—射线追踪(Ray Tracing)模型和基于Markov链的有限元(FEM,Finite Element Model)模型.建模过程和仿真结果表明,Ray Tracing模型的计算复杂度较低,但电磁波"镜面反射"的假设过于理想,模型较为粗糙,只能用于粗略模拟实际情况.而基于Markov链的FEM模型较Ray Tracing模型更加精确.同时,相比于传统的具有高计算复杂度的FEM模型,基于Markov链的FEM模型计算更加简便,利于计算机仿真实现,而且不降低FEM模型的精确度,可以精确模拟实际情况.     

平面P波在弹性介质和非饱和多孔弹性介质分界面上的传播

使用线性粘滞的多孔弹性介质模型,解决在弹性介质和非饱和多孔弹性介质分界面上平面P波的反射与透射问题,这里的非饱和多孔介质中固体骨架被两种相互耦合的流体（液体和气体）所充满．通过势函数的方法得到了振幅反射系数与振幅透射系数．然后推导得到入射波与反射波、透射波之间能量转换情况．研究发现:用振幅比和能量比所表示的反射系数与透射系数是与入射角度、饱和度、入射频率以及上下层介质的弹性常数有关的方程式．数值计算通过图形的形式表达出来,而且入射角度、频率及饱和度对振幅和能量的反射与透射系数的影响分别进行了讨论．证明了在整个波的传播过程中分界处并没有发生能量的耗散．     

有限厚度圆柱壳热冲击问题的广义热弹性解

针对包含有界边界的轴对称结构受热冲击作用的广义热弹性问题进行了研究分析.基于Lord-Shulman广义热弹性理论(L-S理论),构建了热冲击下有限厚度圆柱壳热弹性响应的广义热弹性模型.借助Laplace(拉普拉斯)变换技术以及Bessel函数的渐进特性,推导了温变载荷作用下,圆柱壳内部位移、温度以及应力场的解析表达式.该表达式不仅可以清楚地揭示热冲击下热波、热弹性波在壳体内部的传播、反射以及叠加的作用过程,更可准确地捕捉到热波、热弹性波波前位置处的阶跃现象,并对热冲击诱发的动态热应力峰值进行有效预测.     

散心柱面胞格爆轰演化数值研究

采用有限体积方法,在自适应非结构网格上求解二维含化学反应Euler方程,数值研究了柱面胞格爆轰波演化现象．化学反应计算采用单步可逆总包反应模型．数值结果演示了散心柱面胞格爆轰波演化过程中胞格结构的分裂现象,获得了与实验结果定性一致的结果．胞格结构的分裂演化在点火区近场和远场显示了不同的特点,其中爆轰波传播过程中波阵面当地曲率的变化是控制胞格分裂演化行为的关键因素．数值结果也显示胞格结构的分裂现象来自于爆轰波前锋结构中横波的自组织行为,即沿爆轰波波面传播的小扰动发展成为横波的过程,这种现象与胞格爆轰波的不稳定性密切相关．     

小鼠视觉感受区电位信号(LFP)与视觉刺激之间关系研究

针对小鼠脑电波与行为之间关系研究中涉及弱脑电波信号处理、成分分离及提取、脑电波与对应行为分析等问题,首先处理了脑电波信号,根据呼吸机理建立了"呼"、"吸"动作电位触发下的脑电波模型.其次,通过独立分量分析方法获得睡眠状态小鼠视觉感受区LFP信号模型,采用区间估计和Rayleigh熵检验结合讨论了提取的相关脑电波与呼吸信号的锁相性,分析了LFP信号的周期性,建立了多元线性回归模型,用最小二乘法从理论上讨论了其线性关系,并检验了所建二次模型周期变化的相关性.然后,通过平均经验模态分解及基于互相关性的伪分量检验算法,改善了模态混叠,建立了完整的脑电波信号分离模型.最后,分析图形因素的图像特征,通过提取视觉刺激曲线、功率谱相关频段,采用叠加平均信号法,以LFP节律随视觉信号锁相程度判断不同视觉刺激引起的LFP成分变化.     

Analysis of microwave induced natural convection in a single mode cavity (Influence of sample volume, placement, and microwave power level)

The heating of water layer using microwave oven with a rectangular waveguide has been studied both numerically and experimentally. The mathematical model is validated with the experimental data. The transient Maxwell’s equations are solved by using the Finite Difference Time Domain (FDTD) method to describe the electromagnetic field inside the waveguide and sample. The temperature profile and velocity field within sample are determined by the solutions of the momentum, energy and Maxwell’s equations. In this study, the effects of physical parameters, e.g. microwave power level, placement of sample inside the waveguide, volume of sample, are studied. The distribution of electric field, temperature profile and velocity field are presented in details. The results show good agreement between simulation results and experimental data. Conclusively, the mathematical model presented here correctly explains the phenomena of microwave heating of water layer.     

微波暗室辐射问题的动态平衡解法

利用动态平衡原理,给出了一种微波暗室辐射问题的求解方法,并进行了计算机迭代仿真,证明了该方法的可行性.     

Mathematical model of fuel layer degradation when the laser target is heated by thermal radiation in the reactor working chamber

We propose a mathematical model of the changes occurring in the geometrical properties of the deuterium–tritium layer on the laser target in the process of its insertion into the reactor working chamber. The model is a parabolic equation of general form in spherical coordinates with nonlinear boundary conditions on a moving boundary. We show that under physically justified assumptions this problem may be regarded as a Stefan problem for a singularly perturbed parabolic equation. The first terms of the solution series are written out. Numerical calculations of the fuel layer degradation time are presented for a real target.     

模型燃烧室冷态流场的数值研究

本文采用α-ε湍流模型对模型燃烧室内湍流冷态流场进行了数值分析.采用混合差分格式和SNIP方法进行数值求解.得到了燃烧室内回流区再附长度及速度分布的数值解.计算结果与有关文献的实验数据吻合较好.本文的工作可以作为建立突扩型燃烧室计算模型的基础.     

A complete analysis of axial piston pump leakage and output flow ripples

The paper is focused on understanding the flow losses and the resulting flow/pressure dynamics in a piston pump. Initially, equations to evaluate leakages in all piston pump gaps will be presented and tested against numerical models, later the equations will be linked to determine the general pressure/flow pump dynamic characteristics. The model will also provide the temporal pressure in each piston/cylinder chamber and the temporal leakage in all pump clearances. A test rig able to measure the dynamic pressure inside a piston chamber was build and employed to evaluate pressure ripple dynamics as a function of turning speed, outlet pressure and swash plate angle. The comparison between experimental and simulated results is very good, giving confidence to the model presented. The advantage of using the analytical approach is that explicit equations allow a more direct understanding of the effect of dimension changes and operating conditions on pump dynamics. Fluid used hydraulic oil ISO 32.     

Numerical simulation of methane partial oxidation in the burner and combustion chamber of autothermal reformer

This study aims to model the methane partial oxidation process in the burner and combustion chamber of autothermal reactor. The numerical simulation based on this model offers a powerful tool that can assist in reactor design and optimization and scale up of the process saving expensive pilot work. The steady-state governing equations were solved using the SIMPLE algorithm and the effect of turbulence on the mean flow field was accounted for using the RNG k–ε model. A two-step reaction mechanism was used for the gas combustion with CO as the intermediate species. The reaction rates were modeled using an Eddy-Dissipation Model. In terms of the geometrical model, a 3D model for burner was developed while an axis-symmetric model for the combustion chamber was implemented to reduce the computational costs. The model formulated was validated against a currently operating autothermal reactor and then has been used to investigate different aspects of these reactors. Results show that effect of oxygen to methane ratio is more than that of feed temperature. It is demonstrated that a 60% increase in O₂/CH₄ ratio causes a 15.4% decrease and 42.7% increase in H₂/CO ratio and methane conversion, respectively. In contrast, a 60% increase in feed temperature does not have a significant effect on the process.     

相关光谱式气体检测系统的光学仿真及优化

依据相关光谱式红外气体传感器检测原理,以系统中红外光与气体反应的气室为研究对象,利用光学设计专用软件Tracepro对理想红外朗伯光源在不同反射镜类型下光源的最佳位置进行了仿真分析;通过建立光路传输系统的数学模型,对气室不同结构尺寸下的光功率输出、以及气室内壁反射率对传感系统性能的影响进行了仿真与优化.在理论模型的基础上,根据仿真结果,确定了最佳气室模型的参数组合.实验结果表明,通过对气室中红外光源的位置、反射镜类型、气室长度等部分进行适当的优化修改后,可使探测器接收信号幅度得到明显提高,将有利于后级电路的信号放大、数模转换等处理过程,从而提高气体浓度检测的灵敏度和精度.     

On a structural acoustic model with interface a Reissner-Mindlin plate or a Timoshenko beam

This paper is concerned with a model which describes the interaction of sound and elastic waves in a structural acoustic chamber in which one “wall” is flexible and flat. The model is new in the sense that the composite dynamics of the three-dimensional structure is described by the linearized equations for a gas defined on the interior of the chamber and the Reissner-Mindlin plate equations on the two-dimensional flat wall of the chamber, while, if a two-dimensional acoustic chamber is considered, the Timoshenko beam equations describe the deflections of the one-dimensional “wall.” With a view to achieving uniform stabilization of the structure linear feedback boundary damping is incorporated in the model, viz. in the wave equation for the gas and in the system of equations for the vibrations of the elastic medium. We present the uniform stability result for the case of a two-dimensional chamber and outline the method for the three-dimensional model which shows strong resemblance with the system of dynamic plane elasticity.     

The mathematical and computer modelling of microwave semiconductor devices

The equations governing the modelling of several microwave semiconductor devices are described, with particular attention being paid to the simulation of high speed microwave MESFET and HEMT devices. The MESFET is modelled using the “hydrodynamic equations” consisting of the coupled Poisson, current continuity, and energy transport equations. In order to obtain positive numerical diffusion over all the ranges of the physical parameters, it is found and reported here for the first time that interpolation of the electron temperature between grid points must be done using a logarithmic mean which lies between the corresponding arithmetic and geometric means. In the HEMT simulation, the above equations are augmented by the Schrödinger equation which must be solved self-consistently for the electron energy sub-bands. Simulation results are presented for the above devices, with a multigrid method applied to the HEMT device.