Numerical simulation of the flow within and over an intersection model with Reynolds-averaged Navier--Stokes method

In this study, the Reynolds-averaged Navier--Stokes (RANS) method is employed to simulate the flow within and over an intersection model with three kinds of $k$--$\varepsilon$ turbulence closure schemes, namely, standard model, renormalization group (RNG) model and realizable $k$--$\varepsilon$model. The comparison between the simulated and observed flow fields shows that the RANS simulation with all the three turbulence models cannot completely and accurately reproduce the observed flow field in all details. A detailed comparison between the predicted profiles of wind velocities and the measured data shows that the realizable $k$--$\varepsilon$ model is the best one among the three turbulence closure models in general. However, the extent to which the improvement is achieved by the realizable $k$--$\varepsilon$ model is still not enough to completely and accurately describe the turbulent flow in a relatively complex environment.     

New ways of deriving Arnowitt—Deser—Misner constraint equations in four—dimensional gravity

In this paper,the Arnowitt-Deser-Misner (ADM) constraint equations are naturally derived in two different ways.One method is to construct a one-parametric gravitational action in the Lorentzian spacetime.Hence,the oneparametric ADM constraint equations can be obtained.The other method is to apply the double complex function method to Einstein-Hilbert gravitational fields in Hamiltonian formulation,Therefore the double ADM constraint equations can be obtained,in which the well-known ADM constraint equations are included as a special case.     

The double complex Ashtekar gravitational theory and the corresponding constraint equations with the cosmological term

We establish the double complex Ashtekar gravitational theory with the cosmological term.In particular,by performing the 3 1 decomposition of the double Ashtekar action containing the cosmological term to pass on the Hamiltonian framework,the double Ashtekar constraint equations are derived,which respectively correspond to Lorentzian and Euclidean gravity.     

中高能核物理中的几个问题

简要介绍和评述了近几年来中高能核物理中一些受关注的问题,重点介绍了核子电磁形状因子和K^-介子与原子核相互作用势强度问题。 Several problems in intermediate and high energy nuclear physics have attracted considerable attention in these years: （1） Nucleon electromagnetic form factors. There is a systematic difference between the recently measured data from high energy polarized electron-nucleon scattering experiments and the previous Rosenbluth results. （2） The strength of K^- nucleus optical potentials. It varies qualitatively from different analyyses on K^- nucleus interactions. In this paper, the above problems are discussed and reviewed briefly.     

充液腔体内复杂流动及其系统动力学研究

分别对旋转和非旋转充液腔体,阐述了腔内液体各种流动求解理论与实验研究的进展, 简述了主要数值模拟方法的应用状况,并从系统动力学角度,讨论了充液腔体耦合系统动力 学中的相关问题.     

基于宽范围粒径谱仪的气溶胶探测方法

通过改变基于宽范围粒径谱仪的温度控制方法,研究温度变化对气溶胶浓度的影响和作用.本文首先对宽范围粒径谱仪进样系统温度控制方法进行改进,即采用钽管加热器及不加热切换的温度控制方法来改变宽范围粒径谱仪进样部分温度,使用改进后的系统测量室内外大气气溶胶,对不同温度下大气气溶胶的浓度分布情况进行讨论与分析.结果表明室外粒子数浓度分布的峰值与室内粒子浓度分布的峰值相比,室内空气中粒子数浓度峰值明显要小,即室外大气中的粒径大的粒子更多.证实了宽范围粒径谱仪适用于大气气溶胶的检测,在环境检测中有广泛的应用前景.     

基于小波变换的多频静电力显微镜动态过程测量方法

静电力显微镜(electrostatic force microscopy,EFM)具有较高的灵敏度和空间分辨率,在新能源材料静电性质的测量中被广泛应用.时间分辨静电力显微镜技术主要用于材料的动态电学性质测量,该技术中常用的泵浦探测方法存在设备装置复杂、成本昂贵、测量存在不确定性等问题.本文采用直接时域测量方法,减小了测量的实现复杂度,通过应用多频激励或者激励悬臂高阶模态的方法实现了EFM多种参数同时测量和时间分辨率的提高,达到了微秒级的时间分辨率,并应用小波变换对测量得到的探针信号进行分析,实现了对材料动态电学性质的提取.通过应用该技术进行仿真实验,实现了对模拟样品电势衰减过程中的动态电势变化和模拟电池放电过程中的离子运动特征时间等性质的测量.     

进动圆筒内液体流动不稳定的非线性特征

在建立进动充液圆筒内液体偏差流动方程的基础上，结合液体惯性波和轴向二次流动线性解，通过对定常二次流动的线性稳定性分析，提出了函数空间表达的流动不稳定性非线性分岔分析方程. 对非惯性坐标系下液体流动的Navier-Stokes方程进行了数值求解，并对惯性波发生破裂(实验提供的3种主模态下得出的共振破裂现象)时的压力时间序列进行分析，得出了液体流动不稳定的基本非线性特征.     

制造企业产品质量问题的信号博弈模型北大核心

基于制造企业产品质量信息不对称的特点,以制造企业和客户作为博弈方,以客户服务水平作为产品质量信号,建立信号博弈模型,分析信号博弈的三种贝叶斯均衡及其条件,得出了信息不对称情况下制造企业和客户的应对策略.     

印刷电路板通道的高温传热和阻力特性研究

印刷电路板换热器是超高温气冷堆等高温能量利用与回收领域中极具潜力的备选强化传热技术之一,本文通过数值模拟方法对典型Z字形印刷电路板(PCHE)传热通道的传热和阻力特性开展了相关研究。计算结果表明:层流模型可以较好地获得Z字形PCHE通道的阻力和传热性能,Z字形PCHE通道的阻力系数大于半圆管直通道的阻力系数;当热侧和冷侧通道流体入口温度均不发生改变时,增加两侧流体的质量流量,通道内的阻力系数随之减小,传热效率也稍微减小;当两侧流量及冷侧入口温度均不发生改变时,提高热侧通道入口温度,通道内阻力系数成线性增大,传热效率也随之提高。