捕食者环境容纳量依赖于食饵的食饵-捕食者模型

本文建立捕食者环境容纳量依赖于食饵数量的食饵-捕食者模型,分析了模型解的全局性态.可能同时存在多个正平衡点,它们当中会有鞍点或鞍结点.除鞍点外每个正平衡点都有一个吸引域.高寒草甸植被与高原鼠兔之间的关系可以用所建立的模型来描述,利用模型参数的实际值分析了高寒草甸退化的原因和恢复策略.过度放牧、人类不合理活动的加剧、高原鼠兔天敌的减少都是高寒草甸退化的原因.相应地,通过合理放牧、规范人类活动、保护或招引高原鼠兔的天敌、药物控制可以恢复退化的高寒草甸.     

干旱地区陆面过程耦合模式及应用

本文主要论述干旱地区陆面过程在改善人类生存环境与研究区域经济中的重要地位与研究现状,强调了采用耦合模式的必要性,详细描述了该模式的特点,并将它应用于我国西北腾格里沙漠南缘植物固沙区的水热循环研究,揭示干旱区陆面过程的主要特征,并同现场测试进行了比较。     

大型海湾水交换计算中随机游动方法的应用研究

水交换矩阵是反映大型海湾不同子区域之间水交换关系的有效工具,该文对随机游动方法在水交换矩阵计算中的应用进行了研究．通过与对流扩散模型的对比,指出用随机游动方法计算水交换矩阵比用对流扩散方程求解更快更灵活．同时,为了对不同区域之间的水交换特性进行长期快速预测,采用了Thompson提出的预测矩阵方法．通过理论分析得出,影响该矩阵预测效果的因素主要有:粒子数目、分区数和预测矩阵取值时间．以一个矩形海湾为例,将数值模拟结果和预测矩阵预测结果进行对比,分析了以上各因素对预测效果的影响．结果表明,粒子数目越多,预测矩阵取值时间越长,预测矩阵的预测效果越好．在粒子数目足够多的情形下,分区数越多,预测效果越好．相反,粒子数目如果较少,过多的分区数将可能导致预测矩阵效果太差而不可用．     

高超音速零攻角尖锥边界层转捩的机理

先计算出高超音速零攻角尖锥边界层的定常层流流场．然后在计算域的入口引入两组有限幅值的T-S波扰动,对空间模式的转捩过程进行了直接数值模拟．分析了转捩过程的机理,发现平均流剖面稳定性的变化是其关键．并进一步讨论了不同模态初始扰动在高超音速尖锥边界层中的演化规律．     

超音速平板边界层转捩中层流突变为湍流的机理研究

采用空间模式,对来流Mach数为4.5的平板边界层转捩过程做了直接数值模拟．对结果进行的分析发现,在层流-湍流转捩的突变(breakdown)过程中,层流剖面得以快速转变为湍流剖面的机理在于平均剖面的修正导致了其稳定性特征的显著变化．虽然在层流下第2模态T-S波更不稳定,但在层流突变为湍流的过程中,第1模态不稳定波也起了重要作用．     

应用于非惯性系湍流模拟的扩展内禀旋转张量

研究扩展内禀旋转张量在非惯性系湍流模拟中的作用,特别是对代数Reynolds应力湍流模式(如非线性K-ε模式)的重要性．为此,采用几个近年来发展的非线性K-ε湍流模式模拟旋转坐标系下均匀剪切湍流,并且和大涡模拟的结果进行比较．计算结果和分析表明,需要发展更先进的非线性K-ε模式从而更好地描述非惯性系下的复杂湍流．     

三维矩形槽道中颗粒沉降的数值模拟

采用三维格子Boltzmann方法对矩形通道中的颗粒沉降进行了模拟研究．单颗粒沉降的模拟结果表明,颗粒最终的稳定沉降位置沿槽道中心线,不受颗粒初始位置和直径的影响．颗粒和壁面之间的两体相互效应可以用无因次沉降速度定量描述,无因次沉降速度的模拟结果和实验结果定量上吻合一致．模拟分析了双颗粒沉降的DKT（drafting, kissing and tumbling）过程,探讨了颗粒直径比以及壁面效应对DKT过程的影响．模拟发现当颗粒直径相同时,双颗粒的沉降过程为周期性的DKT过程,从而形成双螺旋形式的沉降轨迹,此螺旋沉降轨迹的频率和振幅受颗粒初始位置影响．从模拟结果中还得到颗粒群的最终稳定构型,并进行了构型对比分析．最后对包含49个颗粒的颗粒群沉降行为进行了模拟,说明多体相互作用在对称性的情况下可以简化．     

黑龙江漠河地区冰雪情自动监测系统的设计与应用

基于空气、冰、雪与水物理特性的差异,设计了适应高寒地区内陆河道冰雪情定点自动检测的传感器和数据采集系统设备.该系统被安装在黑龙江省漠河县北极村水位站黑龙江河道断面,进行了3个半月的冰雪情定点连续监测,获取了系统的黑龙江河道内部冰水情以与积雪深度现场数据.采集数据完整地反映了黑龙江河道冬季冰层和雪层生消变化过程,为分析高寒地区冬季冰雪情变化规律以及春季凌汛预警提供了科学的预测数据,并探索出了一种适合野外恶劣环境下工程应用的冰雪情检测方法和系统设备.     

正变坐标系下完全深度平均湍流方程组

本文针对宽浅型水域，对三维湍流时均方程组逐项进行深度平均，推导出包含自由水面和地形影响的深度平均流动控制方程组．本文还同时获得了深度平均形式的ｋ－ε湍流模型方程组．因计入了水流的三维效应，该模型称为完全深度平均模型．考虑到天然水域几何边界复杂，本文运用较简便的方法，将上述模型方程组交换至正交坐标系下．所得控制方程组可以直接运用于对实际问题的数值模拟．     

角质层渗透性质的有限元数值模拟计算

角质层是皮肤屏障作用的最主要部分，它决定了外界物质对皮肤的渗透情况．在假设角质层细胞为一种三维的十四面体（物理学经典的tetrakaidecahedron体）的情况下，利用有限元法对角质层渗透性质进行了数值模拟研究．为此，首先完成了对角质层空间结构的网格拆分，拆分过程分两步进行：1．对角蛋白细胞的网格拆分；2．对角蛋白细胞周围的网状脂质体的网格拆分．在数值模拟过程中，则用有限元法得到方程离散的格式，用多重网格算法降低高频误差，提高计算精度．最后，给出了数值模拟结果的可视化效果图．     

Modeling of MIG/MAG welding with experimental validation using an active contour algorithm applied on high speed movies

This paper investigates some issues in physical modeling of metal inert gas/metal active gas (MIG/MAG) welding process in the short arc mode. In this mode, a metal supply is molten in the arc state and then transferred to the weld pool during the short-circuit state. A hybrid model having two distinct continuous states whose switchings are controlled by two guard conditions is proposed. Due to the complexity of the physical phenomena involved in the welding process, simplifications are used to obtain a model accounting for the main physical contributions but simple enough to yield an efficient, fast and numerically tractable simulator which can be used intensively for evaluating different control strategies. In an attempt to validate the proposed model, different measurements have been made including supply voltage and current sampled synchronously with high speed digital video. In order to extract some relevant quantities representative of the metal transfer from image sequences, an active contour algorithm is developed and tested. The effectiveness of the proposed model in the prediction of major tendencies of a welding process, especially in the arc state, is shown using experimental data. Some limitations of the model during the metal transfer are also stressed and possible remedies are then proposed.     

A model of the electrothermal ribbon printing process

A sequence of mathematical models describing the electrothermal ribbon printing process is developed. The models describe the electrical processes, heat generation and heat transfer within the ribbon and the transfer of ink from the ribbon to paper. Account has been taken of the nonlinear electrical properties of the ribbon. A model of the paper surface has been developed in order to model the transfer of the ink from the ribbon to the paper. The partial differential equations describing the electric field, the heat flow and the stresses in the ink have been solved using standard techniques.     

Thermal analysis of hot strip rolling using finite element and upper bound methods

Thermal analysis of hot rolling process has been studied in this work. A finite element method has been coupled with an upper bound solution assuming, triangular velocity field, to predict temperature field during hot strip rolling operation. To do so, an Upwind Petrov–Galerkin scheme together with isoparametric quadrilateral elements has been employed to solve the steady-state heat transfer equation. A comparison has been made between the published and the model predictions and a good agreement was observed showing the accuracy of the proposed model.     

Magnetoviscous potential flow analysis of Kelvin–Helmholtz instability with heat and mass transfer

A linear analysis of the Kelvin–Helmholtz instability of interface between two viscous and magnetic fluids has been carried out where there was heat and mass transfer across the interface while the fluids have been subjected to a constant magnetic field parallel to the streaming direction. The viscous potential flow theory has been used for the investigation. A dispersion relation has been obtained and a stability criterion is given by a critical value of relative velocity as well as the critical value of magnetic field. The resulting plots show the effect of various physical parameters such as wave number, viscosity ratio, ratio of magnetic permeabilities and heat transfer coefficient. It has been observed that heat and mass transfer has a destabilizing effect whereas the horizontal magnetic field stabilizes the system.     

Effects of chemical reaction and radiation on an unsteady MHD flow past an accelerated infinite vertical plate with variable temperature and mass transfer

This paper deals with the study of the effects of first order chemical reaction and radiation on an unsteady MHD flow of an incompressible viscous electrically conducting fluid past an accelerated infinite vertical plate with variable temperature and mass transfer. The resulting approximate dimensionless system of governing partial differential equations are integrated in closed form by the Laplace transform technique A uniform magnetic field is assumed to be applied transversely to the direction of the flow. Rosseland model of radiation has been chosen in the investigation, the expressions for the velocity field, temperature field and concentration field and skin-friction in the direction of the flow, coefficient of heat transfer and mass flux at the plate have been obtained in non-dimensional form and these are illustrated graphically for various physical parameters involved in the study. Investigation reveals that the fluid velocity is decelerated in the region adjacent to the plate, due to the effect of first order chemical reaction and the rate of heat transfer (from plate to the fluid) decreases due to the absorption of thermal radiation. The results obtained in this work are consistent with physical situation of the problem.     

A non-local heat transport model in solids with discontinuities via Lagrangian particle method

A new Lagrangian non-local diffusion model, designed as a meshless particle simulation method, is proposed to predict the thermal response of solids involving discontinuous. The main idea is to understand the heat transfer process of solids via a Lagrangian treatment of a particle-discretized system, which provides a more general physical representation of the heat transfer process of solids. In contrast to the traditional differential model, the proposed mathematical model is expressed via an integral form, which can easily handle the case involving discontinuities. The spatial convergence studies show that the proposed model converges to its associated non-local solution while increasing the number of particles in a fixed cut-off radius; and it can reach the local exact solution when the cut-off radius is close to zero. The numerical examples not only verify that the proposed diffusion model converges to the continuum heat conduction model, but also show the capability of its application to heat transfer problems involving discontinuities. In particular, the computational performance of the proposed non-local thermal model is also demonstrated its computational performance in the case with practical crack propagation in a two-dimensional plate. In addition, the specific comparison between the proposed method and the well-developed peridynamics approach for heat conduction problems is carefully described with rigor via mathematical proofs and numerical results. Specifically, the proposed model is shown to be a variation of the corresponding peridynamic-type thermal diffusion model. This model not only has a formulation consistent with that of peridynamics in solid mechanics but also shows a better performance in the case with sharp corners than that of the peridynamic diffusion in [1].     

A simulation code for investigating 2D heating of material bodies by low frequency electric fields

The finite difference method has been used to simultaneously solve in two dimensions Maxwell's equations and the heat transfer equation in forms which are appropriate to modelling low frequency electrical heating of solid materials. The nonlinear coupling of these modelling equations, which is due to temperature dependent electrical conductivities, necessitates the use of an explicit-sequential solution method and the limiting of the timestep size to ensure stability. The finite difference equations were modified to account for sharp electrical conductivity differences between different media in the body being heated.The simulation code was tested by comparison of the simulator predictions with the measured results of a physical scale model experiment. The simulation code was able to accurately predict the resistance between the electrodes used for heating, the energy deposition and the temperature rise in the bulk of the physical model.     

3D finite element analysis of evaporative laser cutting

A three-dimensional computational model of evaporative laser-cutting process has been developed using a finite element method. Steady heat transfer equation is used to model the laser-cutting process with a moving laser. The laser is assumed continuous wave Gaussian beam. The finite element surfaces on evaporation side are nonplanar and approximated by bilinear polynomial surfaces. Semi-infinite elements are introduced to approximate the semi-infinite domain. An iterative scheme is used to handle the geometric nonlinearity due to the unknown groove shape. The convergence studies are performed for various meshes. Numerical results about groove shapes and temperature distributions are presented and also compared with those by semi-analytical methods.     

Numerical simulation of rotational symmetric tube bulging with inside pressure and axial compression

The contribution treats the process of rotational symmetric tube bulging with inside pressure and axial compression. This process enables the standard tubes to be formed into different rotational symmetric hollow parts in such a way that their central part is expanded into a desired shape while the ends remain unchanged. The superposition of axial compression contributes to a more favourable forming stress state, which is reflected in larger forming limits and smaller wall thinning in the widened area. The problems characterizing the process are a limited range of compression stability and difficulties met when establishing and optimizing the technological parameters of the process whose course cannot be defined in an analytical way. Based on a physical model of the forming process, a numerical model was built. Using ABAQUS code the model was simulated over the entire stress/forming region. A comparison of the computer simulated forming process with the experimentally obtained results showed that the model was highly accurate.