Molecular Dynamics Modeling of Diffusion Bonding

Molecular dynamics simulations on diffusion bonding of Cu-Ag showed that the thickness of the interfacial region depended on the stress. The interfacial region became amorphous during diffusion bonding, and it would normally transform from amorphous into crystalline structure when the structure was cooled to the room temperature.

青岛市水资源复合系统动力学模拟研究

水资源是人类赖以生存和发展的最重要的物质资源之一，水资源的可持续利用是实现社会经济可持续发展的必要前提。随着人口的增加，社会经济的发展和人民生活水平的提高，青岛市水资源供需矛盾进一步凸显。本文在分析青岛市水资源利用现状的基础上，运行系统动力学方法，研究了青岛市水资源复合系统可持续利用状况，主要结论如下： （1）采用系统动力学Vensim-PLE软件，以水资源供需平衡为核心，建立了青岛市水资源-社会-经济-生态环境复合系统动力学模型，并以青岛市2000~2008年的统计资料作为基础数据检验了模型的有效性。为了提高模拟效果，分析了模型主要参数的敏感度，发现工业供水比例对模拟结果影响最大，生活供水比例、第三产业供水比例、回用水工业供水比例、污水处理投资系数及回用水生态环境供水比例等参数的敏感度依次减小。 （2）运用所建立的系统动力学模型模拟趋势条件下青岛市2009~2030年的水资源利用及需求状况。结果表明，现状趋势方案下青岛市2009~2030年的总需水量快速增加，2020年、2030年的总需水量分别为96190.6万吨、137300万吨；水资源供需差额急速增加，缺水程度越来越大，到2030年缺水程度达到0.5055；随着青岛市污水处理能力的提高，污水回用量增加，从3000万吨左右增加到接近10000万吨；现状趋势条件下2009~2030年青岛市GDP值快速增长，人口承载力逐渐上升，但上升趋势逐渐趋缓。 （3）假设了5种水资源发展方案：现状趋势发展型，提高污水处理投资系数，提高海水淡化产量，调整各行业用水定额，调整产业结构。用建立的系统动力学模型模拟分析了在这5种方案下的青岛市水资源可持续利用能力。综合比较这5种方案的优缺点，提出优化方案。优化方案1：将污水处理投资系数由0.001提高至0.003；从2009年起海水淡

空间柔性双臂机器人系统建模、控制与仿真研究

空间柔性多臂机器人系统是高度非线性,强耦合的动力学系统,其动力学的研究是比较复杂和困难的问题.本文针对极为复杂的空间柔性双臂机器人系统,建立了其系统动力学模型,利用逆动力学控制算法对该动力学模型实现了轨迹跟踪控制,仿真结果表明该方法具有较好的控制效果,分析了在仿真过程中出现的有关数值算法的问题.

空间对接机构差动式缓冲系统动力学建模与仿真

对接技术是空间交会对接的关键技术，是建立大型空间站，进行复杂空间作业的前‘提，是我国载人航天工程实施需要迫切解决的问题。对接机构缓冲系统是对接机构的核心部分，决定着对接过程的动力学特性，对实现两航大器安全可靠的对接起着至关重要的作用。对接机构缓冲系统结构和工作原理复杂，是一个多自由度、多变量、高度非线性、多参数祸合的动力学系统，对其进行研究是对接机构研制的核心工作，具有重要的理论和实践意义。本文针对我国载人航天工程正在研制的周边式对接机构差动式缓冲系统进行了以’一厂研究：建立了对接机构缓冲系统的运动学模型，利用Lagrange方程建．立了差动式缓冲系统动力学模型，该模型综合体现了缓冲系统参数对缓冲特性的影响。同时，为缓冲系统实时动力学仿真模型的建立奠定了理论基础。首次建立了对接机构缓冲系统全真可视化实时动力学仿真模型，将约束的真实传动特性反映到仿真模型中，解决了多体动力学建模时山一于对杆自差织，合中两根丝杠佩的适动特性进行近似相等假设所造成的缓冲系统动态特J性失真问题，实现了非线性强棍合系统的实时动态仿真。 利用快速极坐标搜索法，进行了对接环工作空间的确定，基于模态测试方法提出了缓冲系统等效特性的虚拟试验方案，得到了系统的动静态力学性能参数，实现了对对接机构缓冲系统设计的验证。总结出对接机构的最大行程和缓冲阻尼特性的变化规律，建立了机构最大工作行程与机构传动链参数和缓冲元件结构参数的关系模型、系统等效缓冲阻尼特性与缓冲元件性能参数的关系模型，对缓冲系统的深入研究有很大的理论价值，对对接机构缓冲系统的工程设计具有重要的指导意义。建立了缓冲系统的缓冲力与对接环相应姿态的高阶关系模型，有效地提高了对接过程动力学仿真的精度。对接机构缓冲系统的动力学建模与仿真，对空间对接技术的研究，载．人航天一L程的顺利实施具有重要意义。对对接机构的研制具有很人的指导意义，为对接过程动力学研究和对接机构的深入设计奠定了坚实的基础。本文的研究结果，己在上海航天局八0五所对接机构的研制中得到了应用。

Structural analysis of human CCR2b and primate CCR2b by molecular modeling and molecular dynamics simulation

CCR2b, a chemokine receptor for MCP-1, -2, -3, -4, plays an important role in a variety of diseases involving infection, inflammation, and/or injury, as well as being a coreceptor for HIV-1 infection. Two models of human CCR2b (hCCR2b) were generated by h

Characterize dynamic conformational space of human CCR5 extracellular domain by molecular modeling and molecular dynamics simulation

The chemokine receptor CCR5 is the receptor for several chemokines and major coreceptor for R5 human immunodeficiency virus type-1 strains entry into cell. Three-dimensional models of CCR5 were built by using homology modeling approach and 1 ns molecular dynamics (MD) simulation, because studies of site-directed mutagenesis and chimeric receptors have indicated that the N-terminus (Nt) and extracellular loops (ECLs) of CCR5 are important for ligands binding and viral fusion and entry, special attention was focused on disulfide bond function, conformational flexibility, hydrogen bonding, electrostatic interactions, and solvent-accessible surface area of Nt and ECLs of this protein part. We found that the extracellular segments of CCR5 formed a well-packet globular domain with complex interactions occurred between them in a majority of time of MID simulation, but Nt region could protrude from this domain sometimes. The disulfide bond Cys20-Cys269 is essential in controlling specific orientation of Nt region and maintaining conformational integrity of extracellular domain. RMS comparison analysis between conformers revealed the ECL1 of CCR5 stays relative rigid, whereas the ECL2 and Nt are rather flexible. Solvent-accessible surface area calculations indicated that the charged residues within Nt and ECL2 are often exposed to solvent. Integrating these results with available experimental data, a two-step gp120-CCR5 binding mechanism was proposed. The dynamic interaction of CCR5 extracellular domain with gp120 was emphasized. (C) 2004 Elsevier B.V. All rights reserved.

Modeling phytoplankton dynamics in the River Darling (Australia) using the radial basis function neural network

A radial basis function neural network was employed to model the abundance of cyanobacteria. The trained network could predict the populations of two bloom forming algal taxa with high accuracy, Nostocales spp. and Anabaena spp., in the River Darling, Australia. To elucidate the population dynamics for both Nostocales spp. and Anabaena spp., sensitivity analysis was performed with the following results. Total Kjeldahl nitrogen had a very strong influence on the abundance of the two algal taxa, electrical conductivity had a very strong negative relationship with the population of the two algal species, and flow was identified as one dominant factor influencing algal blooms after a scatter plot revealed that high flow could significantly reduce the algal biomass for both Nostocales spp. and Anabaena spp. Other variables such as turbidity, color, and pH were less important in determining the abundance and succession of the algal blooms.

Modeling the dynamics of Si wafer bonding during annealing

The bonding behavior of silicon wafers depends on activation energy for the formation of siloxane bonds. In this article we developed a quantitative model on the dynamics of silicon wafer bonding during annealing. Based on this model, a significant difference in the bonding behaviors is compared quantitatively between the native oxide bonding interface and the thermal oxide bonding interface. The results indicate that the bonding strength of the native oxide interface increases with temperature much more rapidly than that of the thermal oxide interface. (C) 2000 American Institute of Physics. [S0021-8979(00)05520-1].

Modeling of Ammonothermal Growth of Nitrides

Bulk single crystals of GaN and AlN can be grown from supercritical fluids using the ammonothermal method, which utilizes ammonia as fluid rather than water as in the hydrothermal process. In this process, a mineralizer such as amide, imide or nitride is used to attack a bulk nitride feedstock at temperatures from 200°C to 500°C and pressures from 1 to 4 kbar. Ammonothermal systems have been modeled here using fluid dynamics, thermodynamics and heat transfer models. The nutrient is considered as a porous media bed and the fluid flow is simulated using the Darcy-Brinkman-Forchheimer model. The resulting governing equations are solved using the finite volume method. The effects of particle size on flow pattern and temperature distribution in an autoclave are analyzed.

Molecular Dynamics Simulation of Barnacle Cement

Barnacle cement is an underwater adhesive that is used for permanent settlement. Its main components are insoluble protein complexes that have not been fully studied. In present article, we chose two proteins of barnacle cement for study, 36-KD protein and Mrcp-100K protein. In order to investigate the characteristic of above two proteins, we introduced the method of molecular modeling. And the simulation package GROMACS was used to simulate the behavior of these proteins. In this article, before the simulations, we introduce some theories to predict the time scale for polymer relaxation. During the simulation, we mainly focus on two properties of these two proteins: structural stability and adhesive force to substrate. First, we simulate the structural stability of two proteins in water, and then the stability of 36-KD protein in seawater environment is investigated.We find that the stability varies in the different environments. Next, to study adhesive ability of two proteins, we simulate the process of peeling the two proteins from the substrate (graphite). Then, we analyze the main reasons of these results. We find that hydrogen bonds in proteins play an important role in the protein stability. In the process of the peeling, we use Lennard–Jones 12-6 potential to calculate the van der Waals interactions between proteins and substrate.

The Head-On Colliding Process Of Binary Liquid Droplets At Low Velocity: High-Speed Photography Experiments And Modeling

The experimental and theoretical studies are reported in this paper for the head-on collisions of a liquid droplet with another of the same fluid resting on a solid substrate. The droplet on the hydrophobic polydimethylsiloxane (PDMS) substrate remains in a shape of an approximately spherical segment and is isometric to an incoming droplet. The colliding process of the binary droplets was recorded with high-speed photography. Head-on collisions saw four different types of response in our experiments: complete rebound, coalescence, partial rebound With conglutination, and coalescence accompanied by conglutination. For a complete rebound, both droplets exhibited remarkable elasticity and the contact time of the two colliding droplets was found to be in the range of 10-20 ms. With both droplets approximately considered as elastic bodies, Hertz contact theory was introduced to estimate the contact time for the complete rebound case. The estimated result Was found to be on the same order of magnitude as the experimental data, which indicates that the present model is reasonable. (C) 2008 Elsevier Inc. All rights reserved.

Nonlinear dynamics of atomic force microscopy with intermittent contact

When the atomic force microscopy (AFM) in tapping mode is in intermittent contact with a soft substrate, the contact time can be a significant portion of a cycle, resulting in invalidity of the impact oscillator model, where the contact time is assumed to be infinitely small. Furthermore, we demonstrate that the AFM intermittent contact with soft substrate can induce the motion of higher modes in the AFM dynamic response. Traditional ways of modeling AFM (one degree of freedom (DOF) system or single mode analysis) are shown to have serious mistakes when applied to this kind of problem. A more reasonable displacement criterion on contact is proposed, where the contact time is a function of the mechanical properties of AFM and substrate, driving frequencies/amplitude, initial conditions, etc. Multi-modal analysis is presented and mode coupling is also shown. (c) 2006 Published by Elsevier Ltd.

Molecular Dynamics Simulation of Barnacle Cement

Barnacle cement is an underwater adhesive that is used for permanent settlement. Its main components are insoluble protein complexes that have not been fully studied. In present article, we chose two proteins of barnacle cement for study, 36-KD protein and Mrcp-100K protein. In order to investigate the characteristic of above two proteins, we introduced the method of molecular modeling. And the simulation package GROMACS was used to simulate the behavior of these proteins. In this article, before the simulations, we introduce some theories to predict the time scale for polymer relaxation. During the simulation, we mainly focus on two properties of these two proteins: structural stability and adhesive force to substrate. First, we simulate the structural stability of two proteins in water, and then the stability of 36-KD protein in seawater environment is investigated. We find that the stability varies in the different environments. Next, to study adhesive ability of two proteins, we simulate the process of peeling the two proteins from the substrate (graphite). Then, we analyze the main reasons of these results. We find that hydrogen bonds in proteins play an important role in the protein stability. In the process of the peeling, we use Lennard-Jones 12-6 potential to calculate the van der Waals interactions between proteins and substrate.

Modeling of Ammonothermal Growth of Gallium Nitride Single Crystals

Ammonothermal growth of GaN crystals with a retrograde solubility has been modeled and simulated here using fluid dynamics, thermodynamics and heat transfer models. The nutrient is considered as a porous media bed and the flow in the porous charge is simulated using the Darcy-Brinkman-Forchheimer model. The resulting governing equations are solved using the finite volume method. For the case of retrograde solubility, the charge is put above the baffle. The temperature difference between the dissolving zone and growth zone is found smaller than that applied on the sidewall of autoclave. The baffle opening has a strong effect on the nutrient transport and supersaturation of GaN species in the growth zone.

Information Preservation Modeling of Rayleigh-Benard Transition from Thermal Conduction to Convection

Onset and evolution of the Rayleigh-Benard (R-B) convection are investigated using the Information Preservation (IP) method. The information velocity and temperature are updated using the Octant Flux Splitting (OFS) model developed by Masters & Ye based on the Maxwell transport equation suggested by Sun & Boyd. Statistical noise inherent in particle approaches such as the direct simulation Monte Carlo (DSMC) method is effectively reduced by the IP method, and therefore the evolutions from an initial quiescent fluid to a final steady state are shown clearly. An interesting phenomenon is observed: when the Rayleigh number (Ra) exceeds its critical value, there exists an obvious incubation stage. During the incubation stage, the vortex structure clearly appears and evolves, whereas the Nusselt number (Nu) of the lower plate is close to unity. After the incubation stage, the vortex velocity and Nu rapidly increase, and the flow field quickly reaches a steady, convective state. A relation of Nu to Ra given by IP agrees with those given by DSMC, the classical theory and experimental data.