Simulation results of arc behavior in different plasma spray torches

Three-dimensional, transient simulations of the plasma flow inside different plasma spray torches have been performed using a local thermodynamic equilibrium model solved by a multiscale finite-element method. The model describes the dynamics of the arc without any further assumption on the reattachment process except for the use of an artificially high electrical conductivity near the electrodes. Simulations of an F4-MB torch from Sulzer-Metco and two configurations of the SG-100 torch from Praxair are presented. The simulations show that, when straight or swirl injection is used, the arc is dragged by the flow and then jumps to form a new attachment, preferably at the opposite side of the original attachment, as has been observed experimentally. Although the predicted reattachment frequencies are at present higher than the experimental ones, the model is suitable as a design tool. This article was originally published inBuilding on 100 Years of Success: Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Protonation patterns in tetracycline:tet repressor recognition: simulations and experiments

Resistance to the antibiotic tetracycline (Tc) is regulated by its binding as a Tc:Mg2+ complex to the Tet Repressor protein (TetR). Tc:TetR recognition is a complex problem, with the protein and ligand each having several possible conformations and protonation states, which are difficult to elucidate by experiment alone. We used a combination of free-energy simulations and crystallographic analysis to investigate the electrostatic interactions between protein and ligand and the possible role of induced fit in Tc binding. Tc in solution was described quantum mechanically, while Tc:TetR interactions were described by a recent, high-quality molecular-mechanics model. The orientations of the amide and imidazole groups were determined experimentally by a careful analysis of Debye-Waller factors in alternate crystallographic models. The agreement with experiment for these orientations suggested that the simulations and their more detailed, thermodynamic predictions were reliable. We found that the ligand prefers an extended, zwitterionic state both in solution and in complexation with the protein. Tc is thus preorganized for binding, while the protein combines lock-and-key behavior for regions close to the ligand's amide, enolate, and ammonium groups, with an induced fit for regions close to the Mg2+ ion. These insights and the modeling techniques employed should be of interest for engineering improved TetR ligands and improved TetR proteins for gene regulation, as well as for drug design.     

A molecular dynamics study of siloxane diffusion in a polyester-melamine solution

This study examines the diffusion of small methylated and hydroxylated siloxanes through solutions of polymer and crosslinker molecules. The diffusivity of the siloxanes was calculated for bulk diffusion and for surface segregation. Our simulations demonstrate that there is significant interaction between 3,3-dimethylhexahydroxytrisiloxane (hexa-OH-TS) and the polar solvent components of the paint matrix at 298 K. The nature of these associations is largely due to hydrogen bonding between the siloxane and the polar solvent molecules. At 573 K the diffusivity of solvent molecules is sufficiently high to disrupt interactions. The smaller number of hydroxyl groups in 1,5-dihydroxyhexamethyltrisiloxane (hexa-Me-TS) results in a weaker interaction with the polar solvent. Surface segregation studies indicate that there is a slight increase in the concentration of hexa-OH-TS in the surface layer of the system as a result of curing and relaxation processes. However, for hexa-Me-TS there is in fact a decrease in the surface concentration of the siloxane relative to the bulk concentration.     

Molecular dynamics of poly(glycolide‐co‐L‐lactide) copolymer during isothermal cold crystallization

Poly(glycolide‐co‐l ‐lactide) (PGA/PLLA) is a random copolymer with 92 wt % PGA, being the basic resin for Vicryl® suture. Molecular dynamics of PGA/PLLA in its wholly amorphous state and during isothermal cold crystallization at 70 and 80°C have been analyzed. Experimental results were generated over a wide range of frequency and temperature by broad‐band dielectric spectroscopy (DRS). The variation of the average relaxation time (defined as τ = [1/2]πf_max where f_max is the frequency at maximum loss for the main α relaxation) has been studied during cold crystallization and the temperature dependence of this average relaxation time for wholly amorphous and crystallized samples has been analyzed. This behavior has been modeled by Havriliak–Negami, Vogel–Fulcher–Tammann, and Kohlrausch–Williams–Watts equations. The evolution profile of the dynamics (frequency at which the maximum loss appears, f_max) depends on the crystallization temperature, being different at 80°C relatively to 70°C, which could reflect different progress of the spherulitic morphology, as it is shown by the evolution of the morphologies obtained during the crystallization processes, followed by optical microscopy. While the loss maximum (and consequently relaxation times) remains almost unmodified during the crystallization process at 70°C, for the process at 80°C the maximum first moves slightly to higher frequencies (shortening of relaxation times) and at the final stages of crystallization it moves to lower frequencies (increasing the relaxation times). Supporting evidence about the thermal behavior of the polymers has been obtained with DSC. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

纳米晶柱热稳定性研究

以纳米晶柱作为表面量子点的模型,以不同截面尺寸的Al纳米晶柱为例,对其在不同温度下的弛豫过程进行了一系列分子动力学模拟,采用了Ercolessi等建立的原子镶嵌势计算原子间的相互作用力.结果表明:对于沿相互垂直的{110}和{211}面切割形成的近正方形截面晶柱.其截面厚度存在一热稳定性转变临界值.小于该值时纳米晶柱迅速失稳,发生熔融-重结晶的过程;大于该值时只发生缓慢的表面原子迁移重组.两种情况下形成的稳定结构均为由{111}和{100}面组成的正多面体纳米岛,只是两种面的相对面积比有所不同;该临界尺寸随温度升高而呈近线性增大.模拟结果还显示,纳米晶柱的高度对其稳定性没有明显影响.     

基于ADAMS的端铣齿轮动力学特性研究

采用端铣加工方法加工渐开线齿轮,运用UG软件对某大模数齿轮轴进行刀具轨迹仿真,并通过数据转换将齿轮和刀具模型导入动力学仿真软件ADAMS中。在ADAMS中建立刀具齿轮铣削系统仿真模型,对端铣刀粗加工切削过程进行动力学分析,得到刀具和工件的位移、速度及加速度变化曲线以及主轴转速和进给速度对切削加工及振动影响的变化规律,为实现端铣齿轮加工技术的实际应用提供参考。     

多自由度串联机器人的高效率反向动力学建模方法

针对多自由度串联机器人的反向动力学建模问题,通过引入解耦的自然正交补的概念和方法,消除了牛顿-欧拉动力学方程中的约束力项,得到了一种基于解耦的自然正交补的高效反向动力学递推建模方法。编制程序对七自由度串联机器人进行了仿真验证,并将其与牛顿-欧拉递推建模方法进行了对比。结果表明,基于解耦的自然正交补的反向动力学递推建模方法可节省约2/3的CPU计算时间,仿真结果正确、合理,从而验证了该方法的可靠性和高效性。     

高速重载下双层保护轴承的最大碰撞力及热特性分析

搭建了转子跌落试验台,研究了A、B两种结构的双层保护轴承在高速重载下的工作性能。建立了动力学模型和热网络模型,理论分析了不同双层保护轴承参数下,磁悬浮轴承失效后转子和保护轴承的动力学响应以及保护轴承内圈的温升情况,并进行了相关的转子跌落试验研究。研究结果表明,B结构双层保护轴承更适合应用于高速重载场合。     

海上浮式风电机组刚柔耦合结构动力学建模与分析

基于凯恩方法和模态叠加法对三叶片水平轴海上浮式风电机组进行了刚柔耦合结构动力学建模和分析。首先建立浮式平台的水动力载荷模型,然后,把塔架和叶片等柔性件离散为有限个刚性单元体,并采用模态叠加法计算每一离散单元的偏速度和偏角速度,最后采用凯恩方法建立刚柔耦合结构动力学模型。以美国可再生能源实验室（NREL）海上5MW半潜式风电机组为算例,结合气动力载荷模型和水动力载荷模型对所建立的模型进行了风、浪响应计算,主要从输出功率、叶尖及塔顶振动信号的耦合特性两个方面对海、陆风电机组进行了对比。     

液压马达式汽车主动稳定杆系统建模与控制

以液压马达式主动稳定杆系统为研究对象,推导得到了液压马达式主动稳定杆的非线性动力学模型。利用线性化反馈的滑模控制方法设计了其控制器,并利用Lyapunov理论对控制系统的稳定性进行了证明。通过CarSim和MATLAB/Simulink对主动稳定杆系统进行了联合仿真,得出安装液压马达式主动稳定杆系统的车辆在双移线工况下运行的响应曲线。为了验证设计的控制方法,进行了实车试验。结果表明,与被动稳定杆相比,主动稳定杆系统具有较好的抗侧倾特性以及乘坐舒适性。