关键词： 移动并联机械手   移动双串联机械手   多体动力学建模   加权初值法   数值仿真  

摘要： 提出一种用于验证移动并联机械手动力学建模及数值仿真正确性与有效性的方法。该方法先将移动并联系统某处运动副打开,用关节支反力替代运动副约束,从而形成移动双串联机械手,并以多体动力学理论为指导,在笛卡尔坐标系下分别进行移动并联、双串联机械手动力学建模,采用加权初值法对移动双串联机械手系统进行数值仿真。最后通过对移动并联、双串联机械手动力学数值仿真比较,验证系统动力学模型构建和数值仿真的正确性和有效性。

关键词： Disturbance   Fire management   Functional traits   Habitat accommodation model   Pyrodiversity  

摘要： Models based on functional traits have limited power in predicting how animal populations respond to disturbance because they do not capture the range of demographic and biological factors that drive population dynamics, including variation in trophic interactions. I tested the hypothesis that successional changes in vegetation structure, which affected invertebrate abundance, would influence growth rates and body condition in the early-successional, insectivorous gecko Nephrurus stellatus. I captured geckos at 17 woodland sites spanning a succession gradient from 2 to 48 years post-fire. Body condition and growth rates were analysed as a function of the best-fitting fire-related predictor (invertebrate abundance or time since fire) with different combinations of the co-variates age, sex and location. Body condition in the whole population was positively affected by increasing invertebrate abundance and, in the adult population, this effect was most pronounced for females. There was strong support for a decline in growth rates in weight with time since fire. The results suggest that increased early-successional invertebrate abundance has filtered through to a higher trophic level with physiological benefits for insectivorous geckos. I integrated the new findings about trophic interactions into a general conceptual model of mechanisms underlying post-fire population dynamics based on a long-term research programme. The model highlights how greater food availability during early succession could drive rapid population growth by contributing to previously reported enhanced reproduction and dispersal. This study provides a framework to understand links between ecological and physiological traits underlying post-fire population dynamics.

关键词： POLYMERIC nanocomposites   POLYURETHANES   METHACRYLATES   MULTIWALLED carbon nanotubes   X-ray scattering  

摘要： Nanocomposites based on polyurethane-poly(2-hydroxyethyl methacrylate) semi-interpenetrating polymer network (PU-PHEMA semi-IPN) doped with 0.01, 0.05, 0.1, or 0.25 wt% of non-functionalized multi-walled carbon nanotubes (MWCNTs) were synthesized. Their nanostructure was studied by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques, and their dynamics and properties were characterized over the temperature range from -100 degrees C to 180 degrees C using dynamic mechanical analysis (DMA) and laser-interferometric creep rate spectroscopy (CRS). It was shown that ultra-low MWCNT contents (0.01%-0.1%) substantially modified matrix nanostructure due to the influence of nanofiller on the processes of micro-phase separation and mixing of matrix components, the changes in the molecular packing density, and good dispersion of MWCNTs. The structural changes were somewhat smaller at 0.25% nanotubes where tendency to formation of their aggregates was observed. Accordingly, the strong changes in dynamic behavior, elastic properties and creep resistance in the studied materials were caused by ultra-low MWCNT additives. The strong effect of extraordinary broadening of the PHEMA relaxation peak in the nanocomposites and, simultaneously, the strongly improved creep resistance over the 80 degrees C-160 degrees C temperature range (at 0.1% MWCNTs) were shown that is of importance for the application of these nanocomposites as perspective damping materials. POLYM. COMPOS., 39:263-273, 2018. (c) 2016 Society of Plastics Engineers

关键词： Cellulose nanocrystal   Liquid silicone rubber   Curing kinetics   Mechanical properties   3-Aminopropyltriethoxysilane  

摘要： To prepare liquid silicone rubber (LSR)/cellulose nanocrystal (CNC) nanocomposite, 3-aminopropyltriethoxysilane (APTES) was used to modify the surface of CNC to improve the interfacial interaction between the hydrophilic CNC and the hydrophobic LSR. Fourier transform infrared spectroscopy and energy dispersive spectrometer results demonstrated that APTES modified CNC successfully. It was found that small amount of modified CNC (M-CNC) had better reinforcement than SiO2 in LSR nanocomposites, the tensile strength and the strain at break of LSR increased over 85% and 44% by adding 1.5 wt% M-CNC. However, DSC and rheology tests indicated that APTES modified CNC increased the curing temperature of LSR and limited the addition of CNC, which was attributed to poisoning of Pt catalyst by APTES. Furthermore, small amount of M-CNC improved the thermal stability of LSR, the TGA results showed that the 10% weight loss temperature (T-10%) of LSR increased 56 degrees C with 1.0 wt% addition of M-CNC.

关键词： Monazite   Sintering kinetics   HT-ESEM   Hot pressing   Texture effects   Microhardness  

摘要： This work focuses on the effect of precursor morphology on the microstructural evolution of monazite-type lanthanum-europium phosphate ceramics during sintering, including grain growth rate, as well as correlations between microstructure, texture effects and their mechanical properties. Sintering kinetics of La0.5Eu0.5PO4 powders with two different grain morphologies (needle-shaped and spherical) was studied by an in situ HT-ESEM method at 1340 degrees C. La0.5Eu0.5PO4 pellets with high density (99% of the theoretical density) were obtained for both precursor powders by hot pressing. Analysis of XRD data collected for the hot pressed pellets obtained from needle-shaped precursors revealed preferential orientation of the grains towards the (100) direction. Mechanical properties of the hot pressed pellets were studied by the Vickers indentation method. The dependence of microhardness and fracture toughness on microstructure and texture was investigated.

关键词： Atomic force microscopy  

摘要： Single-molecule force spectroscopy by atomic force microscopy allows us to get insight into the mechanical unfolding of membrane proteins, and a typical experiment exhibits characteristic patterns on the force distance curves. The origin of these patterns, however, has not been fully understood yet. We performed coarse-grained simulation of the forced unfolding of halorodopsin, reproduced the characteristic features of the experimental force distance curves. A further examination near the membrane-water interface indicated the existence of a motif for the force peak formation, i.e., the occurrence of hydrophobic residues in the upper interface region and hydrophilic residues below the lower interface region. (C) 2017 Elsevier B.V. All rights reserved.

关键词： BVOCs   Camphene   OH radical   IRC   Rate constant   Lifetime  

摘要： Present manuscript represents the DFT studies on the oxidation reaction of camphene initiated by OH radical and fate of product radicals using M06-2X functional along with 6-31+G(d,p) basis set. Intrinsic reaction calculation is done for transition states involving OH-addition reactions which proceed via reaction complexes proceeding to the formation of transition states. The rate constant calculated by using canonical transition state theory at 298 K and 1 atm is found to be 5.67 x 10(-11) cm(3) molecule(-1) s(-1) which is in good agreement with the experimental rate constant. The atmospheric lifetime of the titled molecule has also been reported in our work.

关键词： Patient-specific simulation   Aortic hemodynamics   Aortic valve replacement   Virtual intervention   4D flow MRI  

摘要： Modeling different treatment options before a procedure is performed is a promising approach for surgical decision making and patient care in heart valve disease. This study investigated the hemodynamic impact of different prostheses through patient-specific MRI-based CFD simulations. Ten time-resolved MRI data sets with and without velocity encoding were obtained to reconstruct the aorta and set hemodynamic boundary conditions for simulations. Aortic hemodynamics after virtual valve replacement with a biological and mechanical valve prosthesis were investigated. Wall shear stress (WSS), secondary flow degree (SFD), transvalvular pressure drop (TPD), turbulent kinetic energy (TKE), and normalized flow displacement (NFD) were evaluated to characterize valve-induced hemodynamics. The biological prostheses induced significantly higher WSS (medians: 9.3 vs. 8.6 Pa, P=0.027) and SFD (means: 0.78 vs. 0.49, P=0.002) in the ascending aorta, TPD (medians: 11.4 vs. 2.7 mm Hg, P=0.002), TKE (means: 400 vs. 283 cm(2)/s(2), P=0.037), and NFD (means: 0.0994 vs. 0.0607, P=0.020) than the mechanical prostheses. The differences between the prosthesis types showed great inter-patient variability, however. Given this variability, a patient-specific evaluation is warranted. In conclusion, MRI-based CFD offers an opportunity to assess the interactions between prosthesis and patient-specific boundary conditions, which may help in optimizing surgical decision making and providing additional guidance to clinicians.

关键词： 行星轮系   非线性因素   航天机构   动力学特性   耦合分析  

摘要： 围绕我国航天技术的国家重大需求,空间站大型机械臂、卫星天线等航天机构的应用日益广泛,且执行空间任务的复杂度越来越高,对这类机构的精度、稳定性、可靠性等要求也越来越高。传动关节作为其核心部件,主要完成机械连接、动力传递等工作,对空间机械臂的位置精度、卫星天线的指向精度等具有直接影响。本文针对行星轮系传动系统中存在的多间隙现象,刚柔耦合现象,建立多种动力学模型并进行数值仿真,主要研究工作如下:聚焦于课题研究的意义及相关背景,分析了相关研究的发展状况和优缺点,提出了本文的研究方向及重点。此外,航天机构的动力学研究有助于提高航天机构的设计及分析水平,延缓航天机构的磨损,延长其使用寿命,同时对其他大型机构的研制具有指导借鉴意义。针对行星轮系传动系统中多使用斜齿轮作为传动元件的应用现状,建立斜齿轮传动的弯-扭-轴耦合6自由度动力学模型。考虑斜齿轮传动中的齿侧间隙,时变啮合刚度和啮合阻尼等因素,推导齿轮啮合刚度及间隙函数方程。为分析轮齿修缘对齿轮啮合传动的影响,建立轮齿修缘模型,并用不同曲线分析修缘曲线方程,代入原斜齿轮传动动力学模型,得到系统综合考虑轮齿修缘的5自由度动力学方程。数值仿真结果表明外部激励,齿侧间隙等因素对齿轮传动的振动影响较大,合理的轮齿修缘曲线能够有效减小系统振幅。在此基础上,分析含有多组斜齿轮副传动的行星轮系关节,建立考虑多间隙因素的行星齿轮传动系统动力学模型及柔性臂杆动力学模型,利用拉格朗日乘子法建立含行星轮系关节及柔性臂杆的系统动力学方程。数值仿真中,通过对多间隙模型及刚柔耦合模型的对比,发现柔性臂杆与关节内部多间隙的耦合作用致使传动系统振动剧烈,但也加快了碰撞的衰减,因此,刚柔耦合动力学模型更符合实际工作情况。在刚柔耦合动力学的基础上,改变转速、间隙等参数大小,分析不同因素对系统动力学特性影响。结果表明较高转速时运动偏差波动次数衰减较快但启动时刻的冲击与碰撞程度较大,同时得到了维持系统最佳工作状态时各间隙大小的合理取值。此外,以2K-H行星齿轮传动系统为研究对象,建立平移-扭转耦合模型,将星载天线视为包含卫星本体、行星轮系关节,机械臂杆,天线反射面的多体系统,建立星载天线多体模型。重点分析双行星轮系关节情况下齿轮侧隙、啮合误差、摩擦力、耦合误差、空间温度等非线性因素对天线指向精度的影响。数值仿真结果表明,合适的齿侧间隙能够避免齿轮啮合过程中的卡齿现象,也能保持较高的传动精度,选用泊松比较大的齿轮材料能够减弱齿轮啮合误差造成的影响,在低速、匀速、低摩擦的情况下更易于保持天线的平稳。但由于双行星轮系关节的耦合作用,更容易引起天线指向精度偏差的波动。

关键词： 南水北调   除藻   MATLAB   水动力特性  

摘要： 南水北调工程作为我国的重要战略性工程,解决了我国南涝北旱的问题,对我国的区域经济和环境的发展起到了重要的作用。因此,保证南水北调输水渠道中的水质安全尤为重要。近年来,南水北调的输水渠道中产生了季节性的藻类,不但影响了水质,而且严重时,可使水体发生富营养化,危害人类健康。为此,根据南水北调输水渠道这一特殊工况,设计了依托于止水闸口的除藻设备。根据当前国内外的除藻技术、设备及原理,确定采用物理除藻技术中的机械式除藻法,设计适用于南水北调工况的除藻机械。经过实地考察,记录相关数据,制定技术要求。根据具体功能、工作环境和条件,完成除藻机械的结构设计方案,并分别建立各个方案的Solidworks三维模型。通过对各方案的分析与筛选,确定出最佳方案。结合具体工况,并通过计算与建模,完成除藻机械的总体设计,包括框架结构、传动系统、拦藻清污系统等。结合设备制造工艺与安装流程,对除藻机械进行进一步的改进与完善。利用MATLAB软件对除藻机械进行模糊控制系统的设计,控制拦藻网的拦藻时间和喷水时间,实现除藻机械的自动化。采用Fluent软件对除藻机械的水动力特性进行研究,进一步确定拦藻网参数、验证除藻效果。设备已在南水北调中线渠道运行,为渠道藻类清除提供了重要参考。