关键词： 空间机械臂   动力学   建模   关节  

摘要： 空间机械臂在载人航天活动中有着迫切的应用需求。航天器尺寸、质量等因素的限制使得空间机械臂具有大柔性的特点。文章结合空间机械臂应用环境的特殊性,论述了空间柔性机械臂多体动力学和关节的动力学建模方法,分析比较了不同建模方法的特点。为了满足未来空间机械臂在航天器工程应用的要求,文章指出,空间柔性机械臂宜采用混合坐标法描述臂杆刚性运动与柔性变形的耦合;开展关节精细动力学研究,宜将重点放在齿轮传动系统的低速级上。

关键词： 电火花加工机床   插补方法   上海交通大学   交通部第一交通大学   万生   电弧放电   特种加工机床   多轴联动   数控系统   动力工程   学院  

摘要： 上海交通大学其前身是近代中国历史最为悠久的大学之一,清末洋务派政治家、实业家和福利事业家盛宣怀于1896年创建的南洋公学,经过百余年的不懈努力,如今的上海交大已经成为一所"综合性、研究型、国际化"的国内一流、国际知名大学.在上海交大闵行校区里,在碧绿、清澈和平静的思源湖畔,笔者采访了上海交通大学机械与动力工程学院的赵万生教授.

关键词： Chemical kinetics   Enzyme catalysis   Michaelis-Menten model   Radiobiological models   Pade approximant   Hypofractionated radiotherapy  

摘要： Biophysical models for repair mechanisms for cell surviving fractions after exposure to radiation are studied. The principal focus is on a novel theory, the Pad,-linear quadratic (PLQ), or equivalently, the differential Michaelis-Menten radiobiological model, which predicts as a function of the absorbed dose in the form , with a clear biological and clinical meaning of the three parameters and . It is shown that this functional form in the PLQ model emerges directly from the simultaneous fulfillment of the requirements for the correct asymptotic behaviors of the repair function at low and high doses. Moreover, this automatically secures the purely exponential cell kill modes at both small and large , as also encountered in the corresponding experimental data for cell surviving fractions. Further, it is demonstrated that the PLQ-based repair function, given by a rectangular hyperbola, coincides with the reaction velocity for enzyme catalysis from the Michaelis-Menten mechanism. This repair velocity is the halved harmonic mean of the low- and high-dose asymptotes of the catalytic repair function. Such circumstances constitute a firm mechanistic basis of the PLQ model, which is shown to exhibit excellent agreement with measurements. Robust applications of the PLQ model are anticipated, especially in hypofractionted radiotherapy, such as stereotactic radiosurgery.

关键词： Landscape modeling   Forests   Disturbances   Climate change   Global changes   Mechanistic modeling   Empirical modeling   Phenomenological modeling  

摘要： Researchers and natural resource managers need predictions of how multiple global changes (e.g., climate change, rising levels of air pollutants, exotic invasions) will affect landscape composition and ecosystem function. Ecological predictive models used for this purpose are constructed using either a mechanistic (process-based) or a phenomenological (empirical) approach, or combination. Given the accelerating pace of global changes, it is becoming increasingly difficult to trust future projections made by phenomenological models estimated under past conditions. Using forest landscape models as an example, I review current modeling approaches and propose principles for developing the next generation of landscape models. First, modelers should increase the use of mechanistic components based on appropriately scaled "first principles" even though such an approach is not without cost and limitations. Second, the interaction of processes within a model should be designed to minimize a priori constraints on process interactions and mimic how interactions play out in real life. Third, when a model is expected to make accurate projections of future system states it must include all of the major ecological processes that structure the system. A completely mechanistic approach to the molecular level is not tractable or desirable at landscape scales. I submit that the best solution is to blend mechanistic and phenomenological approaches in a way that maximizes the use of mechanisms where novel driver conditions are expected while keeping the model tractable. There may be other ways. I challenge landscape ecosystem modelers to seek new ways to make their models more robust to the multiple global changes occurring today.

摘要： BACKGROUND: Previous studies reported a good agreement between cardiovascular magnetic resonance (CMR) and transthoracic echocardiography (TTE) for the assessment of the effective orifice area (EOA) of native aortic valves. The objective of this study was to compareCMRandTTE for the estimation of the EOA of aortic bileaflet mechanical valves (BMV). METHODS: 25 asymptomatic patients withBMVin aortic position underwent TTE and velocity-encoded CMR the same day. EOA was estimated by TTE using the standard continuity equation method. Since the LV outflow tract planes were not suitable for analysis due to prosthesis artefacts or poorly defined boundaries, we used a single-plane method for the estimation of the EOA by CMR: i.e. the stroke volume (SV) and the aortic jet velocity time integral (VTIao)were recorded at the same trans-aortic *** phase contrast acquisitions were performed at 4 levels above prosthesis (6, 8, 10 and 12mm). CMR and TTE derived EOAs were then compared to those obtained with an in vitro model. RESULTS: Best correlation between CMR EOA and TTE EOA was obtained using the trans-aortic slice at the 6mmlevel (r=0.62, p<0.001). However, CMR EOA was systematically larger than TTE EOA (mean bias + 0.48 cm2, limits of agreement [-0.27;+1.23 cm2]).CMRSVswere higher compared to TTE SVs (85.9 vs. 73.5ml, p<0.001) andCMRVTIaowere lower (42.6 vs. 48.3 cm, p<0.001). CMR EOA correlated well (r=0.68) with in vitro EOA, whereas TTE EOA did not (r=0.21). CONCLUSION: As opposed to what was reported for native valves, CMR yielded larger EOAs compared to TTE for BMVs. This discrepancy may be explained by: i) underestimation of VTIao by CMR, ii) underestimation of SV by TTE due to non-circular LV outflow-tract geometry, and iii) overestimation of the VTIao by TTE due to localized high velocity within the BMV central orifice.

关键词： HYDROGEN   TUNGSTEN   METALS -- Mechanical properties   MOLECULAR dynamics   RESEARCH   SINGLE crystals   BODY-centered cubic metals  

摘要： The effect of hydrogen on the mechanical properties of single crystal BCC Was well as the intrinsic mechanism was investigated by using molecular dynamics method. The bond energy of tungsten atoms was computed and the results suggest that hydrogen atoms weaken the bonds. By simulating the response of tungsten under uniaxial tensile load, the stress-strain curves for different hydrogen concentration were obtained. It was found that hydrogen has great influence on the elastic modulus and plastic deformation mechanism. Moreover, the interaction between hydrogen atoms and dislocations was simulated. The qualitative relationship between yield stress and hydrogen concentration was obtained. All the numerical results suggest that the retention of hydrogen atoms causes significant degradation of the mechanical properties of tungsten. (C) 2013 Elsevier B.V. All rights reserved.

关键词： Photobioreactor   Hydrogen   Microalgae   Cell trajectory   Light attenuation dynamics  

摘要： Hydrogen productions through biophotolysis by microalgae in photobioreactors (PBRs) were studied using a computational model integrated with fluid dynamics, particle tracking technique, light attenuation dynamics, biochemical kinetics, and mass transport. The trajectories of microalgae entrained in the flow fields within these PBRs were traced by the particle tracking technique and were used to determine the dynamics of light attenuation subjected by the cells, which were analyzed and compared with those obtained from the unstirred PBR under different incident light illuminations. The results show an improvement on the light penetration depth in the mechanically stirred cultures. The dynamics of light attenuation was incorporated into the kinetics equations for the analysis of the inhomogeneous biochemical process for hydrogen production by microalgae. Hydrogen production in the unstirred and the impeller-stirred PBRs were determined under different light illumination conditions and the results show an improvement on hydrogen production in the impeller-stirred PBRs. (c) 2013 Elsevier Ltd. All rights reserved.

关键词： PVAc-MMT-DOAB   dynamic mechanical properties   glass transition   cold crystallization   kinetics  

摘要： To study the dynamic mechanical kinetics of exfoliated nanocomposites using dynamic mechanical analysis, polyvinyl acetate (PVAc)-montmorillonite (MMT)-dioctadecyl dimethyl ammonium bromide (DOAB) was synthesized through five different processes. Different synthesis processes and MMT-DOAB had little effect on the structure of PVAc-MMT-DOAB. MMT-DOAB lowered the glass transition temperature of PVAc, hence improving its low temperature resistance greatly. PVAc and PVAc-MMT-DOAB were homogeneous amorphous linear polymers and they appeared cold crystallized, and their glass transition and cold crystallization kinetics were analyzed using Agrawal integral equation.

关键词： Kinetics   Solvent effect   Solvatochromic parameters   Single electron transfer (SET) pathway   Biphasic concave upward free energy relationship  

摘要： The kinetics and mechanism of the aromatic nucleophilic substitution reactions of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-X-substituted anilines (X = OH, OMe, Me, H, F, I, Cl) were studied in MeOH-Me2SO mixtures and pure Me2SO at 25.0 A degrees C. The second-order rate coefficients depend on the substitutent in aniline and give good Hammett and Bronsted correlations;a polar SNAr reaction is proposed for the reaction in different MeOH-Me2SO mixtures. The measured rate coefficients of the reaction demonstrated dramatic variations for aniline donor with the increasing dimethyl sulfoxide composition in MeOH-Me2SO mixtures. In this case, the Hammett and Bronsted plots are biphasic and concave upwards with a break point at 4-methylaniline. These results indicate a change in mechanism from the polar (SNAr) for less basic nucleophiles (X = 4-Cl, 4-I, 4-F, and H) to the single electron transfer (SET) for more basic nucleophiles (X = 4-OH, 4-OMe and 4-Me). The changes of the structure of the transitions states with substituents and solvent are in accordance with the results of kinetics studies. The solvation model described is well supported by the solvatochromism exhibited by aniline in the solvent mixture under investigation. These results provide an ideal framework for understanding the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent effects.