关键词： 结构动力学   李群分析法   对称性   守恒量   不变解  

摘要： 为给复杂机械系统动力学特性定量和定性分析提供一个强有力的新途径,笔者将现代李群分析方法引入到机械结构动力学中。首先,通过矩阵解耦技术得到有限自由度机械结构振动系统在无限小群变换下导致的守恒量形式;其次,利用机械连续结构体三维弹性动力学方程允许的李点对称构造出方程组的系列群不变解。结果表明借助李群分析可以得到机械系统结构动力学更深层次的力学规律和运动现象。本研究不仅可以得到结构振动响应的精确解,同时也为数值计算提供了有效的验证手段。

关键词： Bileaflet mechanical heart valve   computational fluid dynamics   blood flow shear stresses   damage to blood cells  

摘要： Bileaflet mechanical heart valves (BMHVs) are widely used to replace diseased heart valves. However, patients may suffer from implant complications, such as platelet aggregation and damage to blood cells, which could lead to BMHV failure. These complications are related to the blood flow patterns in the BMHV. A three-dimensional computational fluid dynamic (CFD) model was developed to investigate blood hydrodynamics and shear stresses at different cardiac cycles. A user-defined function (UDF) code was developed to model the valve leaflet motion. This UDF updates the tetrahedral mesh according to the location of the valve leaflet, which enables modeling of complicated moving geometries and achieves solution convergence with ease without the need to adjust the relaxation factor values. The agreement between the experimental and numerical results indicates that the developed model could be used with confidence to simulate BMHV motion and blood flow. Furthermore, valve leaflet and valve pivot were found to be continuously exposed to shear stresses higher than 52.3 Pa which according to previous research findings may cause damage to blood platelets.

关键词： Smoothed Particle Hydrodynamics (SPH)   Dry and saturated soil   Soil-water interaction   Elastic-plastic constitutive model   Seepage flow   Embankment  

摘要： Extreme weather events, such as floods, are becoming more frequent in future with all consequences concerning the safety especially of water retaining structures, in particular, levees used as technical flood protection. The knowledge and correct consideration of coupled hydro-mechanical processes influencing the failure and post-failure behaviour of water saturated soils are of paramount significance for the assessment of hydraulically loaded geotechnical structures. This study focuses on the modelling of the processes governing the behaviour of water saturated soils using Smoothed Particle Hydrodynamics (SPH) on the example of a homogeneous embankment under different loading conditions. This paper aims to improve available SPH framework in order to enhance soil-water interaction in this method for geotechnical engineering investigations. To achieve this goal, the suggested approach is validated intensively by various well-known problems. Finally, the verified tool is used to investigate the hydro mechanical behaviour of homogeneous embankments under simultaneous hydraulic and mechanical loading. The simulation results prove that the suggested approach is capable of simulating relevant hydraulic and mechanical processes governing the deformation behaviour of hydraulically loaded structures beyond the failure point allowing the detailed analysis of the post-failure behaviour characterised by large deformations. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： gamma-TiAl alloy   molecular dynamics   vacancy   temperature   mechanical properties  

摘要： Molecular dynamics simulation is used to analyze tensile strength and elastic modulus under different temperatures and vacancy concentrations. The effects of temperature and vacancy concentration on the mechanical properties of gamma-TiAl alloy are investigated. The results show that the ultimate stress, ultimate strain and elastic modulus decrease nonlinearly with increasing temperature and vacancy concentration. As the temperature increases, the plastic of material is reinforced. The influence of temperature on strength and elastic modulus is larger than that of vacancy concentration. The evolution process of vacancy could be observed clearly. Furthermore, vacancies with different concentrations develop into voids first as a function of external forces or other factors, micro cracks evolve from those voids, those micro cracks then converge to a macro crack, and fracture will finally occur. The vacancy evolution process cannot be observed clearly owing to the thermal motion of atoms at high temperature. In addition, potential energy is affected by both temperature and vacancy concentration.

关键词： MOLECULAR dynamics   NANOPOROUS materials   COPPER   NANOPARTICLES   SINTERING  

摘要： A molecular dynamics (MD) simulation is performed on the coalescence kinetics and mechanical behavior of a thermally sintered nanoporous copper (Cu) nanoparticulate system. To investigate the effect of particle size and sintering temperature on the coalescence of the nanoparticulate system, particles with sizes of 4, 5, and 6nm are sintered at temperatures of 300, 500, and 700K. To determine the thermal sintering process at elevated temperatures and ambient pressure, bulk periodic nanoparticle unit cells consisting of a finite number of nanoparticles are equilibrated through isothermal-Isobaric ensemble simulations. En thermally sintered configurations, uniaxlai tension/compression and shearing simulations are applied at a constant strain rate to derive stress-strain curves. It is found that stacking faults are actively generated in smaller nanoparticles even at a low sintering temperature, while local amorphization and surface and grain boundary diffusion are rather prominent in larger nanoparticles. Even at the same sintering temperature, the density of the sintered nanoparticle Increases as the size of the nanoparticle decreases. In elastic moduli, the same particle size dependency is observed, while no obvious difference Is observed in tension and compression. On the other hand, the yield strengths of the sintered nanoparticles in tension are larger than those in compression. The asymmetric yield strength of the sintered systems Is clarified by addressing the surface stress and surface equilibrium strain of atoms on the surface of nanopores by the evolution of atomic virial stress in tension and compression. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： Notch   Strength   Deformation mode   Stress state   Plastic deformation  

摘要： Metallic glasses (MGs) with various geometrical notches have been one of the most attractive research topics because of their interesting mechanical behavior. In this work, molecular dynamics simulations were employed to examine the effect of internal notch depth on the mechanical behavior of Cu50Zr50 MGs. It is shown that both the strength and the deformation mode are sensitive to the notch depth, and in particular to the inter-void ligament distance. With increasing the notch depth, the notch intensification gets enhanced, while the deformation mode is seen to change from shear banding dominated to mixed mode and then to necking governed. Detailed stress distribution analysis reveals that the notch intensification and the deformation mode transition result from the stress state change induced by the presence of notches. The critical effective shear stress corresponding to local/atomic yielding in the MGs is identified to be around 1.9 GPa. In addition, the intrinsic size confinement effect also influences the deformation process. The findings are expected to provide useful guidance for materials design and investigation of yielding in MGs based on atomistic simulations.

关键词： Cas2   nuclease activity   structure and dynamics  

摘要： Cas2 protein in the CRISPR-Cas system functions as a scaffold for the acquisition of foreign DNA fragments, and as a nuclease against DNA and RNA substrates. Crystal structures of Cas2 have shown catalytically inactive conformational states that do not explain the mechanism of Cas2 nuclease activity. Here, we report that Xanthomonas albilineans Cas2 (XaCas2) assumes an inactive conformation in solution. Residual dipolar couplings and NMR relaxation, however, provide direct evidence on conformational dynamics at the predicted hinge region. Furthermore, XaCas2 transiently associates with metal ions for nuclease activity via highly mobile Asp8. Taken together, the dual function of Cas2 can be explained by a dynamic equilibrium of conformational states that serve as a scaffold or as a nuclease on demand.

关键词： Low-complexity tracking   High-order nonlinear systems   Unknown nonlinearities   Underactuated mechanical systems  

摘要： This paper presents a low-complexity design approach with predefined transient and steady-state tracking performance for global practical tracking of uncertain high-order nonlinear systems. It is assumed that all nonlinearities and their bounding functions are unknown and the reference signal is time varying. A simple output tracking scheme consisting of nonlinearly transformed errors and positive design parameters is presented in the presence of virtual and actual control variables with high powers where the error transformation technique using time-varying performance functions is employed. Contrary to the existing results using known nonlinear bounding functions of model nonlinearities, the proposed tracking scheme can be implemented without using nonlinear bounding functions (i.e., the feedback domination design), any adaptive and function approximation techniques for estimating unknown nonlinearities. It is shown that the tracking performance of the proposed control system is ensured within preassigned bounds, regardless of high-power virtual and actual control variables. The motion tracking problem of an underactuated unstable mechanical system with unknown model parameters and nonlinearities is considered as a practical application, and simulation results are provided to show the effectiveness of the proposed theoretical result.

关键词： Biofilm   Cleaning-in-place   Two-phase kinetics   Wall shear stress   Chemical action   Removal  

摘要： The objective of this study was to investigate the roles of mechanical and chemical actions on the detachment kinetics during CIP of biofilms of Pseudomonas fluorescens at a pilot-plant scale. Biofilms were grown 48 h at 20 degrees C in diluted milk. Shear stress conditions ranged from 0 (static) to 20 Pa and NaOH concentrations from 0.1 to 0.5% w/w. A simple two-phase model was applied to model the removal kinetics and compare detachment parameters. A quick removal of the biofilm during the first phase (4-5 log decrease in three minutes) was followed by a second phase of slow removal of the cells and small aggregates still present at the surface (0-2 log decrease). Hydrodynamics was responsible for removal of both biofilm and single cells while chemicals mainly disrupted biofilm clusters during the first phase. No complete biofilm removal was observed, suggesting a significant role of the interaction forces between bacteria and substrata in the CIP efficiency. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： siRNA   Polyethylenimine   Polymeric nanoparticle   Heparin   Glycosaminoglycan   Polynucleotide delivery   Molecular dynamics  

摘要： Delivery of polynucleotide-based therapeutics into target cells involves interactions with glycoSaminoglycan chains that are located on cell membrane milieu. Mechanisms governing glycosaminoglycan-mediated changes in the nanoparticulate structures of polymer-polynucleotide complexes are unknown, and cannot be fully elucidated without atomistic level details of molecular interactions. We selected a representative nanoparticulate system consisting of a short interfering RNA (siRNA)-poly-ethylenimine complex, and performed all-atom molecular dynamics simulations with the prototypical glycosaminoglycan heparin. We monitored the binding between the complex constituents and the heparin, and identified key features contributing to the response of the siRNA nanoparticles to heparin. We observed three main metastable states that the siRNA nanoparticles might visit in the presence of heparin, which can be translated into different functional outcomes. By correlating our data with the widely different and seemingly contradictory roles previously assigned to glycosaminoglycans, this study provides unique insights into the discrepancies in the experimental literature concerning the role of glycosaminoglycans in the polymeric nanoparticle delivery. (C) 2017 Elsevier Ltd. All rights reserved.