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Multiple time step molecular dynamics simulation for interaction between dislocations and grain boundaries

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Acta Mechanica Sinica 2005年第4期21卷 371-379页

作者： X.Y.Li W.Yang department of engineering Mechanics,tsinghua university,beijing 100084.china department of engineering Mechanics Tsinghua University Beijing China

A multiple time step algorithm, called reversible reference system propagator algorithm, is introduced for the long time molecular dynamics simulation. In contrast to the conventional algorithms, the multiple time method has better convergence, stability and efficiency. The method is validated by simulating free relaxation and the hypervelocity impact of nano-clusters. The time efficiency of the multiple time step method enables us to investigate the long time interaction between lattice dislocations and low-angle grain boundaries.

关键词： Multiple time step Molecular dynamics ,Dislocation Grain boundary

THE EXISTENCE OF J-HOLOMORPHIC CURVES AND APPLICATIONS TO THE WEINSTEIN CONJECTURE

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Chinese Annals of Mathematics,Series B 1999年第4期20卷 425-434页

作者： MA RENYI(department of Applied Mathmatics, tsinghua university, beijing 100084, china) department of Applied Mathmatics Tsinghua University Beijing 100084 China

The author first proves the existences of J-holomorphic curves in the symplectizations of Legendre fibrations and then as an application confirms the Weinstein conjectures on contact manifolds of Legendre fibrations. As a corollary a new proof on the theorem due to Hofer,Viterbo, Gluck, Ziller, Weinstein and Ljusternik-Fet Theorem is provided, which is quite different from their original proofs.

关键词： Symplectic geometry J-holomorphic curves Periodic orbit

Study and Application of Mold Filling Simulation of Shaped Castings

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Journal of Materials Science & Technology 1997年第2期13卷 107-112页

作者： Baicheng LIU Wei QIU Houfa SHEN Zhiqiang GAO (Dept. of mechanical engineering, tsinghua university beijing 100084, china) Dept. of mechanical engineering Tsinghua University Beijing 100084 China

In this paper, an algorithm for simulating fluid flow and heat transfer for mold filling of shaped castings is presented. The main features of the algorithm include: 1) a simple but practical technique based on VOF method to determine free surface, 2) an explicit scheme of enthalpy to solve the energy equation more efficiently, and 3) an effective treatment to modify the flux deviation due to pressure iteration. In order to verify these methods, well controlled experiments have been repeatedly done with both water analog and gray iron pouring experiments to record the flow patterns and temperature variations. The calculated results are in accordance with the experimental ones. For the applications, the simulated initial temperature distribution right after mold filling was used to analyse subsequent solidification and to predict shrinkage defects. Actual castings were poured and tested in a foundry plant. The reuslts show that the defects predication with considering fluid flow effects is more precise than that without considering the effects.

关键词： Simulation Study and Application of Mold Filling Simulation of Shaped Castings Mold

A TWO-SCALE SECOND-ORDER MOMENT PARTICLE TURBULENCE MODEL FOR GAS-PARTICLE FLOW

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Journal of Hydrodynamics 2006年第6期18卷 659-665页

作者： ZENG Zhuo-xiong ZHOU Li-xing department of engineering Mechanics Tsinghua University Beijing 100084 China department of mechanical engineering Nanchang Institute of Aeronautical Technology Nanchang 330034China

A two-scale second-order moment two-phase turbulence model was developed and used to simulate gas-particle flow in a sudden-expansion chamber and a channel. The simulation results were in agreement with the experimental results, and the results were compared with those of the single-scale second-order moment two-phase turbulence model. Several improved features show that the two-scale model is to a certain extent better than the single-scale model, which may be attributed to the fact that particle turbulence is well characterized by the two-scale turbulence model.

关键词： gas-particle flow second-order moment model,two-scale fluctuation

CO_2 Mitigation in Coal Gasification Cogeneration Systems with Integration of the Shift Reaction, CO_2 Absorption and Methanol Production

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Journal of Thermal Science 2004年第3期13卷 193-198页

作者： Yuanyuan DUAN Jin ZHANG Key Laboratory of Thermal Science and Power engineering, department of Thermal engineering, tsinghua university, beijing 100084, china Key Laboratory of Thermal Science and Power engineering Department of Thermal Engineering Tsinghua University Beijing China

Cogeneration of electricity and liquid fuel can achieve higher efficiencies than electricity generation alone in Integrated Gasification Combined Cycle (IGCC), and cogeneration systems are also expected to mitigate CO2 emissions. A proposed methanol-electricity cogeneration system was analyzed in this paper using exergy method to evaluate the specified system. A simple cogeneration scheme and a complicated scheme including the shift reaction and CO2 removal were compared. The results show that the complicated scheme consumes more energy, but has a higher methanol synthesis ratio with partial capture of *** those methanol and electricity cogeneration systems, the CO2 mitigation is not merely an additional process that consumes energy and reduces the overall efficiency, but is integrated into the methanol production.

关键词： cogeneration system methanol synthesis CO2 mitigation exergy analysis

Research on dual-stator winding multi-phase high-speed induction generator with rectifier load

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Science china(Technological Sciences) 2008年第6期51卷 683-692页

作者： WANG XiangHeng1 & WU XinZhen2 1 department of Electrical engineering, tsinghua university, beijing 100084, china 2 department of Electrical engineering, Qingdao university, Qingdao 266071, china department of Electrical engineering Tsinghua University Beijing China department of Electrical engineering Qingdao University Qingdao China

Based on analysis and calculation of the relation between current harmonics and MMF harmonics in the dual-stator winding multi-phase high-speed induction gen-erator with a rectifier load, a new idea, which divides the generator system into two sub-systems to be analyzed individually, is presented. For the sub-system of the 12-phase power winding with a rectifier load, the loop current method is used to establish network equation set in terms of the network graph theory. The numerical stability problem is solved by alternating use of fixed time-step and varied time-step, and the voltage and current with their fundamental components of the power winding are obtained. For the other sub-system of the induction generator with dual-stator winding and solid cage rotor, the electromagnetic field analysis method and the multi-variable optimization approach are combined to get the con-trol winding current and stator frequency. Calculated results well match experiment results, indicating that the new proposed method is of effectiveness and high ac-curacy.

关键词： dual-stator winding induction generator rectifier load MMF analysis loop current method finite element method optimization approach

SURFACE MODIFICATION OF CaCO_3 WITH DENDRITIC CARBOXYLIC ACIDS

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Chinese Journal of Polymer Science 2002年第3期20卷 231-235页

作者： Zhao-xia Guo Jian Yu Institute of Polymer Science and engineering department of Chemical engineering tsinghua university beijing 100084 china Institute of Polymer Science and engineering Department of Chemical Engineering Tsinghua University 北京 100084

Nanometer calcium carbonate was modified with Frechet type polyether dendrons bearing a COOH group at thefocal point by a wet-coating technique in dichloromethane at room temperature. The interaction between CaCO;and thedendritic modifier was proven by IR and mass spectroscopy of the modified CaCO;after thorough extraction withdichloromethane. The amount of tightly bonded organic modifier was estimated by TGA. It was shown that the amount ofbonded molecules decreased with increasing generation number due to the larger number of branches. Compared tounmodified CaCO;, the modified CaCO;possess different dispersibility in water and in organic solvents.

关键词： calcium carbonate surface modification polyether dendrimer dendritic carboxylic acid

A SINGLE-PERIOD INVENTORY SYSTEM WITH A GENERAL S-SHAPED UTILITY AND EXPONENTIAL DEMAND

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Journal of Systems Science and Systems engineering 2010年第2期19卷 227-236页

作者： Wei GENG1 Xiaobo ZHAO2 Deng GAO3 department of Industrial engineering,tsinghua university,beijing 100084,china department of Industrial engineering Tsinghua University Beijing China

We discuss a single-period inventory system with a general S-shaped utility function and exponential demand. It is shown that there exists a state-dependent order-up-to policy to be optimal. Behaviors of optimal polic... 详细信息

关键词： Inventory S-shaped utility function order-up-to policy

DIMENSIONS OF SELF-AFFINE SETS WITH OVERLAPS

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Chinese Annals of Mathematics,Series B 2003年第3期24卷 275-284页

作者： HUA Su department of Mathematics, tsinghua university, beijing 100084, china. department of Mathematics Tsinghua University Beijing 100084 China

The authora develop an algorithm to show that a class of self-affine sets with overlaps can be viewed as sofic affine-invariant sets without overlaps,thus by using the results of [11] and [10],the hausdorff and minkow... 详细信息

关键词： self-affine set overlap hausdorff dimension minkowski dimension