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ISSN 1000-0585
CN 11-1848/P
Started in 1982
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  • Table of Content
      , Volume 57 Issue 5 Previous Issue    Next Issue
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    MECHANICAL ENGINEERING
    Welding deformation predictions of thin-wall cylinders using a local-global method
    ZHAO Haiyan, WU Junwei, LU Xiangming, JIAN Bo, LI Hongwei
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 449-453.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.019
    Abstract   PDF (1549KB) ( 985 )
    Welding structure can be seriously affected by welding deformation. Accurate, efficient predictions of welding deformation are important in engineering projects. A local-global finite element analysis is used to analyze circumferential welding of large-scale thin-wall cylinders. The welding deformation of the cylinder is obtained in 3 sequences. The results show that the deformation of a 1/4 circle weld is less than for a full-circle weld and a 1/2 circle weld. The deformation of a 1/4 circle weld is about 17% less than for a full-circle weld. The local-global method results agree well with results from the thermo-elastic-plastic method and experimental data with the local-global method using about 1/6 of the time of the thermo-elastic-plastic method.
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    Oblique-incidence ultrasonic testing for the adhesion quality of ceramic coatings
    HAN Zandong, LI Yongjie, CHEN Yifang
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 454-458.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.020
    Abstract   PDF (1816KB) ( 740 )
    Non-metallic coatings on the surfaces of metal substrates can improve the corrosion resistance and high temperature performance of the substrates. The adhesion quality of the coatings is an important index for coating performance. Conventional ultrasonic transmission and reflection methods can detect coating de-bonding, but cannot easily detect the adhesion quality of the bonded coating. Non-metallic coatings with different adhesion qualities are evaluated using an oblique-incidence radiative method. The nonlinear coefficients of the oblique-incident wave are calculated using spectral analysis techniques. The results show that the nonlinear coefficients of the oblique-incident wave reflect the adhesion quality of the ceramic coatings. The bonding quality of the coating can then be evaluated by the amplitudes of the fundamental frequency. For a bonded coating, a greater nonlinear coefficient indicates worse adhesion quality.
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    Equivalent amplitude model for a giant magnetostrictive transducer based on an unsteady electromechanical conversion coefficient
    CAI Wanchong, ZHANG Jianfu, YU Dingwen, WU Zhijun, FENG Pingfa
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 459-464.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.021
    Abstract   PDF (2146KB) ( 657 )
    The vibration amplitude model of a giant magnetostrictive transducer was established using an equivalent circuit of the transducer. The model parameters were identified through an impedance analysis. The accuracy of the vibration amplitude model was improved by analyzing the effects of the frequency and amplitude of the excitation voltage on the electromechanical conversion coefficient. The relation between the excitation frequency and the electromechanical conversion coefficient was obtained experimentally. Then, the electromechanical conversion coefficient was calculated for various frequencies using interpolation to relate the vibration amplitude to the excitation current. Comparison with experimental results shows that the vibration amplitude model determined by the impedance analysis can be used to predict the transducer vibration at resonance. The interpolated electromechanical conversion coefficients can be used to calculate the vibration amplitudes so that the theoretical relations between the amplitude and the current for different excitation frequencies are consistent with experimental results, which indicates that the model has the proper relationships between the electromechanical conversion coefficient and the excitation frequency.
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    Corrosion of friction stir welded joints of the 2219-T8 Al alloy in acidic solutions
    KANG Ju, LIANG Suying, WU Aiping, WANG Guoqing
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 465-470.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.022
    Abstract   PDF (1963KB) ( 637 )
    Al alloy (AA) 2219 has potential for use in liquid cryogenic rocket fuel tanks, with friction stir welding (FSW) as a suitable welding method. However, there is limited data on the corrosion of FSW AA2219-T8 joints in acidic solutions. Therefore, the corrosion of an FSW AA2219-T8 joint in an acidic chloride solution (pH 2, 0.5 mol/L NaCl) was characterized using potentiodynamic polarization and corrosion immersion tests. An optical profilometer was used to analyze the corrosion morphology, depth and density of the corroded cross-section surface and identify the corrosion mechanism. The results show that the corrosion resistance differs throughout the whole heat-affected zone (HAZ) with the worst corrosion region far from the welding seam. The corrosion resistance significantly increases in the thermomechanically affected zone (TMAZ) and the weld nugget zone (WNZ). The WNZ, TMAZ and near seam regions of HAZ are less affected because they are cathodes to other regions and are protected from corrosion. Unlike in neutral solutions, the primary reduction reaction is hydrogen evolution, which reduces the corrosion potential and pitting potential in each region.
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    Power loss and temperature rise model for an IGBT in a variable polarity welding power supply with a reverse voltage stabilizer
    YANG Zhongyu, ZHU Zhiming, LIU Bo, TANG Yingying
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 471-475,482.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.023
    Abstract   PDF (2316KB) ( 698 )
    During commutation in a variable polarity welding power supply with a reverse voltage stabilizer, the instantaneous pulse power loss loaded on the insulated gate bipolar translator (IGBT) could cause a jump in the junction temperature that might exceed the maximum allowable junction temperature, threatening safe operation of the IGBT. Formulas were derived to calculate the highest junction temperature using a partial fraction circuit model in the thermal-electric equivalent method. The results were compared with simulation results to verify the formulas. The effects of the main parameters on the junction temperature were analyzed to develop constraint conditions for controlling the IGBT junction temperature in a safe range. These results provide a theoretical basis for choosing reasonable IGBT and its working parameters for the power supply circuits and control strategy.
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    AUTOMOTIVE ENGINEERING
    Piecewise tracking energy optimization approach for an extended-range electric city bus
    XIE Haiming, LIN Chengtao, LIU Tao, TIAN Guangyu, HUANG Yong
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 476-482.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.024
    Abstract   PDF (1517KB) ( 772 )
    A piecewise tracking energy optimization approach was developed to manage the battery state of charge (SOC) consumption and the splitting power to improve the fuel economy of extended-range electric city buses while ensuring their performance. The approach established a stage power splitting optimization model for each control period by constructing a power demand prediction sequence and designing a reference curve to manage the SOC consumption. Model predictive control was introduced for rolling optimization and strategy adjustment. For the Chinese city bus driving cycle, this approach enables a 12 meters extended-range electric city bus to use only 21.8 L fuel and 25.4 kWh electricity per 100 km, which are better than CDCS strategy based results (24.1 L fuel and 25.4 kWh electricity per 100 km). The results show that by preventing the SOC from running out during the route but only reaching its minimum, this approach ensures the dynamic performance and improves the fuel economy.
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    Finite element contact modeling method and error characteristics of partitioned parallel computations for bullet penetration simulations
    LÜ Zhenhua, LIU Sai
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 483-490.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.025
    Abstract   PDF (1364KB) ( 747 )
    Improved ballistic simulations were developed for homogeneous steel sheets against small-caliber bullets. The simulations analyzed the influences of the contact-force penalty function parameters and contact friction coefficients on the contact interface state and ballistic limit velocity. The inconsistencies among computed ballistic limit velocities predicted by different partitioned parallel systems were investigated statistically. The interface stiffnesses within the contact-force penalty function should be defined between different contact pairs rather than assigning an equal interface stiffness to all contact regions to improve the accuracy of the ballistic limit velocity. Different friction coefficients between contact pairs can seriously affect the computed ballistic limit velocity for thicker steel targets against armor piercing bullets. Further study is needed to determine the correct friction coefficients for multiple contact pairs. The exchange of regional data and the use of this data in different sequences make the partitioned parallel computations of repeated computation tasks inconsistent, which affects the ballistic limit velocities predicted by the parallel computations. Therefore, repeated calculations using the same model for the same impact velocity are needed to obtain reliable ballistic limit velocity statistics.
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    Cyclist detection based on detection proposals and deep convolutional neural networks
    LI Xiaofei, XU Qing, XIONG Hui, WANG Jianqiang, LI Keqiang
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 491-496.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.026
    Abstract   PDF (2881KB) ( 990 )
    Cyclist protection systems based on cyclist detection methods are needed to protect cyclists from road traffic. This paper presents a detection proposal method and a cyclist detection method using deep convolutional neural networks to classify and locate cyclists. The detection proposal method uses cyclist shared salient region detection, redundancy-based detection and geometric constraint-based detection. Tests using a public cyclist dataset show that this method significantly outperforms state-of-the-art detection proposals, which verifies the effectiveness of this method.
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    Powertrain design of hydraulic hybrid track-type bull dozers based on planetary gear sets
    QIN Zhaobo, LUO Yugong, ZHANG Donghao, CHEN Long, LI Keqiang
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 497-503.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.027
    Abstract   PDF (1584KB) ( 854 )
    A powertrain design based on planetary gear sets is developed to improve the transmission efficiency of hydraulic hybrid track-type vehicles. An automated modelling framework is built based on the planetary transmission characteristics to characterize the powertrain dynamics. Screening based on the physical property requirements limits the number of feasible designs. The straight driving and steering characteristics of these feasible designs are calculated to select the better designs compared with existing designs. Finally, a dynamic programming analysis of the typical driving cycle of track-type bull dozers is used to calculate the fuel economy. Simulations show that this powertrain gives better traction, steering performance and fuel economy than existing hydraulic track-type bull dozers.
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    Characterization of the fracture of an aluminum alloy anticollision-beam to impact loading
    LAI Xinghua, WANG Lei, LI Jie, JIANG Yazhou, XIA Yong
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 504-510.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.028
    Abstract   PDF (3566KB) ( 1377 )
    Aluminum alloys are important light materials for vehicle weight reduction, but they frequently experience fracture under impact loading. This paper describes experimental and analytical methods for characterizing the fracture of aluminum alloy bumper beams. A test matrix is designed to obtain the material mechanical properties at different tensile strain rates and a variety of stress states, including tension, shear, notch tension, tension-shear and punch. The Swift-Hockett-Sherby law is used to describe the hardening of the material, with different stress states then simulated in the LS-DYNA finite element analysis environment to get a good correlation. Then, the stress triaxialities and lode angles extracted from the simulations are used to calibrate a modified Mohr-Coulomb (MMC) fracture model. Simulations of the material tests and a component bending test with the MMC model correlate well with the test results to support the validity of this method for fracture characterization, as well as the validity of the MMC fracture model for predicting metal fracture.
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    THERMAL ENGINEERING
    Numerical analysis of the drag reduction for turbulent pulsating pipe flows based on large eddy simulations
    NING Tao, GU Chunwei
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 511-515.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.029
    Abstract   PDF (1438KB) ( 755 )
    Large eddy simulations were conducted for turbulent pulsating flows using the commercial solver ANSYS-CFX. The drag reduction and the total energy consumption for pulsating flows were analyzed. The simulations included current dominated and wave dominated pulsating flows. The boundary layer characteristics of the current flow were affected by the superposition of the wave flow. The best drag reduction in the pulsating flows gave a 25% drag reduction when the non-dimensional pulsating amplitude was 5.5. The analysis indicates that the drag reduction is optimized when the pulsating flow is wave dominated and the wave boundary layer is laminar. Pulsating flows with simple sinusoidal pulsating patterns consume much more energy than steady flows.
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    AEROSPACE ENGINEERING
    Optimal analysis of the fuel consumption of combined cycle propulsion systems
    JI Zifei, WANG Bing, ZHANG Huiqiang
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 516-520.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.030
    Abstract   PDF (1332KB) ( 912 )
    The propulsion system fuel consumption must be accurately predicted for aircraft missions. A theoretical analysis of a hypersonic aircraft with a “boost-cruise-glide” flight mission profile powered by a rocket based combined cycle (RBCC) engine is used to predict the fuel consumption of the aircraft and to optimize the fuel consumption. The fuel consumption analysis of periodic hypersonic cruise trajectories shows that the fuel consumption decreases with increasing initial cruise velocity, larger flight-path angles and larger flight-path angles for cruising. As the flight dynamic pressure increases, the fuel consumption first decreases but then increases with a step change. The optimization results show that the hypersonic cruise stage should have 46 skip-periods with a minimum fuel consumption of about 32 tons for a two-hour global-reaching hypersonic aircraft with an initial weight of 100 tons and a lift-drag ratio of 4. The optimal results show that the fuel consumption prediction model is reasonable. The present study can guide the design of combined cycle propulsion systems.
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    ENGINEERING PHYSICS
    Real time trapezoidal shaping algorithm at high count rates
    HUANG Yuyan, GONG Hui, LI Jianmin
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 521-524.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.031
    Abstract   PDF (1123KB) ( 1073 )
    A trapezoidal shaping algorithm was developed for common exponential nuclear signals and the pile-up effect at high count rate was studied. Deconvolution was used for time-of-arrival estimates to effectively improve the ability to recognize pile-up pulses and reduce the pile-up effect on the energy resolution. A complete pulse height analyzer was built in LabVIEW to realize real time trapezoidal shaping, pile-up recognition and spectrum construction in the field-programmable gate array (FPGA). Measurements with an X-ray diffractometer with a silicon drift detector (SDD) give a 2.46% energy resolution at a count rate of 1.8×105 cps (counts per second).
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    Effect of medium size on the γ-ray buildup factor for energy absorption in water
    LI Hua, ZHAO Yuan, LIU Liye, XIAO Yunshi, LI Junli
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 525-529.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.032
    Abstract   PDF (1670KB) ( 864 )
    γ-Ray buildup factors calculated using an infinite medium model can differ greatly for actual finite models. The precision of the buildup factors used in the actual calculations is improved using Monte Carlo simulations to study the variation of the energy absorption buildup factors as a function of the medium size for water using a cylindrical model. The results show that as the cylinder radius increases, the buildup factors in water first increase up to a maximum. The corresponding radius values for the maximum buildup factors are not influenced by the γ-ray energy or the medium thickness, but are only related to the distance from the point source to the measured point on the medium surface. The differences between the buildup factors as the cylinder radius increases are larger than for the maximums, and the variation is linear for some mean free paths. As the medium thickness increases, the corresponding range of the mean free paths where the variation is linear also increases. This work provides a reference for analyzing calculational errors in shielding designs.
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    NUCLEAR AND NEW ENERGY TECHNOLOGY
    Effect of non-condensable gases on steam condensation in a vertical pipe with forced convection
    MA Xizhen, JIA Haijun, LIU Yang
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 530-536.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.033
    Abstract   PDF (1336KB) ( 839 )
    Non-condensable gases have an important influence on steam condensation by increasing the thermal resistance during condensation and decreasing the heat transfer coefficient. A heat and mass analogy model based on the Nusselt's theory is developed for steam condensation in a vertical pipe with forced convection. The predicted effects of the non-condensable gases on the condensation are with agreement with experimental data. The heat transfer coefficient in the inlet varies from 4.8 kW/(m2·K) to 1.2 kW/(m2·K) for inlet air mass fractions from 8.73% to 22.45%. The heat transfer coefficient then decreases along the pipe. Increasing the inlet temperature from 100 ℃ to 140 ℃ reduces the inlet heat transfer coefficient. The research shows that the inlet temperature and the kind and the mass fraction of non-condensable gas are the important factors governing steam condensation rate.
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    Thermodynamic analysis of a Brayton cycle system for a space power reactor
    LI Zhi, YANG Xiaoyong, WANG Jie, ZHANG Zuoyi
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 537-543,549.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.034
    Abstract   PDF (2013KB) ( 1350 )
    The land-use closed Brayton cycle system has high energy conversion efficiencies in a compact configuration. The Brayton cycle can be used in space nuclear plants with some modifications. This paper uses a mathematical model developed using Fortran computer language for a space Brayton cycle system to optimize the working fluid for power conversion units in a space power reactor to improve the design of space power reactors. The results show that xenon reduces the cycle's efficiency and specific power as the xenon mole fraction increases. However, a proper amount of xenon will reduce the enthalpy changes in the turbomachine, which is good for the system's aerodynamics. Helium and xenon have complementary advantages for the system's thermodynamic and aerodynamic performance. By mixing helium working fluid with appropriate amount of xenon, the Brayton cycle system gives better space reactor designs by decreasing the number of compressors and heat exchangers.
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    ECONOMICS AND MANAGEMENT
    Empirical test of the influence of treasury futures trading on interest rate volatility
    HE Ping, LIU Zehao, FAN Zhongjie
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 544-549.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.035
    Abstract   PDF (974KB) ( 733 )
    China reopened the treasury futures market in September, 2013. This study used a difference in difference model and a two-way clustering method to study the influence of treasury futures on interest rate volatility with a propensity score matching method for robust checks. The reopening of the treasury futures market significantly reduced the interest rate volatility. Emulational treasury futures trading also reduced the interest rate volatility, but the effect was weaker than with real trading of treasury futures. The results suggest that more treasury futures contracts can promote the interest rate market stability.
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    COMPUTER SCIENCE AND TECHNOLOGY
    Wavelet-based approach for anomaly detection of online services in cloud computing systems
    LIU Jinzhao, ZHOU Yuezhi, ZHANG Yaoxue
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 550-554.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.036
    Abstract   PDF (1253KB) ( 759 )
    As an increasing number of online services have migrated into the cloud, anomaly detection has now become an important problem. Existing efforts detect anomalies by mining real-time workloads; however, the accuracy of such approaches cannot be assured in case of user spikes and application errors. This paper presents a wavelet-based online anomaly detection approach that uses discrete wavelet transforms to decompose real-time workload traces into multiple curves with different frequencies and then applies statistical analysis to the decomposed traces to detect the workload anomalies. Tests show that this approach is more accurate with a lower false-alarm rate than existing approaches.
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    CIVIL ENGINEERING
    Classification of PM2.5 for natural cities based on co-Kriging and head/tail break algorithms
    LIU Zhao, XIE Meihui, TIAN Kun, XIE Xiaoxiao
    Journal of Tsinghua University(Science and Technology). 2017, 57 (5): 555-560.   DOI: 10.16511/j.cnki.qhdxxb.2017.22.037
    Abstract   PDF (1897KB) ( 810 )
    PM2.5 air pollution is now a hot topic in both social and academic circles. This study investigated the classification of natural cities based on PM2.5 concentrations in Mainland China. Firstly, the PM2.5 data obtained at monitoring stations and aerosol optical depths (AOD) obtained by remote sensing were fused to yield more accurate PM2.5 spatial distributions using a co-Kriging algorithm. Then, the PM2.5 concentrations were classified using the head/tail break clustering algorithm to identify natural cities with high PM2.5 pollution levels. Distribution of natural cities was also analyzed. The results show that the head/tail break algorithm with an appropriate segmentation threshold can efficiently identify natural cities with high PM2.5 concentrations. These classification results can guide policy makers to divide the country into several areas for pollution control.
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