15 November 2018, Volume 58 Issue 11

    

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MECHANICAL ENGINEERING
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  Key techniques in microfluidic flow cytometers

  ZHAO Jingjing, YOU Zheng

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 953-960. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.036

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  Flow cytometers are biomedical instruments for high throughput analyses and sorting of single cells which are widely used in both biomedical research and clinical diagnoses. Advances in microfluidics have led to highly-integrated, compact, fully-enclosed, and no cross-contamination microfluidic flow cytometers. This study focuses on three key techniques with 3-D focusing of the sample flow, laser beam shaping, and on-chip sorting. 3-D focusing microfluidic chips confine the sample flow down to 10 micrometers at a velocity of several meters per second. Specially-designed binary optical elements (BOEs) generate micrometer-scale rectangular quasi-flat spots for exciting fluorescence that replace conventional elliptical Gaussian spots. These give an on-chip sorting mechanism based on the jet-flow by expanding spark-induced cavitation microbubbles. A microfluidic test system is developed by combining these three techniques with performance that is close to that of two commercial instruments.
  
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  Forced vibration response during the milling of thin-walled workpieces

  ZHANG Jie, LIU Chengying

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 961-965. https://doi.org/10.16511/j.cnki.qhdxxb.2018.25.042

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  Strong forced vibrations can easily occur while milling thin-walled workpieces because of their low stiffness. Such vibrations not only reduce the product quality, but also limit the choose of the machining parameters. The forced vibrations of thin-walled workpieces during milling were investigated by using an R-end milling cutter to reduce the cutting forces and the cutting coefficients in cutting experiments. The dynamics of the thin-walled workpiece were analyzed based on modal experiments to obtain the transfer function and modal parameters of the tool-workpiece system. The forced vibration response was then predicted using a time-domain method. The simulations indicate that a proper nose radius can suppress the forced vibration response.
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  Lower bound analysis of plastic limit and shakedown state of orthotropic materials

  QIN Fang, ZHANG Lele, CHEN Min, CHEN Geng

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 966-971. https://doi.org/10.16511/j.cnki.qhdxxb.2018.22.050

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  The purpose of this study is to predict the plastic limit and the shakedown state of orthotropic materials and structures. The Hill yield criterion is used in Melan's theory with the finite element method and large scale nonlinear programing combined to form a model to predict the plastic limit and the shakedown state of complex 3D structures made from multi-orthotropic materials. Several numerical examples are given to verify the accuracy, universality and efficiency of this method. The applicability of using shakedown theory to plastic analyses is extended in this work. This method can be used to design and assess structures made from orthotropic composites in engineering practice .
HYDRAULIC ENGINEERING
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  Precipitation variations in the Yellow River Basin of China

  WANG Yuanjian, FU Xudong, WANG Guangqian

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 972-978. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.043

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  The precipitation will change in the Yellow River Basin due to global climate changes. The flow in the Yellow River depends on the runoff from various regions and during different periods. Engineers need to predict these flow to make decisions about flood-control projects. The present study investigates the daily series of precipitation in the Yellow River Basin from 67 meteorological stations over the past 59 years (1959—2017) using multiple time-series analysis methods. The results show that the upper and middle parts can be divided into 7 regions. In the source region area of the Yellow River Basin (region Ⅲ), the annual precipitation has increased significantly which provides more water for use in the entire Yellow River Basin. However, in the middle reach (region IV), annual number of days with medium or heavy rains in some areas have significantly decreased, which may be one reason for the reduced water flow and sediment load in recent years in the river. The precipitation frequency information shows the rainfall levels have periods of 2~4 years in the Yellow River Basin. These results give evidence for identifying critical areas that impact the runoff in the Lower Yellow River.
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  Analysis of self-compacting concrete workability based on twin-shaft mixing images

  DING Zhongcong, AN Xuehui

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 979-985. https://doi.org/10.16511/j.cnki.qhdxxb.2018.25.043

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  The movement of self-compacting concrete (SCC) during mixing reflects its workability. Therefore, the SCC workability can be quantified by analyzing mixing images. In this study, eight concrete specimens with different workabilities were produced in a twin-shaft mixer. A smart phone was used to record all the mixing processes. Afterwards, images of interest and regions of interest (ROI) were selected from the image sequences for quantitative comparisons. The image of interest was the moment when a blade arm formed a 45° angle with the shaft before the mixer stopped. The SCC contour within the ROI was used as an indicator to indicate the SCC workability. The Y coordinate of the point with an X coordinate of 300 pixels (Y₃₀₀) was used to describe the SCC contour and to estimate SF. The results show that Y₃₀₀ is directly related to SF with a small relative error. This method can be used in a real-time monitoring system to estimate the SCC workability during mixing.
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  Coefficient of restitution of granular material at the seeper condition

  CHEN Yulong, ZHANG Ke

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 986-991. https://doi.org/10.16511/j.cnki.qhdxxb.2018.25.041

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  The motion of a rockfall is strongly dependent on the coefficient of restitution. This study investigates the coefficient of restitution in the presence of seeper. The coefficient of restitution was measured for various rockfall and seeper fall heights. The influences of the seeper depth and the fall height of the glass beads acting as the rockfall on the coefficient of restitution were investigated during normal impact on a flat steel wall with a seeper layer using free-fall experiments with a high-speed camera used to capture the collision mechanics. A collision model was then developed for the glass beads and the seeper on the rigid wall. The model took into account the viscous, surface tension, capillary, contact, drag, buoyancy and gravitational forces acting on the bead. The results show that the seeper significantly reduces the coefficient of restitution. Deeper seeper layers then dissipate more energy during the collisions that gives smaller coefficients of restitution. The coefficient of restitution increased with increasing bead fall height with part of the energy transferred from the glass bead to the seeper.
  
PHYSICS AND ENGINEERING PHYSICS
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  Data assimilation method for atmospheric dispersion based on a Gaussian puff model

  LI Ke, LIANG Manchun, SU Guofeng

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 992-999. https://doi.org/10.16511/j.cnki.qhdxxb.2018.22.049

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  Models are needed to quickly predict the atmospheric dispersion of radioactive material released in a nuclear accident. However, the uncertainties in the source term, meteorological data, and other conditions reduce the dispersion model prediction reliability. Data assimilation (DA) is usually introduced to improve the model predictions. The paper presents a DA method based on a Gaussian puff model to improve the predictions using some observed data. The method modifies the puff parameters to approximate the observed data in an iterative search. The four model parameters modified using particle swarm optimization in this study are the release rate, release height, wind direction, and mean wind speed. The method is applicable to mesoscale atmospheric dispersion with uniform and stable conditions over a flat area. Twin experiments are used to verify this DA method. The correlation coefficient between the experimental group and the control group at the observation points is 0.99. The source estimation in the non-steady condition is also tested with the correlation coefficient of 0.68, slightly better than the ensemble Kalman filter method. The method converges rapidly with good model predictions; thus, this method is useful for data assimilation of atmospheric dispersion.
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  Effect of altruistic scenarios for individual car-following behavior

  XING Han, ZHANG Lin, LIU Yi

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 1000-1005. https://doi.org/10.16511/j.cnki.qhdxxb.2018.21.020

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  Individual car-following behavior during earthquakes is affected by the altruistic scenarios. Driving simulator tests were used to observe car-following behavior for various individuals for the same test conditions for altruistic and neutral driving scenarios. The Gipps model was used to model the car-following behavior with a genetic algorithm used to calibrate the parameters in the Gipps car following model for the various individuals. The tests were then used to compare the model parameters from the two driving task groups, which showed a significant difference between the altruistic and neutral groups. The altruistic group had higher maximum deceleration rates, higher estimates of the maximum deceleration rate of the car in front of them, shorter safety distances and higher maximum acceleration rates than the neutral group. The results show that the altruistic individuals tend to follow closer to the car in front of them with more expectations for driving safety. This study shows that during emergency traffic evacuations, altruistic drivers will exhibit improved driving performance and safety.
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  Accident analysis based on a similarity mechanism safety system model

  JIA Nan, LIU Cheng, QIAN Jing

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 1006-1012. https://doi.org/10.16511/j.cnki.qhdxxb.2018.25.045

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  An accident analysis method was developed to reduce the incidence of accidents that are similar to previous accidents. The method is based on similarity system theory. This paper describes similarity safety system analyses with mathematical expressions for the similarity mechanism model and two examples. A general program was then developed for accident analyses based on the safety system similarity mechanism model. The model was used to analyzed the fire and explosion accident involving the hazard materials warehouse at Tianjin and the Zhongrong Metal Products Co. Ltd to develop methods for accident prevention and safety management. The results show that this method can be used to effectively analyze similar accidents. The research results provide a new accident analysis and prevention method.
AUTOMOTIVE ENGINEERING
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  Vehicle tive vertical vibration energy harvesting using unsprung tuned mass dampers

  BAI Shipeng, HOU Zhichao

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 1013-1020. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.042

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  A quarter car model with tire damping was used to analyze vibration energy harvesting with a tuned mass damper (TMD) system with an unsprung mass. With the road roughness as a stochastic velocity excitation, H₂ optimization of the dynamic vibration absorber (DVA) was used to determine the system parameters. The model predicted vehicle dynamics indices and energy indices defined based on the energy consumption to evaluate the system impact. The energy indices included the overall dissipated energy, the system energy harvesting potential, and the dissipated vehicle energy. The model was also used for a sensitivity analysis of the system parameters. The results show that attaching a system with an unsprung mass can harvest vibration energy from both the suspension and the tire.
THERMAL ENGINEERING
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  Pressure drop and critical mass flux of water flowing upward in a uniformly heated vertical tube

  TANG Guoli, GU Junping, WU Yuxin, LI Zhouhang, LÜ Junfu, LIU Qing

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 1021-1028. https://doi.org/10.16511/j.cnki.qhdxxb.2018.25.039

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  The critical mass flux, G₀, is a key parameter in the design of vertical water wall in once-through boilers. A vertical water wall has positive flow characteristic only when the mass flux inside the parallel tubes is less than G₀. While G₀ has been reported to be 1 000-1 200 kg/(m²·s), there are few studies on the factors influencing G₀ in the literature. This paper presents a hydrodynamic model based on the classical pressure loss computation method and empirical correlations. The model predicts the pressure drop variation for water flowing upward inside a uniformly heated tube at supercritical/subcritical pressures for various heat fluxes, tube lengths and tube inner diameters. The results show that G₀ increases with decreasing heat flux and tube length and decreases with increasing inner diameter.
NUCLEAR ENERGY AND NEW ENERGY
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  AP1000 containment accident transient analysis using MELCOR

  XIAO Hong, CAO Zhiwei, FENG Yingjie, YANG Zhiyi, ZHU Jianmin

  Journal of Tsinghua University(Science and Technology). 2018, 58(11): 1029-1036. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.041

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  The AP1000 containment and passive containment cooling system (PCS) were modeled using MELCOR 2.1 and SNAP with detailed 3D modeling to analyze the convective heat transfer, condensation and film evaporation for containment cooling. The film tracking model was used to simulate the PCS characteristics. The containment thermal hydraulic transients during a LOCA were analyzed to predict the main parameters at each stage after the accident. The effects of the film coverage fraction and the film coverage time were also studied. The results show that the MELCOR program can accurately simulate the thermal hydraulics of the AP1000 passive containment cooling system. The results provide a reference for analyzing the characteristics of the nuclear power plant containment system. This study also independently verifies the safety of this reactor design by using an analysis code that is totally different from the design codes.