15 February 2021, Volume 61 Issue 2

    

• Select all
  |

SPECIAL SECTION: COVID-１９
• Select
  Transmission of COVID-19 viral particles and the risk of infection among passengers in air-conditioned buses

  WU Jialin, WENG Wenguo

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 89-95. https://doi.org/10.16511/j.cnki.qhdxxb.2021.21.002

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  Aerosol transmission of the corona virus disease 2019 (COVID-19) is possible. This study analyzed COVID-19 transmission in an air-conditioned bus in Hunan Province, China. A numerical model was used to predict the transport of droplets and aerosols containing the COVID-19 virus for various air conditioning modes, particle sizes, and source locations. The results were used for quantitative evaluations of the infection risks for all passengers with comparisons to the actual transmission rate. A high proportion of exhaled viral particles from COVID-19 infected people were deposited on the inner wall of the bus and the seat surface. Changing the particle sizes and the outlet locations leads to different aerosol diffusion paths. Small aerosols containing the virus can remain suspended in the air for prolonged periods of time and become widely spread, so these pose a higher risk to passengers seated far from the source. The results are used to develop suggestions for reducing the COVID-19 infection risk in air-conditioned buses.
• Select
  Effects of social isolation on COVID-19 trends

  HUANG Mengyao, HUANG Lida, YUAN Hongyong, LIU Gang

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 96-103. https://doi.org/10.16511/j.cnki.qhdxxb.2021.21.001

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  Social isolation is the most common and effective way to control disease transmission during pandemics with the specific implementations great impacting the results. This study used the SEIRS model to analyze the pandemic conditions in 4 countries to investigate the effectiveness of various social isolation schemes on the spread of the corona virus disease 2019 (COVID-19) to prepare for additional future outbreaks with emphasis on the effects of the isolation duration and degree. The results show that for short-term social isolation, longer isolation time and lower isolation degree worked better for countries with good medical facilities and small populations while shorter isolation time and higher isolation degree worked better for countries with general medical facilities and large populations. For long-term social isolation until COVID-19 is disappeared, a 50% degree of isolation provided effective results. Overall, long-term social isolation is more effective than short-term isolation.
  
• Select
  Analysis of online college teaching data before and after the COVID-19 epidemic

  KONG Xiao, LIU Naijia, ZHANG Menghao, XU Mingwei

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 104-116. https://doi.org/10.16511/j.cnki.qhdxxb.2020.21.017

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  The corona virus disease 2019 (COVID-19) pandemic has led Chinese universities to turn to online teaching. Several researchers have used surveys to study the characteristics and challenges of online teaching. However, these surveys inevitably included subjective bias and sample limitations. This study analyzed the server logs of the Tsinghua University course selection system during online teaching with comparisons with those of the past two years. Since all students must visit the course selection system every semester, the server logs track the students' learning and living habits. After data cleaning, visualization and analysis, the logs show that over 98% of the sessions came from outside the campus during the epidemic. The number of visits to the system decreased by 25%-47%, the number of visits originating from mobile devices decreased by 7%, and the average browsing time per visit declined by 8%. Thus, this study shows evidence that online teaching leads to a notable decline in student engagement with the system. This study then suggests that colleges pay more attention to the student's productivity and engagement, thereby improving the quality of teaching.
• Select
  Comprehensive, resilient disaster and pandemic prevention construction framework for urban underground spaces

  QIU Tong, CHEN Xiangsheng, SU Dong

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 117-127. https://doi.org/10.16511/j.cnki.qhdxxb.2020.21.020

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  After the corona virus disease 2019 (COVID-19) outbreak in late 2019, civil engineering will play important roles in the urban comprehensive, resilient disaster and pandemic prevention construction. This paper combines the demand for urban pandemic prevention and control in China with the advantages of urban underground spaces for disaster prevention and pandemic responses in a comprehensive development plan. Emergency responses and pandemic control in urban underground spaces can provide safety, stability, adaptability to emergency medical conditions, low energy consumption underground emergency material storage, unified dispatching of emergency resources, and adaptability to urban development. An evaluation framework is given for urban underground space disaster and pandemic prevention to evaluate the resilience of such measures. The advantages of using urban underground spaces are illustrated by comparisons with other systems. The four topics, "stock", "increment", "variable" and "unified dispatching" in the disaster and pandemic prevention, are discussed to develop a resilience construction framework for urban underground spaces. Then, a comprehensive, resilient disaster and pandemic prevention construction framework for urban underground spaces is proposed to provide reference for the construction of resilient cities and urban underground spaces.
  
SPECIAL SECTION: SAFETY MONITORING
• Select
  Fire investigation method using completely blocked surveillance cameras

  WANG Guanning, CHEN Tao, MI Wenzhong, KANG Yanwu, DENG Liang

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 128-134. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.029

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  Video analyses are a key part of fire investigations. However, the cause of the fire cannot be easily determined when the surveillance cameras are completely blocked. This study analyzes fire location methods when the surveillance cameras are completely blocked based on video analyses. An oil pan pool fire is used to simulate an indoor fire scenario. The charge-coupled device (CCD) camera views are completely blocked with the videos of the fire scene then collected for analysis. A two-dimensional reconstruction of the fire scene is first created by combining the surveillance videos with the scene information. Then, the fire origin is initially estimated based on optical path analyses and further clarified by spatial illumination analyses. Finally, monocular visual positioning is used to combine the results to locate the fire origin. Tests show that the monocular visual positioning method based on the optical path and illumination analysis using completely blocked surveillance cameras can accurately locate the fire origin as a powerful tool for fire investigations.
• Select
  Instability of flow field in chemical industry park based on wavelet entropy

  ZHOU Chenglong, CHEN Tao

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 135-143. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.032

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  Monitoring and tracing of unorganized volatile organic compounds (VOCs) emissions in petrochemical parks is important to maintaining public safety with the local flow field characteristics at the monitoring points being the key to accurate tracing. A distributed monitoring system was designed to identify unorganized emissions of volatile organic compounds in petrochemical parks with numerical simulations to study the transient flows in equipment areas. The time-varying signal of the measured flow field was processed using wavelet entropy theory to relate the flow field stability to various flow parameters. The results show that the wavelet entropy can characterize the flow instabilities. The correlation analysis shows that the wind speed wavelet entropy and the wind direction variance strongly correlate with the wind direction wavelet entropy and the flow field instabilities. The results also show that the wind speed changes correlate negatively with the wind direction wavelet entropy, while the mean wind deflection and the wind speed variance are not related to the flow instabilities.
• Select
  Speed distribution model of mixed non-motorized vehicles based on video recognition

  LIU Hezi, CHEN Tao

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 144-151. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.026

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  Non-motorized vehicle travel has gradually revived in China to form complex, mixed traffic flows in recent years that lead to traffic movement and safety problems. The characteristics of the mixed non-motorized vehicle speeds need to be accurately modeled to characterize this problem. Deep neural networks are used here for multi-target tracking in videos of the non-motorized vehicle lanes to measure the non-motorized vehicle speeds. A statistical analysis of the vehicle speeds is used to determine the number of components of the Gaussian mixture model for the speeds using information criteria with the maximum likelihood estimate of each parameter calculated using the expectation maximization algorithm. Then, the model parameters are related to the road operating conditions and statistical characteristics. This intelligent video recognition method accelerates data collection to obtain sufficient data and the component determination prior to other parameters improves the modeling efficiency. The fitting results show that the Gaussian mixture model more realistically describes the speed distribution of non-motorized vehicles than the single distribution model. The Gaussian mixture model results are divided into the various types of non-motorized vehicles when the vehicle flow is not obstructed and the parameters are related to the average speed, the standard deviation and the ratio of different vehicle types. The classifications according to flow states are consistent during peak periods with the mean speed of the faster component close to the free flow velocity of the vehicles.
CONSTRUCTION MANAGEMENT
• Select
  Automated mapping from an IFC data model to a relational database model

  GUO Hongling, ZHOU Ying, YE Xiaotian, LUO Zhubang, XUE Fan

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 152-160. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.030

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  Building information modelling (BIM) is important in the construction industry, while industry foundation classes (IFC), an international data exchange model, are widely used for BIM research and applications. However, the limitations of IFC complicate the use of IFC as the main data format for redevelopment projects. Therefore, an automatic mapping method is developed to map an IFC data model to a relational database model. The IFC data model mapping has mapping rules for entity, type, function, rule, property set, and quantity set for the data analysis, data mapping, and information retrieval. Two tests are conducted to evaluate the validity of this method. The results show that the relational database provides better information integrity and data management than the IFC files with the mapping improving the integrity and efficiency of the BIM data management and also promoting the use of intelligent construction methods as well as the development of informatization in the construction industry.
HYDRAULIC ENGINEERING
• Select
  Registration of sand dune images using an improved SIFT and SURF algorithm

  TANG Yingfu, WANG Zhongjing, ZHANG Zixiong

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 161-169. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.031

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  The image registration of a single sand dune is difficult due to the similar colors, similar textures and blurred edges of the dunes. Commonly used methods such as feature extraction and feature point matching tend to have mismatched points. This paper presents an improved feature point screening algorithm with scale invariant feature transform (SIFT) and speeded up robust feature (SURF) based on the similar triangle principle for dune images to improve single sand dune tracking. The algorithm first uses a brutal force method to match the SIFT and SURF feature points, then filters the matching points according to the K nearest neighbor (KNN) algorithm, and finally uses the similar triangle principle to further filter the matching points. This method is then used to register the dunes in the Kumtag desert. The tests show that this algorithm is accurate and can be widely used.
NEW ENERGY ENGINEERING
• Select
  Pore-network model for multiscale analyses of battery thermal management systems

  WANG Yihang, YANG Yuankai, LIU Tong, WANG Moran

  Journal of Tsinghua University(Science and Technology). 2021, 61(2): 170-176. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.018

  Abstract ( ) Download PDF ( ) HTML ( )   Knowledge map   Save

  Industry urgently needs multiscale analyses of battery thermal management systems to improve battery safety. Battery packs are modeled here as porous media to optimize the battery thermal management system as a convective heat transfer process in a porous media. The pore-network model is efficient and strongly couples the multiple physical fields. The pore-network model is then used for multiscale analyses of battery thermal management systems. The validated pore-network model can then be used to relate the thermal characteristics of a single battery to the whole battery pack for multi-scale analyses of battery thermal management systems. The pore-network model is based on the flow and heat transfer processes in porous media at different scales and provides a foundation for future multi-scale engineering application.