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百年期刊
ISSN 1000-0585
CN 11-1848/P
Started in 1982
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, Volume 64 Issue 2
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CONSTRUCTION MANAGEMENT
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Phenomenon of the winner's curse in the Beijing land market
LI Enyuan, LIU Hongyu, ZHU Enwei
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 173-180. DOI: 10.16511/j.cnki.qhdxxb.2023.22.045
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[Objective] The land market plays a key role in achieving a stable and healthy market for real estate development and has a significant impact on macroeconomic conditions, government finances, and overall financial stability. However, many participants in the Chinese land market engage in blind expansion and irrational land acquisition, which interferes with achieving policy objectives such as price stability for land and housing, as well as the healthy development of the land market. This study analyzes market feedback of land auction participants and the factors influencing them. We use auction theory to examine the existence of the winner's curse phenomenon in the Beijing land market and provide policy recommendations for the government to regulate the land market. [Methods] This study is based on micro-level auction data from land sales in Beijing between 2013 and 2018 and from the Wind enterprise database. We first construct models to calculate cumulative abnormal returns for the auction participants' stock prices in the periods following land auctions. We then use an event study to explore the effects of participant, land, and auction characteristics on stock price changes. Particular attention is given to market feedback on the auction winners. The uniqueness of this study lies in the vast amount of data used. We consider factors that are crucial elements of market feedback but have been relatively unexplored in previous studies, such as participants' bidding premiums, past experiences in land auctions, and joint bidding. [Results] We find that:(1) The higher the final bidding premium of land auction participants, the more negative the market's reaction. Previous experience with repeated bidding and joint bidding enables participants to access more market information, helping mitigate irrational bidding. Variables such as land value, bidding intensity, and the frequency of winning bids on a single day that reflect a bidder's economic strength lead to more positive market evaluations. (2) Evidence of the winner's curse phenomenon is observed in the Beijing land market. Although cumulative abnormal returns do not show significant inter-group differences between winners and losers, results of controlling for the final bidding premium reveal that higher bidding premiums result in more negative market evaluations for the winners. Joint bidding helps winners to make rational bids, but the effect of repeated participation in the short term is not significant. (3) The market holds a significantly negative view of bidders who are active over an extended period, and this effect is more pronounced for the winners, providing additional evidence for the existence of the winner's curse phenomenon. [Conclusions] Based on these findings, we recommend the government to enact policies to encourage market participants to make rational bids. This could be achieved by promoting complementary advantages and sharing market information through joint bidding to some extent. The government should also enhance information disclosure through various means to alleviate information asymmetry in the market and strengthen supervision of active market participants' funds and the development and construction processes to reduce irrational bidding behavior.
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Land-buying behaviors of listed real estate firms and their capital market performance
CHEN Zixiao, LIANG Yao, LIU Hongyu
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 181-188. DOI: 10.16511/j.cnki.qhdxxb.2023.22.053
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[Objective] High-quality development and exploration of a new development model for the real estate industry has recently attracted considerable attention. Scale expansion and relevant land-buying are essential components of the development model. Much of the literature investigating land-buying behaviors of real estate firms has focused on the number of lands bought rather than the number of cities entered, which is also an important aspect. Besides buying lands, listed real estate firms must concern the market value management related to financing and investors' valuation. Therefore, this paper investigates the relationship between the land-buying behaviors of listed real estate firms and their capital market performance. Investors can build more efficient stock portfolios and real estate firms can improve land-buying strategies based on the results. [Methods] We employed regression analysis to estimate the influence of land-buying behaviors on the capital market performance of A-listed real estate firms. The data was primarily obtained from the CREIS and CSMAR databases, spanning from 2009 to 2021 and involving 87 firms. The model included the number of lands bought, cities entered, and other aspects of land-buying behaviors. Capital market performance was divided into two aspects:firm investment value and firm risk, which are represented using Tobin's
Q
and Beta coefficients, respectively. The model also controlled firm and manager characteristics, financial ratios, and market environment and employed the two-way fixed effect ordinary least squares (OLS) method. The heterogeneity effect on ownership, firm scale, and year of listing was explored using a regression on the subsample, while robustness testing was conducted by handling extreme values. Finally, we discussed a potential channel through which land-buying behaviors affected the capital market performance. [Results] The results show an inverted U-shaped relation between the firm investment value and number of cities entered; furthermore, the investment value increases when more cities are entered within a certain range throughout the study period. In the market downturn period, i.e., since 2017, the lifting effect due to entering more cities on the firm investment value has decreased, while that on firm risk has increased. Comparatively, the effect of the number of lands bought on capital market performance is not as important as that of the number of cities entered. The capital market performance of non-state-owned, smaller, or shorter-listed real estate firms is more affected by their land-buying behaviors. The impact of land-buying behaviors on capital market performance passes through the land-buying expense and sale revenue ratio channel. [Conclusions] The findings indicate a change in the reaction of the capital market to the regional expansion of land-buying. We recommend that real estate firms respond to feedback from the capital market and adjust their land-buying strategies in time, especially when selecting and entering more cities. Furthermore, investors should pay attention to region-focused real estate firms. The findings also suggest that the government should be cautious about structural changes in the real estate market, and a shift from focusing on the investment demand of real estate firms to focusing on the end-use demand of space is required to coordinate and guide the further improvement of the development model.
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Impact of construction industrial noise on workers' learning efficiency: A study based on electroencephalogram analysis
CAO Xinying, ZHENG Decheng, QIN Peicheng, LI Xiaodong
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 189-197. DOI: 10.16511/j.cnki.qhdxxb.2023.22.050
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[Objective] In this study, the impact of industrial noise on the learning efficiency of construction industry workers was investigated. Twenty-nine workers from a prefabricated component factory were recruited as study participants. [Methods] Industrial noise is generally classified as steady noise and complex noise. Each group of participants was randomly assigned to four different noise interference conditions of one of the two noise types, which were as follows:control group (daily white noise), low noise group (60 dB(A)), medium noise group (70 dB(A)), and high noise group (80 dB(A)). Each experimental session included a 5-minute instructional video and 20 multiple-choice questions related to the video content. The instructional video and test questions were sourced from the National Prefabricated Construction Vocational Skills Competition Exam Question Bank. Furthermore, the content of the instructional videos was designed by training instructors at the component factory to address the practical knowledge needs of the workers, and this content was synthesized into instructional videos. The test questions included knowledge points extracted from the instructional videos. Each instructional video and its corresponding test questions had similar levels of difficulty, with text quantity differences controlled within 10%. During the experiment, electroencephalogram (EEG) data were collected using Emotive-EPOC X EEG equipment. After preprocessing, including filtering, bad segment removal, and independent component analysis, the power spectral density (PSD) values for various frequency bands were extracted from the EEG data. Subsequently, these PSD values were utilized to evaluate the attention and cognitive load levels of the participants during the experiment. Behavioral data (accuracy and reaction time) were collected with psychological software and compared with the cognitive state data. Kruskal-Wallis variance analysis and Mann-Whitney U tests were employed to compare the differences in behavioral data and cognitive state data among the different groups. Pearson's correlation coefficient was utilized to evaluate the relationships between various data sets. [Results] The results of the comparative analysis reveal that, compared to a steady noise environment, a complex noise environment results in lower accuracy and longer reaction times, indicating that complex noise environments are more likely to reduce the learning efficiency of workers. However, within the same noise type, the noise level does not substantially affect the learning efficiency of workers. Moreover, different noise types have no remarkable effect on the cognitive state of workers, and different noise levels in a complex noise environment have no considerable differences in their impact on the worker's cognitive state. In a steady noise environment, different noise levels do not significantly affect cognitive load; however, they do considerably impact attention which significantly decreases when the noise reaches 80 dB(A). Correlation analysis reveals that attention is negatively correlated with cognitive load, and there is no significant correlation between reaction time and accuracy. Attention and cognitive load are not correlated with accuracy. Attention is negatively correlated with reaction time, while cognitive load is positively correlated. This implies that engaging in cognitive tasks increases the workers' cognitive load, leading to decreased attention levels and longer reaction times. Therefore, this study confirms the mediating effect of cognitive states on the relationship between noise and workers' learning efficiency. [Conclusions] This work offers a scientific basis for developing targeted measures to reduce the impact of noise on the cognitive states of construction workers and to enhance their learning efficiency.
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Automatic detection of hazardous scenarios during spatial interaction between tower cranes and workers
ZHANG Zhitian, WANG Yuanyuan, LUO Zhub, GUO Ziyang, GUO Hongling
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 198-204. DOI: 10.16511/j.cnki.qhdxxb.2023.22.047
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[Objective] Tower crane operations are characterized by long durations, extensive moving scopes, heavy loads, and complex spatial interactions with workers. These factors often contribute to construction accidents. Furthermore, construction workers standing under crane hooks and their lifting objects during the lifting process pose high safety risks, often encountering accidents such as collisions and object falling. Information technology plays a crucial role in enhancing tower crane monitoring and reducing workers' safety risks. Although existing studies on tower crane monitoring have made considerable advancements, they primarily focus on the operating state of cranes and overlook safety issues arising from interactions between cranes and workers. This study aims to employ the schedule information extracted from building information modeling (BIM) and computer vision and sensing technologies to propose an automatic hazard detection method for detecting dangerous scenarios during the lifting process in tower cranes. [Methods] This study develops an automatic detection framework for identifying hazardous scenarios involving spatial interaction between tower cranes and workers. This framework comprises four components. (1) Equipment installation and network environment establishment:cameras are installed at elevated positions to monitor the spatial locations of workers under the operating plane of a tower crane in real time. Furthermore, various sensors and cameras are fixed beneath the crane's trolley and cab to collect data regarding its operating status. A local area network is set up on the site to facilitate instantaneous data transmission. (2) Collection of tower crane operating data:the exact spatial location of the crane's hook is calculated using arm tracking and spatial trigonometric relations to determine its operating status. (3) Collection of workers' operational status data:advanced image recognition techniques are used to identify workers' positions, which are then converted into three-dimensional spatial coordinates through coordinate transformation. (4) Spatial relationship analysis and identification:precise spatial mapping of the tower crane's operating status and workers' positions is obtained using a unified BIM, followed by automatic detection according to predefined hazard assessment rules. [Results] The effectiveness and feasibility of the proposed method are validated by implanting it during a one-month real construction project. The analysis of data collected for 15 days reveals that the number of hazardous scenarios fluctuates considerably, peaking 523 times and plunging 35 times. These fluctuations correlate strongly with the number of workers on site, verifying the reliability of the proposed method and highlighting the need for intelligent hazardous scenario detection. Moreover, the results show that construction workers generally lack adequate awareness of the safety implications of tower crane trajectories. [Conclusions] This study successfully integrates BIM, sensing, and computer vision technologies to develop an automatic hazard detection method that focuses on the spatial interaction between tower cranes and workers. The proposed method enhances the timeliness and accuracy of hazard detection and provides innovative perspectives and technical support for construction site safety management. However, this study has certain limitations, such as data interferences caused by minor vibrations during tower crane operations to be further mitigated using noise reduction techniques in future research.
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Effect of expert hazard identification trajectory on construction workers' safety education: Evidence from an eye-tracking experiment
FU Hanliang, TAN Yubing, XIA Zhongjing, GUO Xiaotong
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 205-213. DOI: 10.16511/j.cnki.qhdxxb.2023.22.027
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[Objective] Most accidents in the construction industry are caused by hazards that remain unrecognized due to inexperience and inattentiveness. Novice workers have difficulty learning how to quickly and accurately determine the source of a hazard and avoid it; thus, it is necessary to develop a dynamic hazard identification process-assisted education pattern. Eye-movement modeling examples (EMMEs) are videos of gaze replays of hazard identification trajectory by an expert with a verbal explanation. [Methods] This study constructed the EMMEs of hazard identification by an expert to explore the mechanism of its influence on workers' safety education at different experience levels. We created eight virtual construction sites for hazard identification testing, which mainly included falls, collapses, electric shocks, lacerations, explosions, and unsafe actions. A participant's task was to search for hazards, i.e., to visually inspect construction site scenarios and determine where a safety accident might occur. An eye tracker was used to collect the search patterns of experienced and novice workers before and after EMME training. Eye movement data were collected from 14 novice workers and 10 experienced workers. The study followed a 2? mixed-group design with between-subject factor experience (experienced vs. novice workers) and a within-subject factor case (before vs. after EMME training). Hazard identification accuracy, task completion time, and sequence standardization were used as indicators to measure the identification performance of the participants before and after EMME intervention. [Results] Herein, a
t
-test was used to evaluate the difference between the hazard identification performances of novice and experienced workers, and the interaction effect was used to test the moderating effect of EMMEs on prior experience and hazard identification performance. The main results were as follows:(1) Participants with EMME intervention performed better at hazard identification and showed higher hazard identification accuracy, shorter task completion time, and higher sequence standardization after EMME training. This finding confirmed that instructions comprising EMMEs effectively improved construction safety education. (2) The hazard identification performance of experienced workers was better than that of novice workers in the pretest; compared to novice workers, experienced workers identified more hazards in less time with more standard sequences before EMME training. The experienced workers consistently inspected laborers first, then the equipment or environment, and finally, the entrance. Novice workers typically inspected the hazards in the same order but with a less consistent scan path. (3) The EMME-based safety education mode had the expertise reversal effect. Participants with rich work experience showed insignificant improvement in performance after EMME training, while novice workers benefited far more from EMME intervention than experienced workers. [Conclusions] Our results demonstrate the potential of EMMEs to indirectly teach strategic hazard identification sequences and contribute to deeper safety education, particularly for workers with limited work experience. This study has educational importance as it provides new evidence of the potential of eye-tracking technology as an indirect instruction tool.
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Establishment of an integrated digital delivery (IDD) system under the perspective of new construction industrialization
LU Yujie, LU Jiahui, WANG Shuo, MAO Ye, ZHANG Qilong
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 214-223. DOI: 10.16511/j.cnki.qhdxxb.2023.22.046
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[Objective] China has launched new construction industrialization as a development strategy for promoting the transformation and improvement of its construction industry. The core development demand is to coordinate the whole industry chain and multiple disciplines and enhance the construction industry's productivity through digital technology. This study explores a project management model called integrated digital delivery (IDD), which supports this strategy. IDD uses digital technology to integrate workflows, connect project stakeholders, and meet the inherent needs of new construction industrialization. The IDD implementation in China is currently still in its early stage, facing various challenges, including unclear implementation process, vague user guidance, and low data utilization efficiency. Hence, a theoretical system that can guide and support IDD's engineering practice is urgently needed. [Methods] This study takes China's prefabricated building projects as the main scenario and conducts research from the three following perspectives to cope with the challenges of IDD implementation:process implementation, technology development, and data application. Furthermore, a series of forward-looking theoretical frameworks are designed. [Results] First, the IDD project process framework is investigated, and standardized process nodes for the four stages of digital design, digital manufacturing, digital construction, and digital operation and maintenance are proposed, tightening the digital connection between each stage, breaking through the data silos of each stage, and providing a standardized reference for the IDD project implementation. Second, this work systematically combs through the key technologies involved in the IDD model and determines its core technology system. The IDD key technology system takes building information modeling (BIM) technology as the digital foundation and includes the technical content of two business categories, namely business execution and intelligent management. Finally, a data application model based on the IDD model, which divides 12 lines at the operational level, is proposed to explain the generation, circulation, and application of engineering data from the three aspects of data resource acquisition, data asset processing, and data-driven decision-making. This study analyzes the empowerment of the IDD model for the construction industry based on research results and applies the value chain theory to explore the value-added effect of the IDD model for the construction industry from a theoretical perspective. Similarly, this work lists the application of the IDD model in three typical projects, showing the process and technological innovation brought by IDD and proving the application value of the IDD model from a practical perspective.[Conclusions] This study innovatively applies the IDD model to the engineering management of prefabricated building projects in China and provides solutions to the problems of complicated deliverables, complex production processes, and the highly difficult full-cycle management in prefabricated building projects. The obtained results can be used in prefabricated building projects with complex production processes and complicated deliverables and be referenced and applied in other projects, as well. Providing a scientific basis and practical guidance for promoting the IDD model in China's construction industry, this work carries theoretical significance and application value for promoting the high-quality development of China's new construction industrialization.
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Ontology-based modeling and application of highway engineering safety knowledge
LIU Guangyu, AN Peng, WU Zhen, HU Zhenzhong
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 224-234. DOI: 10.16511/j.cnki.qhdxxb.2023.22.054
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[Objective] Ontology structure has been proved to be particularly important in the construction and organization of a knowledge graph (KG). A comprehensive method for the modeling, updating, and application of KG with the guidance of domain ontology needs to be explored. In view of the common knowledge gap in engineering safety management, this paper aims to propose an ontology-based framework to achieve domain knowledge modeling and updating. Using the highway engineering field as an example, this paper demonstrates how safety knowledge can be automatically extracted from industry-standard text data to facilitate the construction of a domain KG. Subsequently, the safety management scenarios are developed based on the building information model (BIM), and the auxiliary role of intelligent knowledge in safety management is demonstrated to verify the effectiveness of the engineering application of the developed KG. [Methods] This paper used the ontology-guided domain knowledge extraction method to construct the domain KG and proposed a knowledge network-guided method to update the ontology. Specifically, a layered knowledge system with multiple dimensions was summarized as the ontology layer based on the management approach and the established standard specifications within highway engineering. Following the guidance of the ontology layer, a structured knowledge network acting as the data layer was extracted from massive text materials by developing a series of knowledge extraction procedures. Consequently, a knowledge-flowing method from the data layer to the ontology layer was proposed. Three categories of methods based on the essence and composition of entities and the clustering of the entity's core words were summarized to realize the automatic updating of the ontology layer. Finally, combined with the developed highway safety information retrieval and application system, this paper demonstrated the organization and application of the constructed domain KG, thus verifying the effect of introducing ontology in the organization and deployment of knowledge. [Results] The developed ontology of highway engineering safety knowledge was featured as a layered knowledge system with seven levels and 390 nodes connected with~300 000 valid entity nodes in the data layer, facilitating the creation and integration of the KG's logical structures. The proposed method for updating the domain ontology aided by over 1 000 technical terms was demonstrated to be effective, with an increment of 51.5% in the expansion of the nodes to the ontology. The designed method of linking the ontology-guided domain KG with the BIM was validated for its feasibility through practical implementation within a real highway engineering safety management system, displaying the positive impact of ontology's guidance in the organization and expansion of knowledge. [Conclusions] This paper concentrates on the domain of highway engineering safety and presents a comprehensive paradigm for constructing, updating, and applying a domain KG to demonstrate methodological innovation in ontology updating. The results extend the application scope and technical approaches of KG technology, thereby enhancing information technology level in engineering safety management. Moreover, the findings of this research can be used for BIM evaluation and safety guidance in highway engineering construction projects, thereby advancing the level of information technology in construction safety management.
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Influence mechanism of innovation in the construction industry
ZHANG Yakun, ZHANG Xuteng, TANG Wenzhe
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 235-242. DOI: 10.16511/j.cnki.qhdxxb.2023.22.055
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[Objective] During project delivery, construction companies must solve technical problems, coordinate complex relationships among stakeholders, adapt to changing client demands and the external market, and consider environmental protection and social development. To this end, construction companies must innovate to improve their competencies. Achieving innovation has become a critical issue for the construction industry. Most existing studies exploring the promotion of construction innovation lack an overall perspective to analyze the impact of innovation management mechanisms on the integration of innovation resources and activities and rarely consider the critical roles of both engineering project innovation and company innovation ability. Therefore, this study comprehensively and systematically reveals the status of innovation in the engineering construction industry. It analyzes and evaluates the company innovation system, inter-organizational knowledge sharing, project implementation processes, and engineering project innovation. [Methods] From the perspective of the construction industry, this study systematically analyzes construction companies' innovation through questionnaire surveys and interviews to build a theoretical model of the construction industry's innovation. The relationships between company innovation systems, inter-organizational knowledge sharing, project implementation processes, engineering project innovation, company innovation ability, and company performance are revealed. In this study, 118 companies in China from the construction industry were selected, and a total of 500 questionnaires were issued to the owners, the designers, the constructors, and the supervisors from the companies, of which 431 were deemed valid. In addition, 110 technical and management personnel from 12 companies were interviewed via on-site and telephone interviews to determine their perspectives and knowledge of innovation in construction industry to provide in-depth interpretations of the quantitative analysis. [Results] The results show that the company innovation system can affect inter-organizational knowledge sharing and the project implementation processes, promote engineering project innovation, improve company innovation ability, and achieve company performance improvement. [Conclusions] This study systematically analyzes the construction industry's innovation, constructs a theoretical model of construction companies' innovation, comprehensively considers the innovation of engineering projects and company innovation ability, and reveals the influence mechanism of construction companies' innovation and its crucial role in company performance. The model is verified using questionnaire surveys and expert interviews. The results indicate that the company innovation system can affect inter-organizational knowledge sharing and the project implementation processes, promote the innovation of engineering projects, improve company innovation ability, and finally improve company performance. These results provide essential theoretical and practical guidance for construction companies.
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Analysis of factors influencing construdion and operation of Ningxia's “Internet + urban and rural water supply” project and management recommendations
ZHAO Heng, LI Shaoyi, TANG Wenzhe, WANG Zhongjing, HU Senchang
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 243-249. DOI: 10.16511/j.cnki.qhdxxb.2023.22.049
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[Objective] Located in northwest China, Ningxia records low annual precipitation. To address the water supply problems in the region, the Ningxia government actively promotes the "Internet + urban and rural water supply" project, which has become a model for China's urban and rural water supply projects. However, systematic research on the factors affecting the construction and operation of this project, as well as comprehensive research on the improvement and application of the standard system and information technology to this project, are lacking. Thus, the objective of this study is to comprehensively investigate the factors influencing the construction and operation of the Ningxia's "Internet + urban and rural water supply" project for improving the corresponding management system and enhancing project management. [Methods] Questionnaire and interview methods were employed to collect quantitative and qualitative data. First, the factors influencing the Ningxia's "Internet + urban and rural water supply" project construction and operation were identified. Subsequently, to gather quantitative data, questionnaires were given to respondents, namely, owners, designers, constructors, suppliers, supervisors, and users; qualitative data were collected through interviews to verify and supplement the quantitative data. Descriptive statistics was applied to investigate the questionnaire results, while factor analysis was employed to analyze the quantitative results by extracting three key factors from thirteen factors and constructing a factor analysis model. [Results] The results reveal that the factors affecting the construction and operation of the Ningxia's "Internet + urban and rural water supply" project can be reflected in these three extracted key factors, namely, management of water supply project stakeholders, management of water supply information technology, and management of water supply standardization. Moreover, recommendations are proposed for these three key factors. Combining the results of the qualitative research and characteristics of this project, three main recommendations are obtained:(1) It is necessary to improve the management of water supply project stakeholders, build a partnership between project stakeholders, and form long-term project goals based on the residents' demand for water. Based on long-term goals, stakeholders and residents should trust each other, exchange information, and cooperate with each other to improve project performance. (2) It is crucial to enhance the information management of water supply projects by building an intelligent information management platform and using information technology to collect and analyze the data of the entire process behind the "Internet + urban and rural water supply" project. Then, the data is analyzed and used for project optimization, risks prevention, and improving the convenience of urban and rural water use. (3) The standardization system of water supply projects needs to actively summarize the experience of "Internet + urban and rural water supply" projects and apply uniform standards for acceptance. [Conclusions] In this work, the factors influencing the construction and operation of the "Internet + urban and rural water supply" project are presented, and a management system model and empirical evidence for improving the management of this project are provided. This study is conducive to high-quality construction and operation of future urban and rural water supply projects, effectively improving the performance of water supply projects through management of water supply project stakeholders, information management, and construction and application of standardized systems. In addition, this study provides guidance for developing water construction projects, including urban and rural water supply projects.
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Project management of “Internet + urban and rural water supply” based on partnership: Case of Ningxia water supply system
PENG Yingzheng, LI Shaoyi, HU Senchang, WU Zekun, TANG Wenzhe, WANG Zhongjing
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 250-258. DOI: 10.16511/j.cnki.qhdxxb.2023.22.051
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[Objective] In the context of China's promotion of the construction of major water conservancy projects, Ningxia has actively participated in the construction of the "Internet + urban and rural water supply" project. The management of this project is crucial for determining its success or failure. However, current research only analyzes the application of water supply project management to urban and rural water supply projects, lacking empirical analysis on the relationships of water supply project management, partnership, and project performance. This paper uses the Ningxia water supply system as an example to propose a water supply project management model based on stakeholder partnership, which can provide a theoretical foundation for "Internet + urban and rural water supply" project management. [Methods] The present research obtained empirical data using a questionnaire survey method. A partial least square-structural equation model (PLS-SEM) was established using questionnaire data for quantitative analysis to verify the feasibility of the model. Based on the constructed conceptual model and empirical data, a preliminary questionnaire was designed. Further, the Likert 5-point scale method was used to quantitatively assign scores to the questionnaires. Six variables were selected:partnership, demonstration and design management, construction management, operation and maintenance management, "Internet + urban and rural water supply" engineering system operation, and project performance. [Results] This research showed the interaction mechanism between the "Internet + urban and rural water supply" project partnership, project management, and project performance, proving that the stakeholder partnership could strengthen the "Internet + urban and rural water supply" project demonstration and design management, project construction management, and operation and maintenance management, which improved the "Internet + urban and rural water supply" engineering system operation and enhanced the project performance. [Conclusions] Based on the present analyses, the project management of the "Internet + urban and rural water supply" project should be improved from the following aspects:(1) give full consideration to the role of stakeholder partnerships in the early stage of demonstration and design, construction management, and operation and maintenance management of the "Internet + urban and rural water supply" project; (2) utilize information technologies to improve project management, such as building information modeling (BIM) and city information modeling (CIM); (3) establish design, construction, and operation and maintenance management process based on information technology, monitor the status of design, construction, and operation and maintenance in real time, realize dynamic management of design, construction, and operation and maintenance, and ensure achievement of project objectives; (4) utilize the respective advantages of designers, construction parties, and operation and maintenance parties to enhance engineering system operation efficiency, and ultimately achieve project performance.
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Impacts of the irrigation area on extreme rainstorm in the western Tarim Basin
LIU Jiahui, NI Guangheng
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 259-270. DOI: 10.16511/j.cnki.qhdxxb.2023.22.035
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[Objective] The irrigation area in the Tarim Basin is imperative to supporting the local economy and social development; however, frequent and extreme rainstorm events in the basin have posed serious danger to human lives and property. Irrigation areas alter the local near-surface climatic conditions, and irrigation has cooling and humidifying effects and affects local climate. Thus, analyzing the impact of irrigation areas on extreme rainstorm events could improve our understanding of its mechanism and provide reference for irrigation development and flood management. [Methods] This study analyzed the impact of the irrigation area on the extreme rainfall process. The weather research and forecast (WRF) model was used, and the research object was the extreme rainstorm event that occurred in the western Tarim Basin from June 15 to 17, 2021 ("6?5" rainstorm). Furthermore, to explore the mechanism of the irrigation area, a high-resolution comparative simulation experiment was performed using two simulation scenarios set up in WRF, one with irrigation and one without irrigation, to study the impact of the irrigation area on various meteorological elements, and water vapor transport process and formation process in this extreme rainstorm event and reveal the impact mechanism of the irrigation area on the extreme rainstorm. [Results] (1) The Tarim Basin's eastward flow caused by the north-south high pressure brought a substantial amount of water vapor to the basin. Before the precipitation, water vapor was transported along the northern part of the basin to the west and then turned southward to the southwest of the basin, thus forming an extreme rainstorm. (2) The existence of the irrigation area obstructed wind in both the northern and western parts of the basin. The irrigation area caused wind speed to rise in front of the irrigation area, with a decline in water vapor behind the irrigation area leading to a decrease in water vapor transported to the north, and an increase to the southwest providing more water vapor for the extreme rainstorm event. (3) In the main precipitation area, the combined action of the irrigation and mountainous areas enhanced the uplifting effect of the terrain on the wind, forming vertical circulation in front of the mountains, and the precipitation increased due to the high amount of water vapor brought by the eastward flow. [Conclusions] A large amount of water vapor transport is found to be the main cause of the extreme rainstorm event. The obstruction effect of the irrigation area on water vapor transport process results in more water vapor being transported to the extreme rainstorm area. However, the enhanced uplifting effect of the terrain from the combination of the irrigation and mountainous areas causes water vapor to accumulate in the rainstorm area, leading to increased extreme rainstorm. Thus, these results further elucidate the significance of exploring flood management in irrigation areas.
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Water demand dynamic estimation in water distribution network hydraulic models based on Kalman filter
WU Shan, WU Yuchen, HOU Benwei, HAN Hongquan
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 271-281. DOI: 10.16511/j.cnki.qhdxxb.2023.22.037
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[Objective] Nodal water demand dynamic estimation is the main task in the dynamic update of a water distribution network hydraulic model. The data assimilation method based on the extended Kalman filter (EKF) has been widely adopted in dynamic verification and estimation of hydraulic model parameters for pipeline networks. However, the nodal water demand can be divided into different patterns according to user types. Ignoring the influence of demand patterns can cause unreasonable results. Accordingly, this study investigates the effect of demand patterns on the nodal water demand dynamic estimation by the EKF. Additionally, this study proposes an improved extended Kalman filter (IEKF) method and analyzes the main parameters affecting estimation accuracy by comparing different methods. [Methods] The EKF obtains the dynamic water demand of the model through the estimation of multiple time steps, each of which comprises the prediction and correction steps. In the initial time step, the demands are calibrated using an iterative calibration model to avoid state parameter error transmission. In the prediction step, a 24-h demand pattern is adopted as a priori information to predict the nodal water demand at time step
t
+1 with the demand estimation result at time step
t
. In the correction step, the nodal demand predicted at time step
t
+1 is corrected by using the measurement data at time step
t
+1. The abovementioned steps lead to the IEKF. Aiming at the key parameters in the calculation process, including system noise, measurement noise, and sampling interval, the adaptability of several data assimilation algorithms, including EKF and inferred-measurement Kalman filter (IMKF), is analyzed under different parameter settings. The root mean square error (RMSE) and mean absolute percentage error (MAPE) are used to evaluate the accuracy of the results. Additionally, the Nash-Sutcliffe efficiency (NSE) coefficient is introduced to evaluate the similarity between the demand estimation curves and their corresponding demand patterns. The robustness of the IEKF to errors in the demand estimation curves is also analyzed by establishing an error condition. [Results] (1) Compared with the EKF and IMKF, the MAPE obtained by the IEKF is 15.93% and 12.20% lower, respectively. Additionally, the NSE coefficient of the demand estimation curve is improved by 0.40 and 0.35, respectively. The computation time of the IEKF is similar to that of the EKF and is 99.8% lower compared with the IMKF. (2) The IEKF can adapt to larger sampling intervals, offering more advantages over the EKF and IMKF for larger sampling intervals. (3) Within the 24-h estimation period, the IEKF suffers smaller errors at all sampling intervals, and the demand estimation curves match the real demand pattern curves. Compared with the EKF and IMKF, IEKF can more accurately capture the demand change and is robust to the demand pattern curve errors in a priori information. [Conclusions] The proposed IEKF uses demand patterns of different user types as a priori information, and NSE coefficient is introduced to assess the similarity between the demand estimation curves and real demand patterns, which improves the accuracy of dynamic estimation of nodal water demand using data assimilation algorithms. In the estimation of nodal water demand, considering the user water consumption patterns can significantly improve the computational accuracy of the EKF method.
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Greenhouse gas emissions and future forecast of the sewage treatment system in Beijing
WANG Ming, HE Guohua, DOU Peng, QIN Changhai, ZHAO Yong, ZHU Yongnan, JIANG Shan
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 282-293. DOI: 10.16511/j.cnki.qhdxxb.2023.22.034
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[Objective] The low-carbon operation of sewage treatment systems has received significant attention globally as an important source of greenhouse gas emissions. However, existing research shows a deficiency in the prognostic analysis of future trends in urban sewage treatment systems, and insufficient attention is devoted to greenhouse gases from the sewage pipe network and sludge treatment facilities in greenhouse gas accounting. Consequently, the calculated results fall significantly below the actual values, thus impeding the formulation of targeted emission reduction measures. Although Beijing is one of the cities in China with the highest level of wastewater reuse, the large-scale use of wastewater could significantly increase greenhouse gas emissions. Researching greenhouse gas emissions from Beijing's wastewater treatment system could bear reference significance for other cities under the strategic backdrop of China's carbon peaking by 2030 and carbon neutrality by 2060. [Methods] A greenhouse gas accounting system of the sewage treatment system was established to measure the scale of greenhouse gas emissions from 2010 to 2020 based on the sewage treatment process in Beijing. The greenhouse gas accounting system for Beijing's sewage treatment system consisted of three stages:sewage pipeline network, sewage treatment, and sludge disposal. The accounting scope comprised direct emissions of CH
4
, N
2
O, and fossil fuel CO
2
, as well as indirect emissions of CO
2
resulting from equipment operation and chemical consumption. Greenhouse gas emissions for each stage were computed using the emission factor method. Additionally, various calculation formulas and parameters were introduced, such as CH
4
emission factors, N
2
O emission factors, and chemical emission factors. Therefore, the greenhouse gas emissions of each stage were calculated and aggregated, using the sewage treatment plant as the calculation unit, to obtain the overall greenhouse gas emissions of Beijing's sewage treatment system. Additionally, the scenario analysis method was used in this study to predict the characteristics of greenhouse gas emissions under different scenarios in 2035. [Results] The following research results are presented:(1) From 2010 to 2020, there has been a significant increase in the level of greenhouse gas emissions emitted by Beijing's sewage treatment system, rising from 1.191 3 to 2.269 1 million tons. Sewage treatment is the most significant stage, accounting for 51.27% of total emissions, followed by sludge disposal (39.92%) and sewage pipeline network (8.81%). (2) The sources of greenhouse gas emissions vary significantly across different stages. The sewage pipeline network is mainly associated with electricity consumption and CH
4
emission, whereas sewage treatment is primarily associated with electricity consumption and N
2
O emission. The sludge disposal stage mainly contributes to CH
4
and N
2
O emissions. Generally, it can be observed that indirect emissions, which include the consumption of electricity and chemical, constitute 62.79% of the total greenhouse gas emissions from Beijing's sewage treatment system. (3) The emission intensities of different treatment standards and processes are different:the emission intensity of the Beijing landmark exceeds the national standard, and the emission intensity of the membrane bio-reactor technology is 1.77 times that of anaerobic-anoxic-oxic technology. (4) The greenhouse gas emissions of Beijing's sewage treatment system in 2035 will increase in the baseline, high standard effluent, and low carbon emission scenarios by 59.87%, 110.69%, and 41.37% compared with 2020. Although the low carbon emission scenario has the smallest increase, its realization requires an additional investment of 4.995 billion Yuan and an area of 1.92 million m
2
. [Conclusions] The research results show a significant increase in greenhouse gas emissions from Beijing's sewage treatment system and provide pertinent emission reduction recommendations.
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Three-dimensional morphological characterization of sand particles based on a multiangle projection method
XIE Weiqiang, ZHANG Xiaoping, LIU Xiaoli, ZHOU Xiaoxiong, LIU Quansheng
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 294-302. DOI: 10.16511/j.cnki.qhdxxb.2023.21.25
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[Objective] The morphological characteristics (size and shape) of sand particles are critical for the evaluation of the mechanical properties of sand. Accurate quantification of these morphological characteristics is a prerequisite to investigating the mechanical properties of sand. Compared with the three-dimensional (3-D) test method, the two-dimensional (2-D) counterpart has the advantages of rich test types, simple procedures, and low cost. [Methods] To investigate the feasibility of 2-D images for 3-D morphological characterization of sand particles, 3-D laser scanning technology is employed in the present study to obtain the point cloud data of sand particles. Furthermore, a computer algorithm is developed to reconstruct the 3-D models of the sand particles. Thus, the 3-D morphological characteristics of the particles are accurately and quickly obtained. Three 2-D methods, namely the random single projection method, maximum and minimum projected area method (selected from 1 000 projections), and three orthogonal direction projection methods, are proposed for obtaining the 2-D morphological parameters of sand particles. [Results] Computer algorithms for reconstructing the 3-D model of sand particles based on a point cloud were proposed. The algorithms were used to automatically acquire the 3-D and 2-D morphological parameters. Thereafter, the correlations between the 3-D and 2-D parameters were analyzed. A practical method was proposed to obtain the 3-D morphological characteristics of the granular material based on the corresponding 2-D morphological characteristics. [Conclusions] The results reveal that there is a large gap between the 2-D parameters obtained from various angle projections, especially the area parameters in various projections. Notably, the error between the maximum and minimum area is as high as 40%. The random single angle projection method does not provide accurate 3-D information. A strong linear correlation exists between the 3-D and 2-D size parameters of the particles, and a moderate correlation exists between the shape parameters. An empirical relationship for estimating the 3-D morphology of sand particles using 2-D parameters is established. The 3-D parameters, namely the length, width, and thickness of the granular material, can be estimated using the corresponding 2-D parameters (
R
2
=0.735-0.994). The fitting effects of the particle size parameters are better than those of the shape parameters. Additionally, the fitting effects of the maximum and minimum projected area and three orthogonal directional projection methods are better than those of the random single projection method. Among the three 2-D methods, the maximum and minimum projected area and three orthogonal direction projection methods can more accurately estimate the 3-D morphological characteristics of sand particles; thus, these are practical methods for the morphological quantification of granular materials.
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Quantitative review of soil salinization research dynamics based on bibliometric analysis
WANG Zhongjing, YU Suyue, XU Xing
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 303-317. DOI: 10.16511/j.cnki.qhdxxb.2023.21.021
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[Objective] The research on soil salinization is complicated, and the traditional literature review method struggles to grasp the development trend systematically due to its subjective nature. Hence, it becomes important and necessary to seek alternative methods to objectively summarize and analyze the existing research papers and reasonably guide the future development of this field. [Methods] Based on the quantitative analysis of the Web of Science and China's national knowledge network databases related to soil salinization and saline-alkali land management and its utilization in the past 30 years, this paper proposes a quantitative review method for obtaining the research trend in this subject. This paper uses various software tools, including VOSviewer, Citespace, and SPSS, to analyze the number of publications, cooperation networks, and keywords within this field. [Results] The results show a continuous increase in the research on soil salinization at home and abroad. The number of scholars participating in the study of soil salinization has also increased significantly. The primary stage of development was before 1999, the stage of stable development was from 2000 to 2011, and the stage of rapid development was from 2012 to the present. Despite significant progress, according to Kuhn's model, the field remains in the primary science stage, indicating ample room for development. English journals such as
Agricultural Water Management
and
Science of the Total Environment
and Chinese journals such as
Soil Bulletin
and Soil can be regarded as core journals in the field of soil salinization, with the highest number of publications and higher journal impact factors. China, the United States, India, Australia, and other countries (in order of the number of papers published by them) have made outstanding contributions to soil salinization research, with Chinese scholars leading in terms of the highest number of published papers and are the main force in the study of soil salinization. The cooperation network analysis shows the importance of research institutions and direct government agencies in promoting institutional cooperation. However, at present, most cooperations are limited to intra-institutional cooperation, but future efforts should focus on the positive impact of cross-context, cross-institutional, transnational, and interdisciplinary cooperations on leapfrog and diversified development of soil salinization research. Keyword burst detection shows that "biochar", "yield", "salt stress", "quality", "freeze-thaw cycle" and "water and salt transport" are the recent hot spots of concern. Additionally, keyword co-occurrence and trend analyses show the shift in research focus from irrigation and drainage and saline-alkali land improvement in the 1990s to soil-plant salt interaction mechanism, salt-tolerant plant cultivation, soil water and salt regulation mechanism, optimization control technology, research and development technology of saline-alkali soil amendment, large-scale applications, remote sensing monitoring of saline-alkali land, and salinization impact assessment. It represents the developmental trend of research on soil salinization and saline-alkali land management and its utilization from drainage improvement to comprehensive utilization. [Conclusions] The research results of this paper quantitatively highlight the research hotspot and developmental trend of soil salinization and provide a reference for relevant researchers to grasp the developmental trends of the field and explore valuable new research directions.
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Application and research progress of transpiration cooling technology in flight vehicles
CHEN Zhongcan, ZHANG Kai, LI Feng, ZHAO Yue, WU Jianhui, HE Qilian, CHEN Min
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 318-336. DOI: 10.16511/j.cnki.qhdxxb.2023.26.047
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[Significance] Aerospace vehicles have undergone significant modifications in terms of aerodynamic shape, flight speed, flight environment, and flight duration compared with conventional flight vehicles. They must withstand harsh aerodynamic thermal environments for long durations and maintain a sharp leading-edge shape with a high lift-to-drag ratio, imposing extremely stringent requirements on the temperature resistance, durability, structural efficiency, and reliability of the thermal protection system. Traditional thermal protection depends largely on passive methods such as heat insulation, heat sink, and radiation heat dissipation. Although the thermal protection performance of related technologies has improved, which is restricted by several constraints, such as ensuring that the prototype is safe under harsh conditions of extremely high heat flux and ultrahigh temperature along with structural stability, long-term operation, light-weight nature, and repeatability. Thus, a new active thermal protection technology is necessary. In this context, transpiration cooling technology offers the advantage of high thermal efficiency without requiring any changes in the prototype of a vehicle. It has been widely considered a potential active thermal protection technology. However, when transpiration cooling is used for thermal protection of a flight vehicle, some challenges related to the complexity of the system, a mismatch between coolant supply and demand, unstable control of the operation, and development of a high-precision prediction model etc., arise. [Progress] Research on transpiration cooling primarily focused on quick evaluation of performance, numerical simulation of flow and heat transfer, evaluation of cooling mechanism performance, development of optimal control algorithm for efficiency, and optimization of structure form and yielded beneficial results. However, several fundamental scientific issues needed to be urgently addressed to fully realize the engineering application of this technology in aerospace vehicles. In the context of numerical simulation, the accuracy and adaptability of the heat and mass transfer model should be improved. Most existing studies had mathematically described and solved the physical process of heat and mass transfer in porous media at the macroscale. But some parameters related to specific phase change heat and mass transfer (such as evaporation/condensation coefficient and fluid-solid convection heat transfer coefficient) that affect the model's accuracy must be modified through experiments, and the adaptation was partially successful. Most existing models assumed that the temperature of porous media, liquid phases, and gas phases were equal. Although a few models explored the nonequilibrium effect between porous media and fluids, they did not consider the nonequilibrium effect between gas and liquid phases. There were few flight experiments in the research and a large gap between the ground experimental test and practical use conditions. Furthermore, extreme effects related to high-temperature, real, and rarefied gases and shock wave/boundary layer interference during high-speed flight could not be effectively reproduced on the ground. Moreover, there was a lack of experimental data that could be used to verify the accuracy of the heat and mass transfer model. The experimental test method was relatively simple, and the flow and heat transfer process of the liquid in the porous medium could not be obtained. It was challenging to effectively obtain the boundary layer flow law of the liquid when it entered the high-speed mainstream flow from the porous medium. In terms of control strategy, the present research on transpiration cooling control systems lacked a transient simplified mathematical model that could be quickly established, particularly for liquid phase change transpiration cooling with the multiphase flow and phase change process. Simultaneously, there were few transpiration cooling control systems with practical engineering values based on modern control theory, which made it difficult to achieve optimal performance in practical engineering applications. Some adaptive and self-driven transpiration cooling systems had been proposed as new forms of transpiration cooling structures; however, they were still at the mechanism verification stage, and the engineering application effect needed to be verified. [Conclusions and Prospects] Follow-up research will focus on the micro/mesoscale fine numerical calculation model, advanced visual experimental testing methods, rapid response-precise control strategies, self-driven and adaptive structural engineering systems, and combined active and passive thermal protection.
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Numerical simulation of fuel flow and heat transfer in a serpentine tube considering the fuel variable properties
LI Yu, WANG Xiangqin, MIN Jingchun
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 337-345. DOI: 10.16511/j.cnki.qhdxxb.2023.26.044
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[Objective] Air-fuel heat exchangers consisting of serpentine tubes are used to cool aero-engines, whose turbine blades and other hot parts are generally cooled by the air taken from the compressor outlet, which may have temperatures as high as 900 K. A practical approach uses engine fuel, usually at a temperature close to the normal atmospheric temperature, to precool the air taken from the compressor outlet to improve its cooling capacity. This process can be realized in air-fuel heat exchangers. This work aims to analyze the fuel flow and heat transfer in a serpentine tube and explore the influence of fuel inlet velocity, tube wall temperature, and straight section length of the serpentine tube on the flow and heat transfer characteristics, with an emphasis on the differences in such characteristics between the straight and curved tube segments of the serpentine tube and between the constant and variable fuel properties. [Methods] Considering heat exchanger compactness and the possible coking that may take place during the flow of fuel through the heat exchanger, the inside diameter of the serpentine tube is set to be 2.0 mm. To arrange the heat exchanger in the annular space between the combustion chamber wall and the main shaft of the aero-engine, the straight section length of the serpentine tube is set to be 65.000 mm. The serpentine tube is composed of 14 straight and 13 curved tube segments, constituting approximately 7 cycles, provided that 1 cycle is defined to include 2 straight and 2 curved tube segments. When the fuel flows through the serpentine tube, its temperature may increase by several hundred degrees Kelvin. This aspect promptes the consideration of fuel variable properties in the simulation model. The uniqueness of this work lies in the fact that it deals with variable property fuel flow and heat transfer in a thin serpentine tube. The low Reynolds number
k-ω
flow model is employed in the simulations, and calculations are implemented for fuel entering velocities of 1, 2, 3, 4, and 5 m/s, tube wall temperatures of 450, 600, 750, and 900 K, and serpentine tube straight section lengths of 65.000, 32.500, and 0 mm for both constant and variable fuel properties. The inlet fuel temperature is 350 K, whereas the pressure is 5 MPa. [Results] The calculation results revealed that the fuel temperature increased along the serpentine tube for both constant and variable fuel properties but increased more rapidly for variable properties than for constant properties. The fuel velocity remained constant for the constant property but varied nonlinearly for variable properties. The convective heat transfer coefficient remained almost constant for the constant property but exhibited a remarkable increase along the tube for variable properties. Moreover, the curved tube section exhibited a markedly larger convective heat transfer coefficient than the straight tube section. The variable property experienced a noticeably smaller pressure drop than the constant property, similar to the pressure drop in the curved tube section compared to the straight tube section; nevertheless, the pressure drop per unit tube length was considerably larger for the curved tube section than for the straight tube section. The tube wall temperature had a remarkable impact on fuel flow and heat transfer characteristics for the variable properties, whereas the straight tube segment length had a relatively weak influence on such characteristics. [Conclusions] The findings of this study support the fact that neglecting the variable fuel properties leads to an underestimation of the convective heat transfer coefficient and an overestimation of the pressure loss in the serpentine tubes used in air-fuel heat exchangers.
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Numerical study of circulation control phase based on fluidic flight control technology
WANG Ruiwen, BI Dianfang, HUANG Xudong
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 346-357. DOI: 10.16511/j.cnki.qhdxxb.2023.26.032
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[Objective] Circulation control is a typical fluidic flight control technology that is often used to improve the aerodynamics of aircraft and wind turbine blades. The aerodynamic performance of an airfoil can be improved by adjusting the mass flow rate of the jet. The impact of the Coanda effect on the trailing-edge jet produces a wall-attachment effect, decreasing the trend of flow separation. The researchers adjusted the airfoil circulation by controlling the trailing-edge jet separation. However, the relationship between the trailing-edge jet separation position in circulation control technology and the jet position, jet height, and jet intensity is unclear numerically. [Methods] To solve the problem of trailing-edge jet deflection, this paper mainly investigated the influence of the Coanda effect on the flow field distribution and lift-drag characteristics and analyzed the aerodynamic characteristics at different control stages of circulation control to contribute to the promotion of jet flight control technology. Because of the advancements in computer technology, computational fluid dynamics had considerably improved, and the simulation results for the complex structure of the flow field had reflected the real physical laws. In this paper, the NACA0012 airfoil was modified, and two dimensionless parameters, trailing-edge curvature and jet height, were changed to verify the effectiveness of the Coanda effect under circulation control. Furthermore, several key parameters, such as angle of attack, momentum coefficient, and jet velocity, were simulated using the CFD++ aerodynamic simulation software. Referring to other researchers' work, the concept of the Coanda deflection angle was introduced to reveal the triggering and suppressing mechanisms of the Coanda effect numerically. Thus, the separation prediction method was proposed for the modified NACA0012 airfoil and CC-E0020EJ circulation control airfoil (two symmetrical airfoils). For the two control stages in circulation control technology, this paper deconstructed the lift and drag forces through the push-drag decomposition method, explained the supercritical phenomenon of flow field separation and reattachment behind the trailing edge, and proposed a separation region analysis method. [Results] (1) The most important factor affecting the lift-drag ratio of airfoil trailing-edge parameters was jet nozzle height. When the chord position (
x
jet
/
c
) was 0.900, and the nozzle height (
h/r
) was 7.16%, the lift coefficient generated by the Coanda effect considerably increased, and the best effective lift-drag ratio increased to 43. (2) At a zero angle of attack, the variation trend of the Coanda deflection angle with momentum coefficient was highly consistent with the logarithmic relation curve and the same change characteristics of turning point and growth rate were detected on the symmetrical airfoil, indicating that the Coanda deflection angle was a physical criterion for judging the jet strength and increment of the lift coefficient. (3) The concept of "step zone" was introduced at a nonzero angle of attack, and the strong linear relationship between the jet influence zone position and momentum coefficient at 4? 8? 12? and 16?angles of attack was demonstrated. The large area separation in the trailing edge caused a surge in drag coefficient and loss of lift growth. [Conclusions] The analysis method proposed in this paper achieves the expected goal and provides an effective basis for judging the Coanda effect in fluidic flight control.
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Intelligent obstacle avoidance control method for unmanned aerial vehicle formations in unknown environments
HUANG Hao, MA Wenhui, LI Jiacheng, FANG Yangwang
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 358-369. DOI: 10.16511/j.cnki.qhdxxb.2023.27.001
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[Objective] Formations of fixed-wing unmanned aerial vehicles (UAVs), which are commonly used in military, rescue, and other missions, often do not have the ability to hover and have a large turning radius. Thus, when operating in an unknown environment, it is easy for the formations to collide in the presence of obstacles, which will gravely affect flight safety if not guarded against. It is difficult to avoid unknown environmental obstacles using traditional modeling methods. However, artificial potential field methods can address deadlock problems such as target infeasibility and cluster congestion. [Methods] To achieve the cooperation of UAV formations without collision, a deep deterministic policy gradient (DDPG)-based centralized UAV formation control method is proposed in this study, which is designed by combining the centralized communication architecture, reinforcement learning, and artificial potential field method. First, a greedy-DDPG flight control method is studied for leader UAVs, which improves collision avoidance effectiveness. Considering maneuver constraints, reward functions, action spaces, and state spaces are improved. Additionally, to shorten the training duration, the exploration strategy of DDPG is improved using the greedy scheme. This improvement mainly uses the critic network to evaluate the value of random action groups and improves greedy selection to make actions more inclined, thus achieving rapid updates regarding the critic network and accelerating the update of the overall network. Based on this, incorporated with the artificial potential field method and leader-follower consensus, a collision-free control method is designed for followers, which can ensure collision-free following cooperation. [Results] The numerical simulation experimental results show that the improved DDPG algorithm has a 5.9% shorter training time than the original algorithm. In the same scenario, the method that we proposed perceives the same number of obstacles as the artificial potential field method. The artificial potential field method has significant fluctuations in heading angle, while the proposed method has relatively small fluctuations. The DDPG algorithm has a smoother heading angle due to a smaller number of perceived obstacles; however, the minimum distance from the obstacles is only 9.1 m. The method that we proposed here is above 17 m from the obstacles. Furthermore, Monte Carlo experimental data under different scenarios of the long aircraft show that the ability of obstacle avoidance generalization of the proposed method is improved. Moreover, experiments were applied to the proposed formation control method. Under the same scenario and control parameters, the UAV formation control method based on the proposed architecture has lower formation errors during flight, with a maximum error of no more than 10 m. However, the artificial potential field-based formation control method has a maximum formation error of over 25 m. When encountering narrow gaps, our proposed method can quickly pass through without congestion, while the artificial potential field-based formation control method appears to hover in front of obstacles, which is not conducive to flight safety. During the entire flight, this method has a greater distance from obstacles and higher safety. [Conclusions] Compared with the original DDPG algorithm, the improved DDPG algorithm has faster training speed and better training effect. The formation control method can realize the formation flight of unmanned aerial vehicles under unknown obstacles. Compared with the formation control method based on artificial potential field, the formation control method avoids the hovering in place before obstacles, which is of great significance to the formation flight safety of unmanned aerial vehicles.
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Review of light-based host-nondestructive disinfection
ZHANG Wanqing, SU Ping, MA Jianshe, GONG Mali
Journal of Tsinghua University(Science and Technology). 2024,
64
(2): 370-380. DOI: 10.16511/j.cnki.qhdxxb.2023.21.011
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[Significance] Pathogens have caused numerous, large, and deadly outbreaks, and various disinfection techniques have been applied to stop the spread of harmful pathogens. These techniques are classified as chemical and physical disinfection based on the principle of inactivation. However, the frequently used disinfection techniques can cause indiscriminate harm to the host. Ultraviolet (UV) light is the most popular light disinfection technique, followed by chemical, thermal, ionizing radiation, and thermal disinfection, which harm the pathogen-containing biological tissues by altering their protein or nucleic acid composition. These technologies are unsuitable for prolonged disinfection in the presence of human activity. Thus, there is a need to explore a disinfection technology that can be safely used on living beings for a prolonged period. The disinfection technique must inactivate the pathogen and protect the host from the pathogen but cause no harm to the host cells. Light-based host-nondestructive disinfection techniques specifically inactivate harmful pathogens without damaging the surrounding tissues and have important applications in space disinfection, disease treatment, food preservation, and biologics production. [Progress] Currently, far-UVC, antibacterial blue light, and low-power ultrashort pulse laser are some of the most widely used nondestructive light disinfection techniques. Far-UVC damages nucleic acids by forming cyclobutane pyrimidine dimers between thymine molecules in DNA/RNA, thereby eliminating the ability of pathogens to replicate and infect. The 207-222 nm far-UVC is considered safe as the short wavelength penetrates only the stratum corneum, the skin's outermost layer, and the outer surface of the eye. However, research on far-UVC is limited because applicable dose standards and inactivation kinetics are lacking. The disinfection ability of antibacterial blue light is due to the presence of certain endogenous photosensitizers inside microorganisms that absorb light energy, such as porphyrins and flavins. These endogenous photosensitizers convert some substances into reactive oxygen species (ROS), which destroy the internal structures, such as organelles, of the microorganism by oxidizing neighboring biomolecules and subsequently inactivating the microorganism. Currently, research on antimicrobial blue light has focused on bacterial and fungal disinfection, with limited research on the inactivation of viruses and protozoans. Because viruses do not have endogenous photosensitizers, antimicrobial blue light requires exogenous photosensitizers to inactivate the virus through the ROS mechanism. Low-power ultrashort pulse laser creates vibrations on the surface of pathogens using femtosecond-level light pulses to induce protein remodeling, which destroys the surface of pathogens and prevents infection. Different proteins differ in densities and vibration durations, so a low-power ultrashort pulse laser selectively inactivates pathogens by varying the pulse frequency without causing damage to other biological tissues. As the inactivation process does not produce unknown intermediates, it is safe to use in the production of vaccines, sterilization of blood products, and disinfection of cell culture medium. [Conclusions and Prospects] Light-based host-nondestructive disinfection techniques should be evaluated using the actual absorbed dose of microorganisms as the criterion for the inactivation effect, and a comprehensive light energy inactivation rate model should be established. It is necessary to explore the optimal light energy density, optimal inactivation wavelength, and multiwavelength and multimodal synergistic disinfection to promote the application of large-scale energy-efficient light inactivation technology to prevent the spread of pathogens.
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