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百年期刊
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CN 11-1848/P
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, Volume 63 Issue 11
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LOW-CARBON TRANSPORTATION & GREEN DEVELOPMENT
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Application and prospects of the computable general equilibrium model in low-carbon transportation policies
WANG Yan, OU Guoli
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1693-1706. DOI: 10.16511/j.cnki.qhdxxb.2023.26.033
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[Significance] The issue of climate change is extremely complex and encompasses multiple factors such as the environment, economy, society, and related aspects. With the ongoing maturation of complex system modeling technology, low-carbon transportation research using the computable general equilibrium (CGE) model presents a new approach to policy evaluation. The CGE model has three primary advantages for analyzing the economic challenges of transitioning to low-carbon transportation. First, the approach has a solid microeconomic foundation that can directly reflect the mechanism and influence of economic subjects' behavior under the assumption of a rational economic player. Second, CGE models are capable of fully simulating the connections of different economic sectors, which can uncover the transmission effect of transportation policy impact among various sectors, as well as the response of various sectors to the policy impact. Third, the model has two major types, static and dynamic CGE models, which can analyze the short- and long-term impact of different policies, respectively. As an essential prediction tool for policy impact and trend analysis, CGE models can comprehensively reveal the interaction characteristics between the transportation industry and the whole national economy, enabling the prediction of the economic and social impact of low-carbon transportation policies. [Progress] This study investigates contemporary research on transportation policies based on the CGE model. A total of 78 relevant empirical studies are collected from the Web of Science, Science Direct, and China National Knowledge Infrastructure, of which more than 50% focus on predicting the impact of low-carbon transportation policies, indicating that the investigation of traffic-related carbon emissions has gradually become a popular topic of empirical analysis using CGE models. The research topics include: (1) The influence of low-carbon transportation economic incentives, such as carbon tax, emission trading scheme, and transportation subsidies. (2) The application effect of low-carbon technologies, such as electric vehicles and carbon capture and storage. (3) The effect of low-carbon transportation urban planning, including land use, vehicle speed limits, walking-oriented urban design, and bicycle-oriented urban space development. (4) Predicting the economic and social impact of the implementation of nationally determined contributions and fuel economy standards. Previous research establishes a solid foundation for prediction and policy analysis in low-carbon transportation research; however, in the context of China's 2030 carbon peak and 2060 carbon neutrality goals, some issues remain that require further exploration and investigation. [Conclusions and Prospects] First, regarding emissions reduction policies, differing transportation needs, transportation structure, energy structure, technical level, and macropolicies will affect transportation carbon emissions. The carbon emissions reduction potential of various policies requires further study, and it is essential to propose structured solutions referencing the prediction and design of composite system transportation emissions reduction policies. Based on China's 1+
N
policy system for advancing the dual carbon goals, this study constructs a low-carbon transportation policy matrix based on the “avoid/shift/improve-planning/regulatory/economic/information/technological (ASI-PREIT)” structure, producing a proposed “policy basket” for low-carbon transportation CGE modeling. This policy matrix will comprehensively reveal the correlation between policy tools for low-carbon transportation CGE modeling and help put forward structured low-carbon solutions. Second, in terms of model construction, accessibility is the most intuitive factor for transportation. As with other sectors, treating the transportation sector simply as a product production sector risks neglecting network and external benefits; therefore, this study proposes the inclusion of transportation accessibility factors in low-carbon transportation CGE models as spatial computable general equilibrium model to identify regional economic correlations and regional product flow. Third, in terms of synergies, carbon emissions reduction in transportation is crucial to achieving China's dual carbon goals and can advance innovation and economic growth, leveraging a wide range of synergies, including sustainable development, improving public health, and enhancing the overall quality of life. Currently, increasingly severe ecological and environmental challenges are forcing global economies to reassess the GDP-centered development model, seeking balanced and sustainable development strategies that include environment, economy, and society. This study proposes the development of a comprehensive low-carbon transportation CGE model to compare and analyze the optimal solutions for balancing the co-benefits of environment-economy-society from a global perspective and design low-carbon transportation policy combinations to advance sustainable development. In summary, this study endeavors to systematically review the empirical research applying CGE models in the field of low-carbon transportation, provide a reference for expanding the research on low-carbon transportation, and help policymakers and the transportation sector achieve China's dual carbon goals.
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Strategies and pathways of the transport sector for addressing climate change
SONG Yuanyuan, YAO Enjian, XU Honglei, HUANG Quansheng, WU Rui, WANG Renjie
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1707-1718. DOI: 10.16511/j.cnki.qhdxxb.2023.26.021
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[Significance] Climate change is the primary challenge that intensely affects sustainable human development. The transport sector has been one of the major sources of carbon emissions and is considerably affected by climate change. Because of the growth of China's economy and total transport demand, transport-related carbon emissions are also gradually increasing. Moreover, frequent complex and extreme climate events with clear regional differences have negatively affected the construction, maintenance, and operation of the transport infrastructure. Therefore, China's transport sector needs to reduce carbon emissions for green and low-carbon developments and improve its adaptability and resistance to various adverse climatic conditions. However, China's transport sector still faces many challenges in mitigating and adapting to climate change, and its policy tools, measures, and basic capacity to cope with climate change need to be enhanced. Therefore, transport sector-related strategies and routes to adapt to climate change need to be explored. [Progress] First, the policies and measures implemented in different countries to address climate change were introduced from the perspectives of mitigation and adaptation. Second, the advancements made by China's transport sector in mitigating climate change were summarized from the perspectives of the construction of green and low-carbon transport infrastructure, optimization of the transport structures, and promotions and applications of new and clean energy. The measures implemented to adapt to climate change in China's transport sector were summarized from the perspectives of improving the adaptability of the transport infrastructure, strengthening the monitoring and warning systems of climate change, and managing risk. Third, the interactions between each subfield and sublink of the transport system and climate change, as well as the main measures implemented to mitigate and adapt to climate change in the transport sector, were analyzed. Finally, key areas, strategies, and methods to mitigate and adapt to climate change were proposed. [Conclusions and Prospects] Analysis results are provided and discussed. First, the current plan for China's transport response to climate change needs improvement. The capacity to respond to climate change has not been planned at the subfield and sublink level of the transport system. For mitigating climate change, carbon emissions reduction measures, such as the promotion of new energy vehicles and ships, as well as the optimization of the transport structure, are inadequate. Furthermore, the assessment of the effects of the transport infrastructure on climate change is still in its infancy. Second, the direction of the transport system's development should be combined with the strategic requirements of mitigation and adaptation to climate change. Third, in the transport field, the infrastructure, equipment, and transport structure should be improved; moreover, the infrastructure should be adapted to climate change, and emergency support of transport equipment and transportation organization in extreme weather should be optimized to enhance the capability to adapt to climate change. Finally, the following measures are proposed: Mitigation and adaptation to climate change should be jointly and appropriately implemented to comprehensively address climate change in the transport sector. Greenhouse gases and air pollutants should be jointly controlled to realize the goal of “double carbon”. Adaptation to climate change should be applied in conjunction with ecological protection and restoration to strengthen the capacity of the transport sector to adapt to climate change.
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Policy recommendations for green travel in medium cities based on signaling data
LÜ Yuan, GE Haojing, JIAO Pengpeng
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1719-1728. DOI: 10.16511/j.cnki.qhdxxb.2023.26.020
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[Objective] Urban traffic is a vital factor in determining urban competitiveness, which is an essential aspect of determining urban vitality and crucial for improving urban livability. With the increasing demand for urban motorized travel, traffic conflicts and low road efficiency have become more evident in the central parts of cities. These issues hinder the successful development of the city and affect the quality of life of the residents. By further elucidating and analyzing the travel characteristics of different regions, we can implement targeted strategies to improve traffic conflicts, maintain road order, and enhance road efficiency and safety. [Methods] This paper proposes a transportation mode detection method based on signaling data to formulate travel policies to help mid-sized cities for realizing green and sustainable development. The proposed method considers the characteristics of signaling data in mid-sized cities. This paper analyzes the possible range of the traveling time and distance. Moreover, when exploring travel characteristics, the proposed method does not directly trust the signaling timestamp and does not directly use the Euclidean distance between base stations. It neither uses the shortest distance in the road network nor the road network distance based on a certain mapping rule. The traveling speed range is determined according to the base station positioning principle and information reporting mechanism. The map application programming interface (API) is used to obtain the speed of different traffic modes, accurately achieve the travel characteristics of the residents, and formulate targeted policies. When using the map API, the travel strategy of residents, traffic flow information, and network structure are considered. The detection is completed by the speed interval adjustment and the Logit model based on the travel time reliability perception. The limitations of positioning accuracy and time granularity make it a challenging task to mine and verify the characteristics. However, introducing multisource data when the standard dataset has not yet been constructed helps address this issue. [Results] The study used air pollutant emission data and intersection flow data to verify the mode detection results of large-scale areas rather than using only the official mode share statistics for verification. Then the transport modes detection method was used to analyze the travel structure of the central area in Jingzhou. The method could effectively detect transportation modes by extending temporal, and spatial information by considering the road network, and travel strategies of the residents. [Conclusions] The results demonstrate that during peak hours, the high traffic demand of schools and the concentrated parking contradiction around hospitals aggravate traffic congestion. It is necessary to strengthen the traffic organization and management of the academic and medical circles to reduce emissions in the low-speed driving stage of motor vehicles. The attraction of traditional public transportation during peak hours is insufficient, and the local conditions for opening demand-responsive buses are available. The inevitable increase in population will attract more residents who initially used individualized motorized traffic. People have a clear preference for transportation when the origin and destination are the same (interval distance of approximately 3 km) and the travel time of individual motorized travel and cycling is close. Appropriate sharing of nonmotor vehicles, along with traffic organization and management, can encourage specific groups to use nonmotor vehicles.
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Capacity-matching model of landside multiple transport modes for large airports considering the impact of carbon emissions
HUANG Ailing, WANG Zijian, ZHANG Zhe, LI Mingjie, SONG Yue
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1729-1740. DOI: 10.16511/j.cnki.qhdxxb.2023.26.034
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[Objective] In the airport ground-transport system, it is operationally important to match the evacuation requirement of passengers, and the capacity of multimodal transport vehicles is crucial. Numerous studies have investigated single-mode transport capacity allocation; however, research on multimode allocation is scarce. [Methods] To mitigate the difficulty in realizing an exact match between the evacuation demand of passengers and the capacity of multimodal transport, a bi-level programming model for multimodal transport resource allocation is proposed according to the analysis of the interaction between capacity allocation and passenger travel choice. A utility function of multiple travel modes, including airport buses, metro, taxis, and private cars, is formulated with the following four features: travel time, travel cost, punctuality, and comfort. The upper-level objective is to minimize the total enterprise-operation cost, passenger-waiting cost, and carbon emission cost for optimizing the headway of public transit and the taxi arrival rate, which is subject to the capacity of each transit mode, the range of each public-transit headway decided by fixed equipment, and the range of taxi arrival rate decided by the capacity of boarding location. Based on the output of the transport capacity allocation scheme used by the upper level, the low-level objective is to assign the passenger flow toward multiple travel modes according to a stochastic user equilibrium-logit model with a utility function. Furthermore, an improved genetic algorithm combined with method of successive algorithm (MSA) is designed to solve the proposed bi-level programming model. To improve the solving efficiency of the algorithm, a pre-search mechanism is proposed, in which the infeasible solution is filtered out using low-precision MSA to reduce the computational cost of repeatedly calling the low-level model. [Results] The Beijing Daxing International Airport was considered as a case study to illustrate the efficiency and effectiveness of the proposed bi-level programming model in optimizing transport capacity allocation in airport ground-transport centers. The transport capacity allocation scheme obtained via the proposed model reduced the average passenger-waiting time and the total carbon emission of the system by 14.08% and 6.21%, respectively, while increasing the operation cost by only 1.32%. Moreover, the optimized capacity allocation scheme resulted in the switching of 6.7% of passengers who availed taxis and private cars to buses and metro, which were more environmentally friendly. The proposed solution algorithm could efficiently solve the bi-level model. Under the pre-search mechanism, the generation time of the scheme was 217.6 s, which could meet the production demand within the acceptable time. [Conclusions] Results show that the optimized scheme obtained from the bi-level model and algorithms is considerably better than before. The proposed scheme reduces passenger-waiting time and the carbon emissions of the multimodal transport system at a negligible cost. Using the optimized scheme, the organizers of airport ground-transport centers can coordinate the capacities of landside multiple transport modes and guide passengers reasonably. This will reduce operation costs, improve airport landside traffic structure, and encourage green and low-carbon travel.
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Low-carbon-oriented pricing strategy of multi-mode transportation service
WANG Yue, YAO Enjian, HAO He
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1741-1749. DOI: 10.16511/j.cnki.qhdxxb.2023.26.024
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[Objective] Optimising travel structure, improving travel efficiency, and reducing transport carbon emissions are essential paths to green and low-carbon transport development. Research into fine-grained carbon management has received much attention in recent years. However, the implementation is complex, and setting a price on carbon estimation tends to elicit negative feelings from travellers. [Methods] In the concept of mobility as a service (MaaS), the service can provide an end-to-end travel service by the combination of multi-transport modes, including roads and public transport, as well as many new forms of transportation. Thus, the service provider can realise flexible price adjustments for multi-transport modes and sections in a single trip. Consequently, this paper proposes a low-carbon-oriented pricing strategy for the service provider. From the different perspectives of the MaaS servicer, travellers and the environment, we propose a multi-objective optimisation model. The object includes maximising service providers' revenue and minimising network travel time and transportation network carbon emissions. The model is a two-layer planning model. The upper layer of the model is the process of finding decision variables to calculate the objective function. The lower layer is the joint traffic mode and route choice process, as well as traffic equilibrium allocation in a multi-modal transportation network. In this model, the joint choice of mode and route of travellers depends on the upper-layer decision variables. Then, to solve the above optimisation problem, the reference point based non-dominated sorting genetic algorithm (NSGA-Ⅲ) and the method of successive algorithm (MSA) are introduced. [Results] The case study was conducted on an example network with 1 origin-destination pair, 16 sections in 3 traffic modes (travel by car, bus, and metro), and 6 nodes. Three representative strategies of Pareto solutions were selected, including optimise service provider benefits (OP-
S
), optimise network travel time (OP-
T
), and optimise transportation carbon emissions (OP-
C
). Furthermore, the original (OR) state was also presented as the background. The result showed that the travel price significantly increased in OP-
S
, which was unfriendly to travellers. In contrast, OP-
T
and OP-
C
were respectively metro-friendly and public transport-friendly strategies. Compared with the OR state, service benefits and carbon emissions were optimised, which means that the service provider could achieve emission reductions in multi-modal transport networks while ensuring their own profitability through rationalised regulation of service pricing. The traffic volume analysis also proved that the service provider could optimise the network travel mode structure, thereby reducing road congestion and increasing the share of public transport. By comparing the results of the optimisation strategies under different demands, we found that with the travel demand increased, the service provider benefits continued to grow (especially in OP-
S
). Although traffic carbon emissions increased, the optimisations could always reduce the traffic carbon emissions of the system. [Conclusions] This paper validates the feasibility of travel service pricing strategies in multi-modal network traffic optimisation and low-carbon transport development. Service providers should not only seek to maximise their own revenue but also take into account the cost of travel and its impact on the transport environment and take responsibility for the coordination and reduction of transport system emissions. This paper identifies the profitability and responsibilities of travel service providers in the green and low-carbon development of transport and provides a basis for service pricing strategies.
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Optimization model of electric vehicle charging induction based on comprehensive satisfaction of users
BI Jun, DU Yujia, WANG Yongxing, ZUO Xiaolong
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1750-1759. DOI: 10.16511/j.cnki.qhdxxb.2023.26.038
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[Objective] With the increasing prevalence of electric vehicles (EVs) in urban transportation systems, charging guidance service has become an effective means to solve the charging problem in the context of insufficient charging infrastructure. However, for optimizing the charging station selection decision-making plan of users, most existing research studies aim to minimize travel costs, which rarely considers the charging experience of users during travel and ignores the interaction of charging station selection decision-making between multiple users. To enhance the charging experience of users, based on the analysis of charging satisfaction of EV users, an EV charging guidance optimization model that integrates user satisfaction with detour distance, queuing time, and charging cost is proposed in this study. The model aims to maximize the average comprehensive satisfaction of multiple users. [Methods] To quantify the comprehensive satisfaction of users with charging stations during charging processes, evaluation indicators of detour distance, queuing time, and charging cost are constructed. To accurately deduce the queuing time of users at charging stations, this study fully considers the interaction influence of charging station selection decision-making between multiple EV users. Prediction models of the charging station operation state are established by considering several charging scenarios based on the arrival patterns of two successive users. According to the characteristics of the proposed model, an immune algorithm and the Floyd shortest path algorithm are applied to optimize the decision-making plan of charging station selections and the travel paths of multiple users, respectively. A numerical example with multiple charging requests is designed to confirm the feasibility and effectiveness of the proposed model and the algorithms. [Results] The experimental results indicated that optimal charging station selections and driving paths of multiple EVs to maximize average comprehensive satisfaction could be obtained by solving the optimization model. Compared with models with single optimization objectives, namely, minimum detour distance, shortest queuing time, and least charging cost, the average comprehensive satisfaction of EV users was increased by 15.0%, 17.8%, and 11.4%, respectively. The results also showed that average driving speed was a critical factor affecting optimal charging station selection and average comprehensive satisfaction of EV users. By analyzing the arrival patterns of two successive users at the same charging station under different charging scenarios, their queuing time after arriving at the charging station could be accurately obtained. Subsequently, optimal charging station selections by multiple users could be determined by considering the interaction that influences their selections. [Conclusions] The proposed optimization model provides multiple EV users with decision-making support for selecting charging stations by considering their interaction influences. Provided that the threshold values of each satisfaction indicator remain unchanged, the comprehensive average satisfaction obtained by the proposed model is considerably higher than that obtained by models with single objectives: minimum detour distance, shortest queuing time, and least charging cost. Thus, the proposed method can enhance the charging experience of EV users during travel.
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Eco-driving evaluation and trajectory optimization based on vehicle specific power distribution
ZANG Jinrui, JIAO Pengpeng, SONG Guohua, WANG Tianshi, WANG Jianyu
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1760-1769. DOI: 10.16511/j.cnki.qhdxxb.2023.26.029
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[Objective] Eco-driving is an important way of reducing emissions and conserving. However, in previous research, the evaluation and trajectory optimization techniques of eco-driving behavior have primarily been based on traffic simulation and driving simulator technologies, considering less the actual driving characteristics of human beings. In practice, eco-driving optimization curves are difficult for drivers to follow. The purpose of this study is to propose a novel quantitative evaluation method of eco-driving behavior based on the difference of vehicle specific power (VSP) distributions between a large number of drivers and an individual driver and to develop an eco-driving trajectory optimization model that conforms to human driving habits. [Methods] First, the baseline speed-specific VSP distributions are developed based on 754 000 records of second-by-second vehicle activity data of driving trajectories from 159 drivers on expressways in Beijing. The individual driver's speed-specific VSP distributions are developed for comparison to the baseline VSP distributions. Based on the discovered variations, a model is proposed to assess eco-driving behaviors based on the identified differences to quantify the ecological level of driving behaviors for various speed ranges. Then, based on the eco-driving assessment model suggested in this study, a significant number of real eco-driving trajectories are found. The back propagation (BP) neural network, polynomial, and sine function fitting techniques are used, and the fitting accuracy is assessed using the goodness of fit and root mean-square error. The optimum fitting approach is used to build the eco-driving trajectory fitting method. Finally, to prove the viability of the approach suggested in this case study, the non-environmental trajectories are optimized using the ecological trajectory optimization model as a case study. [Results] The results showed that: (1) The differences between the baseline and individual VSP distributions effectively evaluated ecological driving behavior, and the scoring method for ecological driving behavior was constructed to quantitatively evaluate the ecological degree of driving behavior ranging from 0 to 10. (2) The goodness of fit of the quintic polynomial of the sine function to the actual ecological driving trajectory was 0.999 8, which was the highest of the three fitting methods. The sine function polynomial fitted the acceleration and deceleration trends of the eco-driving trajectory well. (3) The eco-driving trajectory optimization method had a good fuel-saving effect, and the overall fuel consumption of non-environmental trajectories was reduced by 7.63% on average.(4) The case study showed that the fuel consumption of non-environmental trajectories was reduced, and the stability of non-environmental trajectories was improved after the optimization of ecological trajectory curves developed in this paper. By analyzing the differences between the baseline and individual VSP distributions, the ecological degree of the driving behavior could be quantitatively evaluated, and the actual eco-driving trajectory could be effectively identified. The eco-driving trajectory optimization model proposed in this paper had a good effect on reducing fuel consumption. [Conclusions] The conclusions in this research fill the gap left by the existing trajectory optimization models that neglect to consider human driving factors. In order to assist in reaching carbon peaking and carbon neutrality, this paper offers practical ecological driving recommendations that take into account the driving characteristics of human beings, are easy to implement, and help to achieve carbon peaking and carbon neutrality.
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Cluster characteristics analysis and critical node identification in ecologically integrated transport networks in urban agglomerations
MA Shuhong, YANG Lei, CHEN Xifang, ZHU Min
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1770-1780. DOI: 10.16511/j.cnki.qhdxxb.2023.26.031
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[Objective] To achieve the integrated development of transport, economy, and ecological environment quality, it is necessary to identify the core clusters and key nodes properly in urban agglomerations. [Methods] In this paper, the Guanzhong Plain urban agglomeration is taken as the research object, and weights are assigned to the functional attractiveness of nodes, radiance, and carbon emission intensity through the entropy method. A modified gravity model based on the multidimensional characteristics of the integrated transport network nodes is constructed to calculate the strength of the spatial connections between districts and counties within the urban agglomeration. Furthermore, a model of spatially linked networks is constructed by combining the spatial structure theory with complex network theory. This model takes the skeleton of the comprehensive transportation network as the main body, the districts and counties as the nodes, and the indicators of transportation network level, functional attractiveness and radiance, and carbon emission correlation intensity as the connected edge weights having multidimensional and multilevel characteristics. Additionally, unnecessary parameter calculations are removed to improve the INFORMAP algorithm by combining the centralities of degree, betweenness, and closeness obtained from the complex network theory. This improved INFORMAP algorithm classifies regional clusters separately in terms of traffic, function, and carbon emissions. The result reflects the strength of the spatial linkages between the districts and counties of the urban agglomeration in different dimensions. Finally, based on the results of regional grouping and the hypergraph theory, we construct a hypergraph network model of ecologically integrated transport networks in urban agglomerations, and key indicators such as neighborhood hyper degree and neighborhood influence entropy are proposed to identify the key nodes of ecologically integrated transport networks in urban agglomerations. [Results] The high-speed railway, motorway, and mainline railway networks of the Guanzhong Plain urban agglomeration were divided into two clusters. The western cluster had a considerably lower frequency of intercity travel than the eastern cluster, and the Xi'an cluster had an increased transport network and frequency of intercity travel. The result of the high-level division of the transport network into clusters was mostly centered on prefecture-level cities. The spatial distribution pattern in terms of functions and carbon emission links had one pole and many cores, with Xi'an at the core. In terms of the relationship between cluster and administrative divisions, some districts and counties had broken through the constraints of higher administrative divisions to form independent groupings. Xi'an and Xianyang were key nodes in the construction of the ecologically integrated transport network of the city cluster. The importance of the nodes in the eastern cluster was found to be greater compared to the western part. [Conclusions] To achieve an integrated development pattern of transport, economy, and ecological environment quality in urban agglomerations, we optimize the layout of the existing transport network, increase the proportion of the low-grade transport network, supplement and connect the high-grade transport network through the articulation role of the low-grade transport network, and create an integrated multilevel transportation network pattern.
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Analysis of urban subway passengers' detention behavior based on big data
WANG Jing, LIU Kai, WANG Jiangbo, GONG Lei
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1781-1790. DOI: 10.16511/j.cnki.qhdxxb.2023.26.028
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[Objective] As an irreplaceable green travel mode, urban subway involves considerable daily travel. However, the passenger flow of subway systems has recently increased remarkably, resulting in overcrowding in space and time. Many trains operate at the highest frequency and maximum capacity during peak hours and still cannot meet passenger demand. Thus, detention is becoming increasingly severe, affecting the basic performance indicators of the subway systems while reducing the service quality and reliability perceived by subway passengers. However, research on the behavioral mechanism of detention is currently inadequate. Based on bus smart card data, supplemented by data mining technology, this study aims to quantify the difference between the rational travel time and the actual travel time of passengers. It reproduces the waiting process of each passenger as realistically as possible and predicts the occurrence and duration of the detention. The study provides a reference with application value for improving the future subway operation status evaluation index system and lays a research foundation for refined passenger flow distribution simulation, depending on the individual level of passengers. [Methods] The bus smart card data record the spatiotemporal information of each subway passenger's travel start and end station at a low cost. Furthermore, data mining technology is used to mine the travel information of passengers from the bus smart card data. This study uses the Shenzhen Tong data from september 2019. First, a suitable pretreatment method is adopted for the problems existing in the original data. Through an analysis of the fluctuation of each time component of the travel time, the travel time is redivided into the entry time, waiting time, in-car time, and outbound time. Second, the rational travel time value of the passenger is obtained by estimating the time of each part individually. Then the passenger's detention is identified based on the maximum rational travel time of the passenger. The influencing factors of detention are excavated from the three levels of individuals, stations, and lines. Frequent passengers are selected as research objects, and nonequilibrium panel travel data are used. Finally, a panel logistic regression model and panel data random effect regression model are established to achieve a prediction of detention and detention time. [Results] The study results, based on Shenzhen Metro Line 1, showed that the average detention rate of each station was 12.0%, the detention time was concentrated in 1.00-8.00 min, and the average detention time was 4.20 min. The travel time in the downward direction of the evening peak and the upward direction of the morning peak increased by 0.61 and 0.88 min, respectively, while the travel time in the downward direction decreased. The study also found that long-distance passengers had longer detention time than others, and they were willing to sacrifice their time in exchange for a comfortable ride. They had more choices at the starting station and were willing to wait for one or more trains to obtain seats. However, the passengers who took the subway less had cognitive bias, leading to the detention time increased. [Conclusions] Detention is a common problem during the morning and evening rush hours, which affects a passenger's travel time. This research clarifies the mechanism of detention behavior, provides a basis for generating operation management plans and issuing customized guidance strategies and is conducive to maximizing the operational efficiency of a subway transit system.
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A review of supercritical carbon dioxide centrifugal compressor flow characteristics
YANG Zimu, JIANG Hongsheng, ZHUGE Weilin, QIAN Yuping, ZHANG Yangjun
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1791-1807. DOI: 10.16511/j.cnki.qhdxxb.2023.26.023
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[Significance] The supercritical carbon dioxide (S-CO
2
) Brayton cycle is a power cycle at intermediate temperature and high pressure. This cycle is considered an important solution to improving the efficiency of traffic power systems such as gas turbines and internal combustion engines by recycling exhaust energy at high temperatures. The compressor is considered one of the most important components of this cycle. Its efficient and stable operation plays an important role in cycle performance. [Progress] In this paper, the research progress on S-CO
2
centrifugal compressor flow characteristics was reviewed from four aspects: experiment, one-dimensional flow analysis, three-dimensional flow characteristics, and flow control. Researchers learned from the experimental studies of the S-CO
2
centrifugal compressor that the special thermophysical properties of S-CO
2
, particularly their dramatic change near the critical point, brought great challenges to the design and stable operation of this compressor. Therefore, the problems of compressor flow caused by the drastic physical properties change near the critical point of the working medium, and the related research contents were emphatically expounded. The current research on one-dimensional flow analysis of the S-CO
2
centrifugal compressor is mainly conducted by the one-dimensional mean streamline method considering the special thermophysical properties of S-CO
2
fluid. The preliminary aerodynamic design of the S-CO
2
centrifugal compressor was conducted using one-dimensional flow analysis. This method is limited by its prediction accuracy under off-design conditions. In addition, the flow details inside the compressor could not be obtained by this method. To reveal the flow mechanism of the S-CO
2
centrifugal compressor, its three-dimensional flow characteristics must be deeply understood, and its internal flow field information must be obtained. The research on the three-dimensional flow characteristics of the S-CO
2
centrifugal compressor was mostly conducted by the computational fluid dynamics (CFD) numerical simulation method, which can be used to obtain the flow field of the centrifugal compressor and present the relevant flow phenomenon. Because of the drastic variations in the thermophysical properties of S-CO
2
fluid near the critical point, special consideration was taken in the process of the CFD simulation of the flow inside the centrifugal compressor. By applying CFD to S-CO
2
centrifugal compressor three-dimensional flow characteristics, researchers found that this special thermal physical property also brought special flow phenomena inside the flow domain of the S-CO
2
centrifugal compressor. The research on S-CO
2
centrifugal compressor three-dimensional flow characteristics mainly focused on its steady flow and needs to further reveal deeply and comprehensively the flow mechanism of the S-CO
2
centrifugal compressor under various unsteady working conditions. The flow control of the S-CO
2
centrifugal compressor was mainly by the passive flow control method using the relevant control method of the air compressor for reference, and its effect was remarkable. As for the active flow control method, few studies have heeded its effect on the S-CO
2
centrifugal compressor. [Conclusions and Prospects] In this paper, the flow characteristics of the S-CO
2
centrifugal compressor are summarized, and their research prospects are proposed. These flow characteristics are considerably different from those of centrifugal compressors with conventional fluids, mainly because of the special physical properties of S-CO
2
fluid. In the future, more advanced research methods are expected to be used, such as visual flow experiments, one-dimensional flow analysis incorporating machine learning algorithms, and active flow control, to conduct more in-depth and comprehensive studies of S-CO
2
compressor flow characteristics.
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Assembly of spacecraft components based on adaptive compliance control
CHEN Shuqin, LI Tiemin
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1808-1819. DOI: 10.16511/j.cnki.qhdxxb.2023.26.014
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[Objective] The assembly of spacecraft components plays an important role in their production, and the quality and efficiency of assembly have a direct impact on the quality and efficiency of their production. Currently, spacecraft components are often constructed by hand, which results in low accuracy and efficiency. The aerospace industry's research focus is on utilizing robots to complete the assembly tasks of spacecraft components, which can improve the quality and efficiency of their production. The current assembly robots mostly use the position control mode, which measures the relative pose between the assembly features of two spacecraft components and then moves the robot to complete the robotic assembly tasks according to the measurement results. In this control mode, assembly errors are unavoidable due to measurement and robot motion errors, which will result in a huge contact force between the two contact surfaces of the spacecraft components. Excessive contact forces can damage the surface quality and coatings of spacecraft components, ultimately affecting their service lives. Therefore, the contact forces are required to be controlled by compliance control. The control parameters in the current study of compliance control are established based on the operator's experience, which is closely related to the contact forces. Because the spacecraft components are manufactured in small batches, pre-assembly cannot be used to determine the control parameters without damaging their surface quality and coatings. And improper control parameters can lead to uncontrolled contact forces. [Methods] To address this issue, a compliance control method is proposed in this paper based on the classical admittance control, which can adaptively adjust the control parameters according to the contact forces and system status. In this adaptive compliance control, the target pose and stiffness matrix are changed during the assembly process. This research examines the control effects of adaptive compliance, position, and classical admittance controls to validate the practicality of this strategy. Taking the control moment gyroscope (CMG) assembly task as an example, this research designs and develops a CMG robotic assembly prototype. The
F/T
sensor is installed between the CMG and the robot's end-effector to measure the contact forces during the assembly process. And Kalman filtering is utilized in this paper to filter the measurement noise of the
F/T
sensor. [Results] The position and orientation of the CMG were modified according to the adaptive compliance control presented in this study. After adjusting the position and orientation, the CMG's contact surface and the mounted base's contact surface were fitted together, and the contact forces of the two surfaces were guaranteed to be small. [Conclusions] The outcomes of the simulation and experiment results show that adaptive compliance control has advantages, including fast convergence, minimal residual contact force, and adaptive adjustment of the control parameters. Additionally, the adaptive compliance control suggested in this study can be quickly applied to various spacecraft component assembly tasks. This method establishes the theoretical and technical foundation for autonomous robotic assembly of spacecraft components and is expected to be employed for real-world spacecraft component assembly tasks.
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Numerical simulation and analysis of multiphase flow through fiber array structure in extracorporeal membrane oxygenation
JIAN Meng, ZHANG Mingkui, HUANG Jianbing, LUO Xianwu
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1820-1832. DOI: 10.16511/j.cnki.qhdxxb.2022.25.024
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Objective] Extracorporeal membrane oxygenation (ECMO) is an effective life support treatment for severe cardiopulmonary failure and is widely used as supportive therapy for COVID-19. The design and assessment of ECMO oxygenators, which consist of thousands of 3D hollow fiber bundles, are essential to expanding their clinical applications. This study aims to investigate the effects of 3D fiber membrane array structure on the hemodynamic loss and gas transfer efficiency in ECMO. An axial slice of a commercial oxygenator is selected as a simplified model. [Methods] The immersed boundary (IB) method code was developed to simulate the two-dimensional steady laminar flow, and a segregated solver was implemented during the coupled multi-component gas transfer in ECMO. A grid independence test was carried out to ensure that the computational results were not influenced by the grid size. Results obtained from the IB method, commercial computational fluid dynamics (CFD) software Fluent with a body-fitted mesh, and the reference showed a good agreement, validating the accuracy of the IB method and the gas transfer solver. Seven array arrangement schemes with constant porosity were simulated at
Re
=5, and the results of permeance, wall shear stress, vortex distribution, and entropy generation rate were compared. [Results] Numerical results showed that when porosity was constant, different fiber array arrangements and angles between the odd and even row fibers could significantly change the flow state and gas transfer performance by affecting the relative value of axial and radial permeability. Inline arrangements and small angles between the odd and even row fiber arrangements deteriorated the uniformity of the flow state, consequently enlarging flow separation zones and causing peak wall shear stress. The array staggering from the axial direction and large angles between the odd and even rows could be used to avoid the large-scale vortex at the outlet of the ECMO and reduce the risk of blood damage. For all fiber array configurations, the head loss values predicted by entropy generation theory were smaller than the calculated results. As for gas transfer, in regions near the oxygenator inlet, outlet, and fluid retention zones, gas was mainly transferred through diffusion, controlled by the concentration gradient. In the middle stream regions, convection dominated the gas transfer. Further analysis showed that the gas transfer performance was mainly affected by the array arrangement. Compared with inline arrays, staggered arrangements increased the gas transfer rate on the upstream side and gaps of adjacent fibers. For the staggered arrangement where the flow passage had no evident periodic contraction and expansion, the flow retention area was small. Thus, the effective oxygenation area was the largest, and the gas was transported mainly by convection, which resulted in a high gas transfer rate. In addition, the gas transfer efficiency improved by increasing the angle between the odd and even rows. [Conclusions] The simulated pressure drop results of the simplified model are within the clinical operating range, but its gas transfer rate is lower than that of commercial oxygenators. Our results can serve as a reference for further 3D pore-scale numerical simulation and as a scientific basis for the structural optimization and clinical applications of ECMO.
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Optimization of thermogravimetric method for measuring very low saturation vapor pressure
LI Zhongwei, LI Xiaofei, Tang Zuozhou, XU Wenting, SONG Qiang
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1833-1843. DOI: 10.16511/j.cnki.qhdxxb.2023.26.030
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[Objective] Vapor pressure is a fundamental thermodynamic property, the measurement of which is particularly important. Coal-fired pollution control research needs basic data on the vapor pressure of heavy metals, but it is very low and is difficult to measure. A common method for measuring very low vapor pressure is thermogravimetric analysis, wherein vapor pressure is estimated using the evaporation rate. The key factors affecting the measurement accuracy are the conditions under which the linear relationship between the vapor pressure and the evaporation rate is established and the similarity of the calibration constants
k
of different substances. [Methods] Taking the TA Q500 thermogravimetric analyzer as an example, this paper establishes a mathematical model for isothermal evaporation in a thermogravimetric analyzer. The thermogravimetric analyzer’s flow field and evaporation process are analyzed via computational fluid dynamics (CFD) method. Numerical simulations are conducted for six organic substances and 160 model substances under various temperature and carrier gas flow conditions. The independence of the grids used in the numerical simulations is verified through examination of the
x
-direction velocities,
y
-direction velocities, and mass fraction distributions for different numbers of grids. The reliability of the calculated results is verified using the experimental results obtained for the vapor pressure of benzoic acid. [Results] A comparison of the mass distribution diagrams of organic substances revealed that the evaporative mass transfer in the thermogravimetric analyzer was due to the combined effect of molecular diffusion and convective transport. The evaporation process, which was typically analyzed using the Langmuir equation, was based on molecular diffusion, which meant that the Langmuir equation was not be applicable to describe the evaporation process inside the thermogravimetric analyzer. The experimental conditions (carrier gas flow rate and temperature) and substance properties (molar mass, vapor pressure, and diffusion coefficient) would affect the evaporation and mass transfer of the substance and further affected the calibration constant
k
. A numerical simulation of the isothermal evaporation process of 160 model substances revealed that the difference in the physical properties of these substances could result in significant differences in
k. k
increased with decreasing molar mass and diffusion coefficient and increasing vapor pressure. The dimensionless analysis of the governing equations showed that the evaporation process was determined by the dimensionless quantities
Re
(Reynolds number),
Pe
(Peclet number), and
w
i
(the dimensionless form of the sample vapor mass fraction on the crucible surface). Through the dimensionless analysis of the governing equations, the nonlinear relationship between evaporation rate and vapor pressure was obtained via fitting. When the molar mass and vapor pressure of the substance were small, the relationship between the vapor pressure and the evaporation rate was more linear. The deviations obtained from the different calibration-constant calculation methods were compared. The results confirmed that the calibration constant
k
was related to the vapor pressure. The results also proved that the key influencing parameters obtained through the dimensionless analysis of the governing equation were reliable. The influence of physical properties on pressure measurement deviation was analyzed, and the results revealed that the closer the molar mass and diffusion coefficient values between the substance to be measured and the calibration substance, the smaller the difference in
k
between the two substances. [Conclusions] Based on the analysis of the results, it is found that: The relationship between evaporation rate and vapor pressure is approximately linear only when the molar mass and vapor pressure of the substance are small. When choosing a calibration substance, in order to reduce the measurement deviation of vapor pressure, the substance with the diffusion coefficient and molar mass of the substance to be measured should be selected as close as possible.
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Mechanism of corrugation on the track with Cologne egg fasteners based on transient contact characteristics
WANG Zhiqiang, LEI Zhenyu
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1844-1855. DOI: 10.16511/j.cnki.qhdxxb.2022.25.025
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[Objective] Rail corrugation is a problem that needs to be addressed urgently and is one of the common technical issues limiting the development of contemporary rail transit. This study uses the finite element method to analyze the formation process of rail corrugation from the wheel-rail transient contact stick-slip vibration to provide new insights into the mechanism of rail corrugation and to understand the phenomenon of rail corrugation on the metro line. [Methods] This study examines the formation mechanism of rail corrugation using field measurements and numerical simulation. First, according to the on-site corrugation situation, a three-dimensional wheel-rail rolling contact model is developed using the finite element software ABAQUS, and its effectiveness is established. The contact stick-slip state is then analyzed during the wheel operation, and the influence of the rail surface and no rail surface defect on it is discussed. Furthermore, the relationship between stick-slip characteristics and corrugation formation is examined. Finally, the inherent characteristics of the wheel-track system and longitudinal wear characteristics of rail are analyzed using the complex modal theory and the Archard wear model to explain the formation mechanism of rail corrugation. [Results] The results revealed that when the wheel rolled over the smooth rail, the adhesion area was at the front edge of the contact area, and the middle and rear edges were the slip area, which was closer to the steady state dynamic calculation results, verifying that the established finite element model was effective. Moreover, the wheel-rail contact was always in a stable rolling state, indicating that the wheel-track system was not easily unstable, consequently making corrugation generation difficult. When the wheel rolled through the squat defect, the contact area was shown as two slip areas surrounding the squat; after the wheel rolled through the squat defect, the area of the wheel-rail contact patch decreased, and almost all of it showed slip. The squat defect changed the stick-slip state of wheel-rail rolling contact and promoted the slip of the wheel-rail interface, which induced the instability of the wheel-track system and caused the wear of the rail surface material; this might eventually form rail corrugation. The complex modal analysis showed that the rail surface defect exacerbated the inherent unstable vibration characteristics of the wheel-track system, and the unstable vibration frequencies fell within the measured corrugation passing frequency range. [Conclusions] The analysis results of wheel-rail contact stick-slip and complex modal reveal that the formation mechanism of rail corrugation can be attributed to the inherent unstable vibration of the wheel-track system caused by the excitation of the rail surface defect, and the unstable vibration is represented by the vertical bending vibration of the rail relative to the track slab. Thus, when the wheel passes through the squat defect, it will stimulate the transient fluctuation wear, which results in wavy wear on the rail surface. The characteristic wavelength of the longitudinal wear on the rail surface is 40~50 mm, which is consistent with the corrugation wavelength on the actual line; thus, the formation mechanism of rail corrugation is further validated in the process of quantifying rail corrugation formation.
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Kinematic characteristics and stability analysis of four-cable suspension system for full-model flutter wind tunnel test
JIANG Hailong, WANG Xiaoguang, WANG Jiajun, LIU Ting, LIN Qi
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1856-1867. DOI: 10.16511/j.cnki.qhdxxb.2023.26.035
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[Objective] Full-model flutter test is crucial for the aeroelastic design and verification of aircraft. One of the key challenges of the test is ensuring that the model's suspension design meets the natural frequency and motion adjustment range requirements. This study proposes a cable suspension system with four cables/three springs for the full-model flutter wind tunnel test under transonic conditions to address the current research gap in verifying suspension systems other than the existing two-cable or three-cable suspension mechanisms. The designed four-cable suspension method is expected to offer distinct advantages for transonic wind tunnel tests, such as suitability for the static unstable aircraft models and their intelligent controls. [Method] The stability and kinematic characteristics of the proposed four-cable suspension system are analyzed and validated through a series of methods. First, the stiffness expression of the mechanism is established based on the differential kinematics and used for deriving the stability criterion in light of the principle of virtual work by considering the system dynamic equations and the aerodynamic model of the aircraft. Subsequently, the eigenvalues of the stiffness and aerodynamic derivatives matrix are determined, and the pose variations of the aircraft model subjected to aerodynamic forces are numerically investigated to demonstrate the suitability of the suspension system for static unstable aircraft models. Additionally, the system impact response and the factors influencing its frequency are studied, proving that the four-cable suspension system meets the natural frequency requirements of the full-model flutter wind tunnel test. Numerical calculations and Adams software simulations are performed to verify that the four-cable suspension system can achieve effective adjustment of the aircraft model pose by controlling the cable length and manipulating the aileron and rudder surfaces. Finally, a simple prototype is built for modal frequency experiments to verify the feasibility of the proposed theoretical method. [Results] The simulation and numerical calculation results demonstrated that the proposed four-cable suspension method was a viable solution for the full-model flutter wind tunnel test under transonic conditions, providing five degrees of freedom to the model. The high-speed incoming flow dynamic response results revealed that the four-cable suspension system exhibited outstanding stability, with the largest magnitude observed in the centroid displacement along the sideslip direction of the aircraft model, which was less than 0.04 m, while the rotational angle amplitudes did not exceed 15.0°. The initial pre-tension force could be adjusted to ensure that the cable continuously remained in tension. Furthermore, the natural frequencies of the mechanism in the three rotation directions were approximately 0.8~1.0 Hz, and the natural frequencies in the sideslip and heave directions were within 3.0 Hz. The study also examined the influence of different traction positions and spring numbers on the natural frequency and revealed that the attitude angle adjustment range of the four-cable suspension system with three springs could meet the requirements of the test through cable length adjustment and rudder surface control. The simple prototype frequency experiment demonstrated that the roll, pitch, and yaw direction modal frequencies were less than 3.0 Hz. [Conclusion] This study demonstrates the feasibility of using the proposed four-cable suspension system for transonic full-model flutter wind tunnel testing through numerical calculations, software simulations, and prototype experiments, providing a approach for the model suspension technology in transonic full-model flutter test.
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Stress uniformity of spacecraft multilayer thermal insulation components
LIU Yue, SUO Shuangfu, GUO Fei, HUANG Min, SUN Weiwei, TAN Botao, HUANG Shouqing, LI Fangyong
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1868-1877. DOI: 10.16511/j.cnki.qhdxxb.2023.26.027
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[Objective] Multilayer thermal insulation components are key parts for thermal insulation and depressurization on the outer surface of a spacecraft, with the structure comprising primarily of multilayer thin films and nylon nets. However, during the process of spacecraft launch, the internal gas in the multilayer thermal insulation components rapidly flows out because of the rapid decrease in external pressure. Furthermore, the internal flow field changes considerably, resulting in the deformation of the multilayer thermal insulation components. Under certain pressure differential conditions, the stress on each thin film is nonuniform, and structural failure may occur on a certain thin film layer due to extreme stress. Thus, the stress uniformity of the multilayer thermal insulation components is a key indicator of structural failure. This paper proposes an evaluation indicator to assess the thin film stress uniformity of the multilayer thermal insulation components. [Methods] Three-dimensional slice models of the multilayer thermal insulation components are established, and the computational fluid dynamics method is adopted to analyze the internal flow field distribution and stress on each thin film during the depressurization process. Through the systematic analysis of the fluid pressure differential on each thin film, the thin film pressure differential coefficient is proposed as an evaluation indicator for stress uniformity. Furthermore, four typical structural parameters, namely the number of layers, thickness of the film, diameter of the hole, and distance of the hole, are selected within the extreme design range of various structural parameters of the multilayer thermal insulation components, and the orthogonal experimental design method is employed to analyze these structural parameters and determine the influence law of the structural parameters on the thin film pressure differential coefficient. Finally, a mathematical analytical model for calculating the thin film pressure differential coefficient is proposed based on the influence law. [Results] The orthogonal experimental results revealed that the four typical structural parameters had different degrees of influence on the thin film pressure differential coefficient. The thickness of the film had the highest degree of influence, whereas the diameter of the hole had the lowest degree of influence. The results of the mathematical analysis and computational fluid dynamics methods were compared, and the results revealed that: (1) For a single-hole thin film structure, the maximum error in results between the mathematical analytical model and the computational fluid dynamics model was 4.5%. (2) For a double-hole thin film structure, the maximum error in results between the mathematical analytical model and the computational fluid dynamics model was 5.3%. (3) The mathematical analytical method was accurate and fast. [Conclusions] This paper reveals the internal flow field distribution and the film stress of multilayer thermal insulation components using a three-dimensional slice model and proposes the thin film pressure differential coefficient as a key indicator of stress uniformity. Furthermore, this paper proposes a feasible and effective mathematical analytical model to rapidly evaluate the thin film stress uniformity by exploring the influence law of the four typical structural parameters on the thin film pressure differential coefficient through the orthogonal experimental test. The proposed mathematical analytical method can be used to rapidly calculate the pressure differential coefficient, which provides a basis for judging the stress uniformity of the multilayer thermal insulation components during spacecraft launch, thereby preventing the failure of the multilayer thermal insulation components.
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Thermal design and optimization of south-oriented combined external window for a typical office building in frigid plateau region
LIU Yiming, XU Peiqi, LIU Nianxiong
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1878-1886. DOI: 10.16511/j.cnki.qhdxxb.2023.22.008
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[Objective] Existing design standards for energy efficiency impose rigorous static restrictions on the shape coefficient and envelope performance of buildings. However, these requirements are incompatible with dynamic adjustment to real-time changes in the weather. Thus, the loss of energy-saving effect and indoor use quality of the building proffers the potential for improvement. Therefore, this study introduces the south-oriented combined external window design for buildings based on the principle of leap heat transfer. This design improves the solar heat gain during the daytime and the insulation performance at night, which is a feasible strategy to dynamically improve the indoor thermal environment and reduce the building load. However, the effect of this design strategy on the Tibetan plateau region still needs further investigation. To improve the energy efficiency and indoor usage quality of office buildings, this study takes a typical office building in Xigaze as the object to explore the more suitable design strategies for south-oriented external windows. [Methods] This study compares nine building cases with distinct south-oriented external window designs. By quantifying the differences in building performance induced by each window design, the most efficient south-oriented external window design in the Tibetan plateau region is identified. The triple-glazing low-E windows in the baseline case meet the thermal performance requirements. In the rest of the design cases, the external window components consist of a 6 mm glass curtain wall with heat-break bridge aluminum alloy, a normal insulating window or low-E insulating window composed of two pieces of 6 mm glass and 12 mm air interlayer, and an insulating cotton curtain. DesignBuilder 6.1 is used in this study to analyze the cool and heat loads, hourly operating temperature, and thermal comfort of the primary rooms in a dynamic simulation throughout the year. Consequently, the comparison of the performance scores of nine building cases serves as the foundation for case validation. [Results] The results indicate the followings: (1) In the absence of heating and air conditioning, the indoor operating temperature fluctuates less in summer and more in winter. (2) In the absence of heating and air conditioning, the average time percentage of indoor temperature (from 18 ℃ to 26 ℃) varies significantly depending on various types of windows. The dynamically adjustable external window allows for a longer period of comfort in the office and a relatively long period of comfort in the dormitory. (3) The combination of a 6 mm glass curtain wall with heat-break aluminum alloy, normal insulated windows consisting of two pieces of 6 mm glass and 12 mm air interlayer, and thermal insulation curtains presents the lowest heat load and total load in the design case. (4) Compared with the baseline case, the proposed external window design enables a higher level of annual thermal satisfaction and indoor thermal sensation. [Conclusions] For office buildings in the frigid plateau region, glass curtain walls and heat-collecting walls should be used to fully capture solar radiation during the daytime. Meanwhile, thermal insulation should be employed to reduce heat dissipation at night. The proposed external window design presents the most significant effect on reducing building load and improving thermal comfort. Moreover, compared with windows employing low-E glass, this design reduces the construction cost and can be considered a more efficient choice for the south-oriented external window design of office buildings in the frigid plateau region.
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Calculation method for stormwater network design flow based on kinematic wave simulation
WU Shan, ZHAO Yujie, WANG Hao, WANG Qiang, LIU Zilong
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1887-1896. DOI: 10.16511/j.cnki.qhdxxb.2023.26.017
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[Objective] The pipe section collection time is typically based on the theory of steady full pipe uniform flow when using the reasoning formula method to calculate the design flow of a storm pipe network, but the actual water flow in the storm pipe is non-steady, causing errors in the calculation of the design flow that, when applied to a larger scale pipe network, gradually reduce the calculation accuracy. In this context, the paper suggests a design flow computation method for storm pipe networks based on kinematic wave simulation. [Methods] In this paper, the design flow of pipe sections is solved using kinematic waves under the condition of ensuring the equivalent setup of model parameters and storm pipe network starting design parameters. This paper combines the Horton infiltration model and the
φ
index method to calculate infiltration intensity in the surface rainwater runoff stage. The runoff generation is calculated with the objective of achieving equivalence of the volumetric runoff coefficient and discharge runoff coefficient. Taking surface catchment time and linear confluence curve type as input, and coupling with the isochrones model, the equivalence setting of design conditions and stormwater outlet inflow process line calculation are completed. In the pipe section confluence process, the pipe section flow process line is calculated by inputting the corresponding stormwater inlet inflow process line into the node inflow mode and computing the pipe section confluence process using the stormwater management kinematic wave model. The stormwater inlet inflow process line of the designed pipe section and the upstream pipe section flow process line connected with it are superimposed to complete the calculation of the pipe section design flow process line. Combined with the hydraulic design of the stormwater pipe section, the whole storm pipe network design is realized based on the geospatial data abstraction library development technology process. [Results] The results of a storm pipe network example in a particular area (with a total size of 4.506 km
2
) showed that: (1) When compared to the reasoning formula method, the stormwater pipe section created using the kinematic wave simulation approach had a quick catchment time and a greater design flow rate. (2) The flow calculation difference between the two approaches increased over time as catchment time and catchment area increased, reaching a maximum increase of 39.45%. (3) Under the 10-year rainfall scenario, the design storm pipe network obtained by the two calculation methods of equivalent design conditions reduced the number of overflow nodes, total overflow volume, and length of pipe section overload by 8.57%, 28.57%, and 38.48%, respectively, compared to the reasoning formula method. [Conclusions] By comparing the differences in the design results obtained by the two calculation methods for different catchment times and catchment areas, it can be seen that for large projects, it is advisable to use the kinematic wave simulation method to calculate the design flow of the storm pipe network. In a simulated analysis with a 10-year exceedance of rainfall, the storm pipe network designed by the kinematic wave simulation method has better flood prevention performance.
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Analysis of depth and width correction factors for bearing capacity of foundation soil
JIE Yuxin, FU Zhibin, WANG Yangqiang, YIN Changyun
Journal of Tsinghua University(Science and Technology). 2023,
63
(11): 1897-1908. DOI: 10.16511/j.cnki.qhdxxb.2023.25.033
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[Objective] The determination of bearing capacity of foundation soil is one of the classical research fields of soil mechanics. For large area artificial fill projects, the current specifications of China do not consider the correction factor of foundation width in the calculation of bearing capacity. This paper discusses this problem, and investigates the definition and measurement methods of the foundation bearing capacity, related theoretical principles, and the method of determining allowable bearing capacity. We also discuss and speculate the possible basis for the selection of correction factors of the bearing capacity of foundation soil made of artificial fill and soft underlying stratum. It may provides theoretical guidance for width and depth correction of bearing capacity of foundation soil in large area artificial fill under current technical conditions. [Methods] In this paper, two main approaches for calculating bearing capacity of foundation soil are studied. One is to determine the bearing capacity of the foundation soil according to the extension range of the plastic zone; the other is to use the ultimate load as the ultimate bearing capacity, and then divide it by the safety factor to obtain the bearing capacity. The width and depth correction of bearing capacity is also based on these theories. We reviews the sources of the calculation method of bearing capacity together with the width and depth correction factors in the current specifications of China. On the basis of deriving the calculation formula and analyzing the principle of bearing capacity of foundation soil, two cases are investigated in this paper: one is a homogeneous foundation soil, and the other is that the shear strength of the soil outside the foundation boundaries is lower than the foundation soil. The possible essence for the correction factors of the bearing capacity is then discussed in order to guide the determination of the width and depth correction factors for large area artificial fill. [Results] For large area artificial fill projects, the correction factor of width is not considered in current specifications of China. The main reasons may be as follows: 1) The compaction quality of the foundation soil is not be able to guarantee easily. 2) The quality control standard of the soil under the foundation is stricter than that outside the foundation boundaries. 3) Post-construction settlement may occur for artificial fill. Since that the existing construction technology and quality control level has made great progress compared to the past, it is theoretically feasible to increase the correction factors of width and depth for large area artificial fill such as in island and reef. However, the following items are needed: 1) The degree of compaction should meet the requirement in the fill site. 2) The engineering quality of the fill outside the foundation boundaries should also be guaranteed not to be lower than the foundation soil. 3) Settlement calculation of the buildings are necessary. [Conclusions] Based on the theories of determining the bearing capacity of the foundation soil, this paper investigates the method of selecting the values of correction factors of width and depth. It is thought that the correction factors of width and depth can be appropriately increased under certain conditions for large area artificial fill. This will have good economic benefits, especially for offshore islands and reefs.
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