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201.
E-R模型特殊超类/子类结构转换方法研究 总被引:1,自引:0,他引:1
分析了该系统的概念结构中出现的特殊超类/子类结构,即:超类实体对不同的子类实体具有不同的主码,其主属性仅仅是所有子类实体共同的主属性,各子类实体拥有各自特定的主属性. 相似文献
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《国际交通安全学会研究报告》2022,46(3):329-344
Road traffic accidents (RTA) are a prevalent cause of fatality with African countries having the highest fatality index (25–34 per quota). The World Health Organization estimates Kenya's fatality rate due to RTA at 28 per quota. From literature, the country's fatality and injuries have increased by 26% and 46.5%, respectively, since the year 2015. The country is faced with incomplete RTA data capturing, hindering effective planning and policy adjustments to curb the menace. In this paper, we scrapped user-generated data (Twitter) and national transport and safety authority's (NTSA) reports to shed light on traffic safety, practices, and cultures in the country. To this end, we gathered 1,000,000 tweets and 8000 speeding entries between 2015 and 2021 and performed natural language processing (NLP) and quantitative study of the data. We applied NLP and n-gram search of keywords to categorize data into 8 topics: traffic, public service vehicle (PSVs), policing, accident, infrastructure, recklessness, robbery, and corruption. From the data, policing, which touches on all police and law-enforcement-related activity was found to be highly correlated with PSVs, recklessness, accidents, traffic congestion, robbery, infrastructure, and corruption with indices of r(76) = 0.92, 0. 91, 0.87, 0.82, 0.81, 0.76, and 0.70, respectively with p < 0.001. The topic modeling confirmed the identified topics to be the latent discussion issues affecting the public. From the study, PSVs, policing and traffic flow were isolated as key issues that ought to be addressed immediately. The research recommended the integration of driver monitoring systems to strengthen policing. The research, which utilized unstructured data, points to the utility of data mining which would greatly benefit traffic research, particularly African-based studies, that suffer from data inadequacy. 相似文献
204.
艇体变形是影响轴系校中质量的重要因素,以深水潜器为研究对象,通过建立潜器的三维有限元模型,提出利用弹簧约束调节潜器重力与浮力平衡的方法,分析潜器处于正浮状态时,在重力、浮力和静水压力作用下的艇体变形,得出潜器轴系各个轴承的位移数据,并分析轴承位移造成的轴承负荷变化。为艇体变形影响下的潜器轴系校中提供依据。分析结果表明:耐压艇体内的轴承位移要小于耐压艇体外,支撑轴承的艇体结构的差异会导致轴承位移大小的不同,从而导致各个轴承负荷变化也不一样,艇内液舱的不对称布置会导致位于该液舱上轴承产生较大的横向位移和横向轴承负荷。 相似文献
205.
改进最短路径算法在最佳航线选择中的应用 总被引:7,自引:1,他引:7
最佳航线选择不仅是船舶驾驶员经常关注的问题之一,而且还是船舶经营公司时常面临的重要问题之一。现利用改进的最短路径算法对该问题作出定量描述。较详细地阐述了Dijkstra算法和改进算法,并通过最佳航线选择示例说明了算法的应用。通过比较,改进算法在搜索效率上优于Dijkstra算法。 相似文献
206.
依据材料疲劳损伤累积理论对汽车驱动桥半轴的疲劳寿命进行预估,结合随机载荷谱处理技术给出了室内试验强化系数的理论模型,研制了汽车驱动桥室内强化试验系统,并对某公司5 t ZL50DWB装载机车桥等多个产品进行了试验。分析表明理论模型和试验数据吻合较好。 相似文献
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This paper develops a multi-level decision making approach for the optimal planning of maintenance operations of railway infrastructures, which are composed of multiple components divided into basic units for maintenance. Scenario-based chance-constrained Model Predictive Control (MPC) is used at the high level to determine an optimal long-term component-wise intervention plan for a railway infrastructure, and the Time Instant Optimization (TIO) approach is applied to transform the MPC optimization problem with both continuous and integer decision variables into a nonlinear continuous optimization problem. The middle-level problem determines the allocation of time slots for the maintenance interventions suggested at the high level to optimize the trade-off between traffic disruption and the setup cost of maintenance slots. Based on the high-level intervention plan, the low-level problem determines the optimal clustering of the basic units to be treated by a maintenance agent, subject to the time limit imposed by the maintenance slots. The proposed approach is applied to the optimal treatment of squats, with real data from the Eindhoven-Weert line in the Dutch railway network. 相似文献
210.
Experimental studies showed that infiltration and passive ventilation are important air exchange mechanisms inside vehicles but previous mathematical models did not consider either one. In this study, we incorporated infiltration and passive ventilation to advance the existing mathematical models and evaluated how different transport mechanisms affect passenger exposures at increasing speeds. Infiltration was formulated using Bernoulli’s equation and passive ventilation was derived empirically. The new model describes ultrafine particle (UFP) and carbon dioxide (CO2) transport for a wide range of driving speed under any ventilation conditions. Unlike statistical models, this mathematical model can also provide vehicle-specific and transport mechanism-specific information. The model predictions were in a good agreement with data collected from 10 different vehicle models with an average discrepancy of less than 16% for UFPs and less than 3% for CO2. Under outdoor air (OA) mode, when the fan is off, the model simulation showed that the infiltration and passive ventilation can substantially increase the UFP I/O (in-cabin/on-road concentrations) ratio from 0.15 at 0 km/h to 0.57 at 130 km/h. At medium fan setting, mechanical ventilation dominates and UFP I/O stays at 0.58 regardless of driving speed. Under recirculation (RC) mode, infiltration increases and the RC-mode filtration only removed 44% and 69% of the infiltrated particles at the lowest and medium fan settings, respectively. Model simulations under OA mode show that infiltration starts to occur above 115 km/h with the lowest fan setting; whereas, medium and higher fan settings prevent infiltration up to 145 km/h. 相似文献