查看更多>>摘要：Bike-sharing systems (BSSs) have rapidly attracted worldwide interest for their success in improving quality-of-life in metropolitan areas. One kind of BSS requires docking stations to avoid misplacements of bikes. Such a BSS is faced with challenges caused by imbalanced demands across stations and frequent failures of bikes and docks. To achieve a high level of customer satisfaction, timely bike rebalancing and system maintenance must be performed. In this work, a simulation framework is proposed for evaluating different rebalancing and maintenance strategies. The framework can be integrated with any multi-vehicle static or dynamic rebalancing optimization model. An optimization model solved by an enhanced k-means clustering method (EKM) and an Ant Colony Optimization (ACO) algorithm is provided as an example for demonstrating such integration. A case study based on the configuration and historical data of Citi Bike in New York City is conducted for simulation model validation and for illustrating the managerial impacts of different rebalancing and maintenance strategies on the investment, operation, and service levels of such a large-scale BSS. The application of the proposed simulation framework is not limited to BSSs but also can be extended to other types of shared transport systems with non-floating stations where rebalancing and maintenance optimization are critical for efficient and healthy operation.
查看更多>>摘要：The cutter ring is the core part of the disc cutter. The parametric model of the variable cross-section(VCS) cutter ring is designed according to the cutter ring's failure mode and the rock's mechanical properties. The finite element method is used to carry out modal frequency, penetration, and rolling simulations of constant cross section(CCS) and VCS cutter rings. Finally, the experimental results of the two are compared and analysed. The results show that the natural frequencies of CCS and VCS cutter rings of the same type have little difference, the mode shapes and amplitudes are almost the same, and the replacement of CCS and VCS cutter rings will not cause the change of vibration field. Unlike the CCS cutter ring, the VCS cutter ring applies a periodic force on the rocks in the rolling process. The energy output from the VCS cutter ring can adapt to the characteristics of rock materials, promotes the development of rock cracks, and accelerates the rate of rock breakage. The VCS cutter ring is subjected to the reaction force of rock mass, which reduces the local wear failure caused due to low friction.
查看更多>>摘要：The most important performance merits of inductive wireless power transfer links (IWPTL) operating at load-independent-voltage-output (LIVO) frequency are efficiency and output characteristics. It was recently demonstrated that none-series (NS) compensated IWPTL operating at LIVO frequency outperforms its series-series (SS) and series-none (SN) compensated counterparts in terms of efficiency. In order to complete the comparison, this paper presents output characteristics modeling and analysis (namely, relation between output voltage and load power) of a practical NS-compensated IWPTL operated at LIVO frequency. It is demonstrated that even though the system is operated at LIVO regime, its output voltage remains load-dependent due to practical issues. The lower bound of the output voltage range corresponds to operation under rated loading and is derived using phasor domain equivalent circuit, assuming continuous conduction mode (CCM) of the receiving-side diode rectifier without neglecting equivalent series resistances. On the other hand, upper bound of the output voltage range corresponds to operation under zero loading (inevitably imposing discontinuous conduction mode (DCM) of the receiving-side diode rectifier) and is obtained using time-domain solution of corresponding differential equations system due to the fact that classical phasor analysis fails to accurately describe DCM operating NS-compensated IWPTL. The proposed method is applied to a 1 kW, 400 V NS-compensated IWPTL operated at LIVO frequency and is well-verified by simulations and experiments. Comparison with output voltage characteristics of SS and SN-compensated IWPTLs assert the advantage of NS-compensated topology.
查看更多>>摘要：In the process in which high-density passengers need to be evacuated from a multi-exit subway station, not all the exits can be effectively utilized, and the bottlenecks near the exits are often crowded. Therefore, the utilization of different exits and the optimization of congestion at bottlenecks were investigated in this study. To alleviate the congestion of the evacuation in a large-scale and multifunctional subway station, the first underground floor of the station was divided into four regions, referring to the idea of fire compartment division, in which passengers were evacuated from local exits. The results reveal that the evacuation time increases as the initial passenger density increases. The railings have a positive effect on balancing the proportion of different exits for the region with fewer exits in the early evacuation stage. For regions with several exits well-distributed, the symmetrical structure is beneficial to the evacuation and the effective use of different exits. What's more, setting railings can reduce crowding at bottlenecks to a certain extent. The results of the study can guide passenger evacuation in typical multi-exit subway stations.
查看更多>>摘要：Shot peening (SP) parameters are optimized and their effects on fatigue life are investigated using a response surface methodology (RSM) for AA7B50-T7751. Three parameters (SP velocity, nozzle distance (hereafter called distance), and coverage) are optimized using the finite element method (FEM) to obtain the appropriate residual stress, average surface roughness (SR), grain-refinement, and micro-strain. The grain-refinement and induced micro-strain are calculated based on the full width at half maximum (FWHM). Moreover, the FEM results are consistent with the experiments. Optimized parameters are obtained by employing the Box-Behnken design (BBD) response surface methodology (RSM) based on the FEM results. Finally, fatigue experiments show improved fatigue lives for the treated specimens using the optimal scheme. It noted that the optimized parameters of SP velocity, distance, and coverage were 100 m/s, 14.2 cm, and 300%, respectively. Further, the fatigue lives of the AA7B50 samples peened using optimized parameters improve to 314.1%, 173.1%, and 23.9%, respectively, compared to three control groups.
Amel KhelifaRiad MokademTarek HamrouniFaouzi Ben Charrada...
21页
查看更多>>摘要：Federated cloud is a promising solution for cloud providers that enables collaboration and leasing resources among multiple cloud providers. However, resource management in such system is very challenging, as each cloud provider must maintain its own economic profit while meeting the requirement of service level agreement (SLA). In this respect, we propose a dynamic and periodic data replication strategy in federated cloud systems. It aims to guarantee the monetary profit of a cloud provider while satisfying its users' requirements in terms of response time and minimum availability. To identify replicas, we perform a periodical analysis of the users' tasks using the spectral clustering technique to extract the existing correlations between remote data related to SLA violations. The adaptation of such correlations can significantly reduce data transfer amount and the time required to transfer it, thereby reducing tasks' response time and the number of future SLA violations. Then, we rely on a fuzzy inference system to place the groups of replicas considering four main parameters to choose replicas placements among his owned or leased resources from other providers. Furthermore, a replicas number adjustment is performed when SLA is satisfied over time. To demonstrate the efficiency of our strategy, performance of the proposed strategy are compared alongside existing single-clouds-based and interconnected-clouds-based data replication strategies. The obtained results indicate that our strategy decreases the amount of SLA violations while preserving the monetary profit of providers.
查看更多>>摘要：To enhance the performance of heat exchangers (HEs), it is important to make the flow uniform and is to reduce the pressure. To achieve them simultaneously, baffle with small holes is often installed in the header of HEs, but the configuration is completely determined by the engineer's experience. In this paper, the baffle configuration under 2D fluid flow is optimized so as to minimize the flow nonuniformity and the pressure drop. Therefore, multi-objective design optimization is performed. The computational fluid dynamics (CFD) is so intensive that sequential approximate optimization that response surface is repeatedly constructed and optimized is adopted to identify the pareto-frontier. It is found from the numerical result that the optimal baffle configuration can drastically improve the flow nonuniformity and the pressure drop, compared to the conventional one. Compared to the conventional configuration, 34% improvement of the flow nonuniformity and the 23% improvement of the pressure drop are achieved by the design optimization. In addition, the size of large vortex can drastically be reduced. In particular, it is clarified that the inclined baffle plays an important role for improving the flow nonuniformity.
查看更多>>摘要：A session-based software system provides various services to its end users through graphical user interfaces. A novice user of a service's user interface takes more think time - the average time to comprehend the content and its layout on the interface-in comparison to expert users. The think time gradually decreases as she repeatedly comprehends the same interface over time. This decrease in think time is the user learning phenomenon. Owing to this learning behavior, the proportion of users - at various learning levels for different services-changes dynamically leading to a difference in the workload. Traditionally though, workload specifications (required for system performance evaluation) never accounted for user learning behavior. They generally assumed a global mean think time, instead. In this work, we first report an experimental study that demonstrates the impact of user learning of the graphical user interfaces of session-based systems. We measure the performance data of a real system while it is getting used by synthetic users (who are) at various learning levels. Statistical tests on our measured data reveal that different human learning curves, combined with different amounts of users arriving the system with various learning levels for different services, produce different system performance. Next, we explain a queueing network (QN) performance model called CogQN that accounts for user learning. It is a multi-class QN model where each service and its learning level constitute a class of users for the service. We present the concept of learning-level dependent, class-switching probabilities to model the probabilistic transition of learning levels of users for different services. This novel concept enables us to avoid the exponential explosion of classes that would otherwise be suffered by traditional class-switching theory, had it been used in this work. We solve the model using discrete-event simulation and validate it against the empirical data. Our CogQN model captures the effect of user learning on overall mean response times across different learning conditions within 10% absolute error in comparison to empirical data.
查看更多>>摘要：Today's energy market is increasingly integrating time-varying tariffs, peak demand charges, and/or export tariffs. In this context, intelligent charging scheduling can considerably reduce the plug-in electric vehicle (PEV) charging cost. This is especially the case as more and more PEVs are charged in buildings that are also equipped with grid-connected intermittent energy resources (IERs) (e.g., photovoltaic systems and wind turbine generators). In this work, we propose a novel and complete intelligent PEV charging scheduling system (tailored for domestic settings) that can account for peak demand charges, time-varying tariffs, and/or export tariffs, appropriately considering both potential IER generation and the rest of a building's consumption. The backbone of our approach builds on adaptive model predictive control, and includes an efficient depth-first-search-based PEV charging planning algorithm that we propose. Importantly, our approach does not rely on a simplified linear modeling of the charging dynamics, which is a typical and limiting assumption of such systems. We evaluate our approach with real data, considering both solar and wind IER generation capacity, to show that it can reduce the cost of charging by up to ～45% and ～35% in the United States and the United Kingdom domestic settings, respectively, compared to standard PEV charging practices.
查看更多>>摘要：Container-as-a-service (CaaS) is a business model that facilitates software developers in organizing, running, managing, and deploying applications using container-based virtualization. The CaaS market is a fast-growing one, leading infrastructure-as-a-service (IaaS) providers into offering CaaS solutions built on top of their IaaS platforms. Given the increasing competition among providers, pricing strategies play a key role in determining the business prospects of CaaS. In this work we propose a methodology to analyze the pricing strategies of the providers, based on a hedonic regression. Following that, we also develop a hedonic price index, based on data collected from popular providers, by identifying a wide set of architectural requirements influencing the pricing strategy. We further analyze the correlations between these requirements and define the subset to be included in the regression analysis, interpreting the results in view of the underlying CaaS market characteristics. The model implementation and the data set are freely available on the web under an open-source license.
NSTL
Elsevier