基于电子商务的集群式供应链跨链库存控制

以电子商务环境下集群式供应链库存控制为对象,建立了时变需求下跨链合作的交叉库存模型,并用H∞控制中抑制外部干扰的对策理论来寻求最优订货策略,以减小库存、订货波动和提高供需网快速响应市场的能力.文中采用对比分析方法分别针对供应链间有合作与无合作情形对模型做了仿真实验,仿真表明,实施H∞控制和交叉紧急库存补充策略可以有效避免货物积压或缺货现象,提高供应链及集群的竞争力.     

Approximate evaluation of multi-location inventory models with lateral transshipments and hold back levels

We consider a continuous-time, single-echelon, multi-location inventory model with Poisson demand processes. In case of a stock-out at a local warehouse, a demand can be fulfilled via a lateral transshipment (LT). Each warehouse is assigned a pre-determined sequence of other warehouses where it will request for an LT. However, a warehouse can hold its last part(s) back from such a request. This is called a hold back pooling policy, where each warehouse has hold back levels determining whether a request for an LT by another warehouse is satisfied. We are interested in the fractions of demand satisfied from stock (fill rate), via an LT, and via an emergency procedure from an external source. From these, the average costs of a policy can be determined. We present a new approximation algorithm for the evaluation of a given policy, approximating the above mentioned fractions. Whereas algorithms currently known in the literature approximate the stream of LT requests from a warehouse by a Poisson process, we use an interrupted Poisson process. This is a process that is turned alternatingly On and Off for exponentially distributed durations. This leads to the On/Off overflow algorithm. In a numerical study we show that this algorithm is significantly more accurate than the algorithm based on Poisson processes, although it requires a longer computation time. Furthermore, we show the benefits of hold back levels, and we illustrate how our algorithm can be used in a heuristic search for the setting of the hold back levels.     

Optimal Solutions in the Multi-location Inventory System with Transshipments

We consider a single-period multi-location inventory system where inventory choices at each location are centrally coordinated. Transshipments are allowed as recourse actions in order to reduce the cost of shortage or surplus inventory after demands are realized. This problem has not been solved to optimality before for more than two locations with general cost parameters. In this paper we present a simple and intuitive model that enables us to characterize optimal inventory and transshipment policies for three and four locations as well. The insight gained from these analytical results leads us to examine the optimality conditions of a greedy transshipment policy. We show that this policy will be optimal for two and three locations. For the n location model we characterize the necessary and sufficient conditions on the cost structure for which the greedy transshipment policy will be optimal.      

On Solving Quickest Time Problems in Time-Dependent, Dynamic Networks

In this paper, a pseudopolynomial time algorithm is presented for solving the integral time-dependent quickest flow problem (TDQFP) and its multiple source and sink counterparts: the time-dependent evacuation and quickest transshipment problems. A more widely known, though less general version, is the quickest flow problem (QFP). The QFP has historically been defined on a dynamic network, where time is divided into discrete units, flow moves through the network over time, travel times determine how long each unit of flow spends traversing an arc, and capacities restrict the rate of flow on an arc. The goal of the QFP is to determine the paths along which to send a given supply from a single source to a single sink such that the last unit of flow arrives at the sink in the minimum time. The main contribution of this paper is the time-dependent quickest flow (TDQFP) algorithm which solves the TDQFP, i.e. it solves the integral QFP, as defined above, on a time-dependent dynamic network, where the arc travel times, arc and node capacities, and supply at the source vary with time. Furthermore, this algorithm solves the time-dependent minimum time dynamic flow problem, whose objective is to determine the paths that lead to the minimum total time spent completing all shipments from source to sink. An optimal solution to the latter problem is guaranteed to be optimal for the TDQFP. By adding a small number of nodes and arcs to the existing network, we show how the algorithm can be used to solve both the time-dependent evacuation and the time-dependent quickest transshipment problems. This revised version was published online in August 2006 with corrections to the Cover Date.     

Real-time control of freight forwarder transportation networks by integrating multimodal transport chains

Western European freight forwarders are continually being forced to increase the efficiency of their transportation processes because of the liberalization and deregulation of the European transport market. This paper proposes a new real-time-oriented control approach in order to expand load consolidation, reduce empty vehicle trips, and handle dynamic disturbances. This approach integrates multimodal transportation and multiple transshipments for the first time. Thus, it enables the flexible generation and adaptation of transportation processes. In order to be able to handle occurring disturbances, an optimization procedure that adapts the transportation processes is continually applied. Vehicle breakdowns or deceleration of vehicles, traffic congestion, and street blockages are integrated as possible disturbance scenarios. At the same time, dynamically incoming transportation requests are also dealt with. Moreover, cooperative agreements between freight forwarders, which are gaining increasing importance, are integrated by mapping hubs and external services. The efficiency of the new real-time approach is validated by several computational experiments. In particular, the use of the entire execution time for plan adaptation as well as the integration of multiple transshipments has shown promising results.     

Infinite horizon optimal policy for an inventory system with two types of product sharing common hardware platforms

We consider a periodic review inventory system and present its optimal policy in the infinite horizon setting. The optimal inventory policy that maximizes the infinite horizon expected discounted profit for the model is analytically obtained by relating to the finite horizon setting using results from variational analysis. Results are provided that elucidate the operations of the inventory system in the long run.     

Optimizing yard assignment in an automotive transshipment terminal

This paper studies a yard management problem in an automotive transshipment terminal. Groups of cars arrive to and depart from the terminal in a given planning period. These groups must be assigned to parking rows under some constraints resulting from managerial rules. The main objective is the minimization of the total handling time. Model extensions to handle application specific issues such as a rolling horizon and a manpower leveling objective are also discussed. The main features of the problem are modeled as an integer linear program. However, solving this formulation by a state-of-the-art solver is impractical. In view of this, we develop a metaheuristic algorithm based on the adaptive large neighborhood search framework. Computational results on real-life data show the efficacy of the proposed metaheuristic algorithm.     

DMF运载5-氟尿嘧啶在大鼠体内分布浓度的定量检测

用溴尿嘧啶作内标,制备生物样本、沉降蛋白、酯萃法提取待测组分,建立氟尿嘧啶浓度测定方法,检测"右旋糖酐-磁性层状复合氢氧化物-氟尿嘧啶"(Dextran-Magnetic LDH-Fluorouracil,DMF)给药干预大鼠体内组织中的氟尿嘧啶浓度。结果表明溴尿嘧啶内标HPLC法回收率高(>90%)、精密度好、结果重现性好(RSD<3%)、测试稳定性高;在外加磁场和多次给药干预条件下,测得不同实验组动物组织的药物浓度有统计学意义,满足验证DME磁靶向缓控释性能的分析要求。     

Shunting yard operations: Theoretical aspects and applications

Almost ever since freight has been transported via rail, shunting yards (also called classification or marshaling yards) are operated in order to separate freight trains and reassemble new trains. The efficient use of shunting yards has a deep impact on the efficiency and reliability of rail freight services. Thus, much research on shunting yards has been published, starting from the 1950s. Lately, several publications mostly focusing on the sorting procedures have livened up research on shunting yards. This paper reviews the literature on the operational processes at shunting yards over the last 40 years and discusses the operational challenges of freight transshipment. The approaches are classified according to different sorting strategies which allows an easy access to the models for both, researchers and practitioners. The paper concludes with an overview on future research challenges.     

具有单向转载的Newsboy型产品订货决策的博弈模型

研究了允许单向横向转载的Newsboy型产品的订货决策问题。以两个Newsboy型零售商为研究对象,在允许单向横向转载存在的前提下,构建了以二者期望利润最大化为目标的非合作博弈模型,讨论了模型存在纳什均衡解的条件并证明了解的存在性与唯一性,给出了均衡解的解析表达形式,进一步分析了转载价格对均衡的影响。最后的算例分析则从期望利润、均衡订货和转载价格三个方面验证了横向转载策略的优势。研究表明:(1)与传统报童模型相比,横向转载策略提高了零售商的期望利润;(2)转载价格显著影响横向转载策略下零售商的订货决策。