Resource-constrained production planning in semicontinuous food industries

The resource-constrained production planning problem in semicontinuous multiproduct food industries is addressed. In particular, the case of yogurt production, a representative food process, in a real-life dairy facility is studied in detail. The problem in question is mainly focused on the packing stage, whereas timing and capacity constraints are imposed with respect to the batch stage to ensure the generation of feasible production plans. A novel mixed discrete/continuous-time mixed-integer linear programming model, based on the definition of families of products, is proposed. Timing and sequencing decisions are taken for product families rather than for products; thus, reducing significantly the model size. Additionally, material balances are realized for every particular product, permitting the detailed optimization of inventory and operating costs. Packing units operate in parallel and share resources. Qualitative as well as quantitative objectives are considered. Several industrial case studies, including also some unexpected events scenarios, have been solved to optimality.     

A management and control architecture for providing IPdifferentiated services in MPLS-based networks

As the Internet evolves toward the global multiservice network of the future, a key consideration is support for services with guaranteed quality of service. The proposed differentiated services framework is seen as the key technology to achieve this. DiffServ currently concentrates on control/data plane mechanisms to support QoS, but also recognizes the need for management plane aspects through the bandwidth broker. In this article we propose a model and architectural framework for supporting DiffServ-based end-to-end QoS in the Internet, assuming underlying MPLS-based explicit routed paths. The proposed integrated management and control architecture will allow providers to offer both quantitative and qualitative services while optimizing the use of underlying network resources     

Algorithms for precomputing constrained widest paths and multicast trees

We consider the problem of precomputing constrained widest paths and multicast trees in a communication network. Precomputing and storing of the relevant information minimizes the computational overhead required to determine an optimal path when a new connection request arrives. We evaluate algorithms that precompute paths with maximal bandwidth (widest paths), which in addition satisfy given end-to-end delay constraints. We analyze and compare both the worst case and average case performance of the algorithms. We also show how the precomputed paths can be used to provide computationally efficient solutions to the constrained widest multicast tree problem. In this problem, a multicast tree with maximal bandwidth (widest multicast tree) is sought, which in addition satisfies given end-to-end delay constraints for each path on the tree from the source to a multicast destination.     

Energy-Aware Broadcast Trees in Wireless Networks

In this paper we address the problem of broadcasting in wireless networks, so that the power consumed by any node is as small as possible. This approach is motivated by the fact that nodes in such networks often use batteries and, hence, it is important to conserve energy individually, so that they remain operational for a long time. We formulate the problem as a lexicographic node power optimization one. The problem is in general NP-complete. We provide an optimal algorithm which runs in polynomial time in certain cases. We also provide a heuristic algorithm whose performance relative to the optimal one is fairly satisfactory. We next show that these algorithms can also be used to solve the problem of broadcasting so that the residual energy of any node after the broadcast process is as large as possible. Finally, we discuss the issues of implementing the above algorithms distributively, as well as their multicast extensions.     

Multicast tree structure and the power law

In this paper, we investigate structural properties of multicast trees that give rise to the so-called multicast power law. The law asserts that the ratio R(n) of the average number of links in a multicast tree connecting the source to n destinations to the average number of links in a unicast path, satisfies asymptotically R(n)/spl ap/cn/sup /spl phi//, 0相似文献   

A system dynamics model for dynamic capacity planning of remanufacturing in closed-loop supply chains

Product recovery operations in reverse supply chains face continually and rapidly changing product demand characterized by an ever increasing number of product offerings with reduced lifecycles due to both technological advancements and environmental concerns. Capacity planning is a strategic issue of increased complexity importance for the profitability of reverse supply chains due to their highly variable return flows. In this work we tackle the development of efficient capacity planning policies for remanufacturing facilities in reverse supply chains, taking into account not only economic but also environmental issues, such as the take-back obligation imposed by legislation and the “green image” effect on customer demand. The behavior of the generic system under study is analyzed through a simulation model based on the principles of the system dynamics methodology. The simulation model provides an experimental tool, which can be used to evaluate alternative long-term capacity planning policies (“what-if” analysis) using total supply chain profit as measure of policy effectiveness. Validation and numerical experimentation further illustrate the applicability of the developed methodology, while providing additional intuitively sound insights.     

A heterogeneous dynamic model for the simulation and optimisation of the steam methane reforming reactor

In the current paper the dynamic behaviour of an industrial heterogeneous catalytic packed-bed reactor for the steam reforming of methane is examined. The model consists of a set of partial differential equations describing the physico-chemical processes that take place both in solid and gas phases accounting for diffusional limitations within the catalyst particles. The model was validated against literature data, while the heat provided to the reactor wall was optimised in terms of the optimal H₂ yield using a quadratic wall temperature profile. The values of the physico-chemical properties were adjusted to the severe operating conditions (high pressures and temperatures) of the reactor accounting for multicomponent gas mixture properties. It is shown that the 2-phase reactor concept along with the optimised wall temperature profile capture very well the dynamic conversion, the temperature and the partial pressure profiles both at bed and at particle level.     

Water and solute active transport through human epidermis: Contribution of electromigration

A triphasic, coarse-grained model of mass transport through the human epidermis is developed, consisting of free extracellular water, live cells (keratinocytes), and inert extracellular matrix. The model accounts for the superposition of active transport of Na⁺, K⁺ and Cl^? ions across the membrane of keratinocytes, and electromigration driven by an externally imposed electrostatic potential difference. Local cell volume is regulated by the transmembrane fluxes of water and ions according to a time-delay scheme which aims to keep the volume between certain thresholds. Numerical simulations reveal that either weak hyposmotic shocks or negative potential gradients smaller than one millivolt/micrometer across the epidermis can generate travelling waves in extracellular ion concentration. By monitoring the transmembrane (Na⁺?K⁺ ?ATPase) pump flux, we have found that maintaining a higher transepidermal potential gradient requires faster active transport through the cells.     

Computer modelling of impact on curved fibre composite panels

This paper presents results of a finite element (FE) analysis study into low energy impact on curved composite panels. The aim of this study was to determine the accuracy and efficiency of a non-linear explicit FE code, MSC.Dytran, and compare results to published experimental data. The study also looked at impact response as a function of composite panel curvature, composite mesh density, impactor weight, velocity and size, and various suggestions are made for improving the accuracy and efficiency of FE analysis procedures in composite low energy impact studies. The paper presents 265 explicit computer simulation results, which show that non-linear FE analysis does provide accurate, efficient and conservative solutions provided various guidelines are followed.