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"Production control Mathematical models."
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Handbook of Stochastic Models and Analysis of Manufacturing System Operations
This book applies stochastic models and analytical techniques to such areas of manufacturing as facilities planning, transportation and material handling systems, production scheduling, logistics, supply chain management and productivity and quality models.
eBook
Handbook of Stochastic Models and Analysis of Manufacturing System Operations
This book applies stochastic models and analytical techniques to such areas of manufacturing as facilities planning, transportation and material handling systems, production scheduling, logistics, supply chain management and productivity and quality models.
Book
On time-inconsistent stochastic control in continuous time
In this paper, which is a continuation of the discrete-time paper (Björk and Murgoci in Finance Stoch. 18:545–592,
2004
), we study a class of continuous-time stochastic control problems which, in various ways, are time-inconsistent in the sense that they do not admit a Bellman optimality principle. We study these problems within a game-theoretic framework, and we look for Nash subgame perfect equilibrium points. For a general controlled continuous-time Markov process and a fairly general objective functional, we derive an extension of the standard Hamilton–Jacobi–Bellman equation, in the form of a system of nonlinear equations, for the determination of the equilibrium strategy as well as the equilibrium value function. The main theoretical result is a verification theorem. As an application of the general theory, we study a time-inconsistent linear-quadratic regulator. We also present a study of time-inconsistency within the framework of a general equilibrium production economy of Cox–Ingersoll–Ross type (Cox et al. in Econometrica 53:363–384,
1985
).
Journal Article
Declining oxygen in the global ocean and coastal waters
As plastic waste pollutes the oceans and fish stocks decline, unseen below the surface another problem grows: deoxygenation. Breitburg et al. review the evidence for the downward trajectory of oxygen levels in increasing areas of the open ocean and coastal waters. Rising nutrient loads coupled with climate change—each resulting from human activities—are changing ocean biogeochemistry and increasing oxygen consumption. This results in destabilization of sediments and fundamental shifts in the availability of key nutrients. In the short term, some compensatory effects may result in improvements in local fisheries, such as in cases where stocks are squeezed between the surface and elevated oxygen minimum zones. In the longer term, these conditions are unsustainable and may result in ecosystem collapses, which ultimately will cause societal and economic harm. Science , this issue p. eaam7240 Oxygen is fundamental to life. Not only is it essential for the survival of individual animals, but it regulates global cycles of major nutrients and carbon. The oxygen content of the open ocean and coastal waters has been declining for at least the past half-century, largely because of human activities that have increased global temperatures and nutrients discharged to coastal waters. These changes have accelerated consumption of oxygen by microbial respiration, reduced solubility of oxygen in water, and reduced the rate of oxygen resupply from the atmosphere to the ocean interior, with a wide range of biological and ecological consequences. Further research is needed to understand and predict long-term, global- and regional-scale oxygen changes and their effects on marine and estuarine fisheries and ecosystems.
Journal Article
Distributionally robust multi-item newsvendor problems with multimodal demand distributions
We present a risk-averse multi-dimensional newsvendor model for a class of products whose demands are strongly correlated and subject to fashion trends that are not fully understood at the time when orders are placed. The demand distribution is known to be multimodal in the sense that there are spatially separated clusters of probability mass but otherwise lacks a complete description. We assume that the newsvendor hedges against distributional ambiguity by minimizing the worst-case risk of the order portfolio over all distributions that are compatible with the given modality information. We demonstrate that the resulting distributionally robust optimization problem is
NP
-hard but admits an efficient numerical solution in quadratic decision rules. This approximation is conservative and computationally tractable. Moreover, it achieves a high level of accuracy in numerical tests. We further demonstrate that disregarding ambiguity or multimodality can lead to unstable solutions that perform poorly in stress test experiments.
Journal Article
Grassland ecosystem services: a systematic review of research advances and future directions
ContextGrasslands provide a variety of ecosystem services (ESs) for humans. While much ES research has focused on forests and wetlands, synthesizing the currently somewhat sporadic studies of grassland ecosystem services (GESs) is much needed.ObjectivesWe aimed to review the scope, major methods, and key findings of GESs, and identify knowledge gaps and future directions.MethodsWe conducted a systematic review of articles published during 1970–2018 (including 380 peer-reviewed articles from Web of Science and 32 book chapters from Google Scholar).ResultsThe number of GES studies has accelerated in recent decades, with China (31%) and the United States (18%) together accounting for almost half of them. A total of 33 GESs were mentioned in the searched articles, of which carbon sequestration, forage production, and water erosion control had the highest frequencies. Methods for evaluating GESs include field survey, field experiments, and statistical and process-based modeling. Grasslands are the primary source of meat and dairy products, account for about one-third of the total carbon of all terrestrial ecosystems, and provide numerous other ESs, such as night cooling, soil erosion control, and flood mitigation.ConclusionsThis review presents the state-of-the-science of GESs, and identifies several future research directions. To move forward, we propose a framework with a 3-M methodology: (1) “Multi-scales”—understanding GESs from various spatiotemporal scales; (2) “Multi-methods”—evaluating GESs with multiple statistical and modeling techniques using multiple data sources; and (3) “Multi-perspectives”—assessing GESs from ecological, social, and economic perspectives for sustainability.
Journal Article
Fused deposition modeling: process, materials, parameters, properties, and applications
In recent years, 3D printing technology has played an essential role in fabricating customized products at a low cost and faster in numerous industrial sectors. Fused deposition modeling (FDM) is one of the most efficient and economical 3D printing techniques. Various materials have been developed and studied, and their properties, such as mechanical, thermal, and electrical, have been reported. Numerous attempts to improve FDM products’ properties for applications in various sectors have also been reported. Still, their applications are limited due to the materials’ availability and properties compared to traditional fabrication methods. In 3D printing, the process parameters are crucial factors for improving the product's properties and reducing the machining time and cost. Researchers have recently investigated many approaches for expanding the range of materials and optimizing the FDM process parameters to extend the FDM process’s possibility into various industrial sectors. This paper reviews and explains various techniques used in 3D printing and the various polymers and polymer composites used in the FDM process. The list of mechanical investigations carried out for different materials, process parameters, properties, and the FDM process's potential application was discussed. This review is expected to indicate the materials and their optimized parameters to achieve enhanced properties and applications. Also, the article is highly anticipated to provide the research gaps to sustenance future research in the area of FDM technologies.
Graphical abstract
Journal Article
The effect of processing parameters on the mechanical characteristics of PLA produced by a 3D FFF printer
3D printing by fused filament fabrication (FFF) provides an innovative manufacturing method for complex geometry components. Since FFF is a layered manufacturing process, effects of process parameters are of concern when plastic materials such as polylactic acid (PLA), polystyrene and nylon are used. This study explores how the process parameters, e.g. build orientation and infill pattern/density, affect the mechanical response of PLA samples produced using FFF. Digital image correlation (DIC) was employed to get full-field surface-strain measurements. The results show the influence of build orientation and infill density is significant. For on-edge orientation, the tensile strength and Young’s modulus were 55 MPa and 3.5 GPa respectively, which were about 91% and 40% less for the upright orientation, demonstrating a significant anisotropy. The tensile strength and Young’s modulus increased with increasing infill density. In contrast, different infill patterns have no significant effect. Considering the influence of build orientation, based on the experimental results, a constitutive model derived from the laminate plate theory was employed. The material parameters were determined by tensile tests. Results demonstrated a reasonable agreement between the experimental data and the predictive model. Similar anisotropy to tension was observed in shear tests; shear modulus and shear strength for 45° flat orientation were about 1.55 GPa and 36 MPa, whereas for upright specimens they were about 0.95 GPa and 18 MPa, respectively. The findings provide a framework for systematic mechanical characterisation of 3D-printed polymers and potential ways of choosing process parameters to maximise performance for a given design.
Journal Article