385 research outputs found

    Basics of Dynamic Programming for Revenue Management

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    International audienceThe Revenue Management (RM), namely the pricing and the inventory control of a perishable product, is usually used to improve services marketing efficiency. While booking a flight, the manager has to allocate seats to various fare classes. Then, he has to assess the consequence of a current decision on the future stream of revenue, i.e. accept an certain incoming reservation or wait for a possible higher fare demand, but later. Since its practice becomes omnipresent this last decade, this paper presents some basics of Dynamic Programming (DP) through the most common model, the dynamic discrete allocation of a resource to n fare classes. The properties of the opportunity cost of using a unit of a given capacity, the key of any RM optimizations, are studied in details

    Price Fairness versus Pricing Fairness

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    International audienceThis research note discusses the distinction between these two concepts of perceptions of fairness, based on the theory of distributive justice and procedural justice, in order to helps understand consumer behavior. With a sample of 250 tourists in French Polynesia and a structural equation model, tourists do not confuse price fairness and pricing fairness. The theoretical implications are that future research should use two distinct scales. For managers, the study suggests that the attention devoted to explaining the fairness of the pricing has more impact on consumer satisfaction than some attempts to explain the price

    Modifications et stabilité du phosphore échangeable d'un ferralsol ingéré par un ver géophage

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    L'un des paramètres des cinétiques de dilution isotopique du phosphore est modifié par l'ingestion, du ferralsol étudié, par le ver géophage #Pontoscolex corethrurus (#Glossoscolecidae, #Oligochaeta$). La concentation d'ions phosphate de la solution du sol et le compartiment d'ions immédiatement échangeables associé au sol sont nettement augmentés. Divers mécanismes pouvant expliquer ces modifications sont discutés. La minéralisation du phosphore organique s'effectue au profit des formes les plus rapidement échangeables. (Résumé d'auteur

    Predicting the coefficient of permeability of soils using the Kozeny-Carman equation

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    Résumé: La conductivité hydraulique saturée d'un sol peut être prédite par des relations empiriques, des modèles capillaires, des modèles statistiques et des théories de rayon hydraulique. Une relation bien connue entre perméabilité et propriétés des pores fut proposée par Kozeny et modifiée par Carman. L'équation résultante est largement connue sous le nom Kozeny-Carman (KC), bien que ces auteurs n'aient jamais publié ensemble. Dans la littérature géotechnique, il existe un large consensus à l'effet que l'équation de Kozeny-Carman s'applique aux sables mais pas aux argiles. Cependant, cette opinion n'est appuyée que par une démonstration partielle. Cet article examine les fondements et la validité de l'équation KC à l'aide d'essais de perméabilité en laboratoire. Les résultats d'essais proviennent de diverses publications qui ont fourni toute l'information requise pour faire une prédiction : indice des vides et soit la surface spécifique mesurée pour les sols cohérents, soit la courbe granulométrique pour les sols pulvérulents. L'article montre comment calculer la surface spécifique d'un sol pulvérulent à partir de sa courbe granulométrique. Les résultats présentés ici indiquent qu'en général, l'équation de Kozeny-Carman prédit assez bien la conductivité hydraulique saturée de la plupart des sols. Plusieurs des divergences constatées peuvent être reliées soit à des raisons pratiques (e.g. valeur imprécise de la surface spécifique, régime permanent pas établi, échantillons non saturés, etc.) soit à des raisons théoriques (une partie de l'eau est immobile, et l'équation de prédiction est isotrope alors que la conductivité hydraulique est une propriété anisotrope). Ces aspects sont discutés dans l'article en relation avec la capacité de prédiction de l'équation de Kozeny-Carman. --------- Abstract: The saturated hydraulic conductivity of a soil can be predicted using empirical relationships, capillary models, statistical models and hydraulic radius theories. A well-known relationship between permeability and properties of pores was proposed by Kozeny and later modified by Carman. The resulting equation is largely known under the name of Kozeny-Carman, although these authors never published together. In the geotechnical literature, there is a large consensus that the Kozeny-Carman (KC) equation applies to sands but not to clays. Such opinion, however, is supported only by partial demonstration. This report evaluates the background and the validity of the KC equation with laboratory permeability tests. Considered test results were taken from publications that provided all information needed to make a prediction: void ratio, and either the measured specific surface for cohesive soils, or the gradation curve for non-cohesive soils. This report shows how to estimate the specific surface of a non-cohesive soil from its gradation curve. The results presented here show that, as a general rule, the KC equation predicts fairly well the saturated hydraulic conductivity of most soils. Many of the observed discrepancies can be related to either practical reasons (e.g. inaccurate specific surface value, steady flow not reached, unsaturated specimens, etc.) or theoretical reasons (some water is motionless, and the predictive equation is isotropic whereas hydraulic conductivity is an anisotropic property). Theses issues are discussed in relation to the predictive capabilities of the KC equation

    Shakespeareʼs Complete Works as a Benchmark for Evaluating Multiscale Document-Navigation Techniques

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    International audienceIn this paper, we describe an experimental platform dedicated to the comparative evaluation of multiscale electronic-document navigation techniques. One noteworthy characteristics of our platform is that it allows the user not only to translate the document (for example, to pan and zoom) but also to tilt the virtual camera to obtain freely chosen perspective views of the document. Second, the platform makes it possible to explore, with semantic zooming, the 150,000 verses that comprise the complete works of William Shakespeare. We argue that reaching and selecting one specific verse in this very large text corpus amounts to a perfectly well defined Fitts task, leading to rigorous assessments of target acquisition performance. For lack of a standard, the various multiscale techniques that have been reported recently in the literature are difficult to compare. We recommend that Shakespeare's complete works, converted into a single document that can be zoomed both geometrically and semantically, be used as a benchmark to facilitate systematic experimental comparisons, using Fitts' target acquisition paradigm

    Development of a model to predict the water retention curve using basic geotechnical properties

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    Résumé: La courbe de rétention d’eau (CRE) est devenue une fonction clé pour définir le comportement non saturé des sols et d'autres matériaux meubles. Dans beaucoup de cas, il peut être très utile d'avoir une évaluation de la CRE dans les pr emières phases d’un projet, lorsque peu ou pas de résultats d'essais sont disponibles. Des modèles prédictifs, basés sur les propriétés géotechniques de base, peuvent aussi être utilisés pour évaluer comment le changement des paramètres (en termes de porosité ou de granulométrie) affecte la CRE. Dans cet article, les auteurs présentent un ensemble d'équations développées pour prédire la relation entre la teneur en eau volumique θ (et le degré de saturation S r correspondant) et la succion ψ . Le modèle proposé pour la prédiction de la CRE est une version modifiée du modèle de Kovács (1981), qui fait une distinction entre la rétention d’eau due aux forces capillaires et celle par adhésion. Ce jeu d'équations est donné avec des relations complémentaires développées pour des applications spécifiques sur des matériaux granulaires et sur des sols (argileux) plastiques/c ohérents. Il est montré que le modèle constitue un moyen simple et pratique pour estimer la courbe de rétention d’eau à partir des propriétés géotechniques de base. Une discussion suit sur les capacités et les limitations du modèle. ---------- Abstract: The water retention curve (WRC) has become a key material function to define the unsaturated behavior of soils and of other particulate media. In many instances, it can be very useful to have an estimate of the WRC early in a project, when little or no test results are available. Predictive models, based on easy to obtain geotechnical prope rties, can also be employed to evaluate how changing parameters (e.g. porosity or grain size) affect the WRC. In this paper, the authors present a general set of equations developed for predicting the relationship between volumetric water content θ (and the corresponding degree of saturation S r ) and suction ψ . The proposed WRC model is a modified version of the Kovács (1981) model, which makes a distinction between water retention due to capillary forces and retention by adhesion. The complete set of equations is given together with complementary relationships developed for specific applications on granular materials and on plastic/cohesive (clayey) soils. It is shown that the model provides a simple and practical means to estimate the water retention curve from basic properties. A discussion follows on the capabilities and limitations of the model

    Calibration Optimization Methodology for Lithium-Ion Battery Pack Model for Electric Vehicles in Mining Applications

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    Large-scale introduction of electric vehicles (EVs) to the market sets outstanding requirements for battery performance to extend vehicle driving range, prolong battery service life, and reduce battery costs. There is a growing need to accurately and robustly model the performance of both individual cells and their aggregated behavior when integrated into battery packs. This paper presents a novel methodology for Lithium-ion (Li-ion) battery pack simulations under actual operating conditions of an electric mining vehicle. The validated electrochemical-thermal models of Li-ion battery cells are scaled up into battery modules to emulate cell-to-cell variations within the battery pack while considering the random variability of battery cells, as well as electrical topology and thermal management of the pack. The performance of the battery pack model is evaluated using transient experimental data for the pack operating conditions within the mining environment. The simulation results show that the relative root mean square error for the voltage prediction is 0.7–1.7% and for the battery pack temperature 2–12%. The proposed methodology is general and it can be applied to other battery chemistries and electric vehicle types to perform multi-objective optimization to predict the performance of large battery packs
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