L'apport de la gestion de production aux sciences agronomiques. Le cas des ressources fourragères

International audienceLe niveau d'intégration des connaissances des sciences biotechniques (agronomie et zootechnie) ne leur permet pas de définir seules les nouvelles façons de produire imposées par la redéfinition des fonctions de l'agriculture. À partir de la gestion des ressources fourragères, nous montrons l'intérêt des concepts de la gestion de production (gestion de flux, planification et pilotage) pour organiser et orienter la production de connaissances et d'outils devant accompagner ce changement de fonction. Nous proposons ainsi un modèle générique de la gestion des surfaces fourragères qui prend en compte les caractéristiques des surfaces et les objectifs environnementaux. Ce modèle est illustré sur deux exemples de systèmes herbagers en transformation. Nous terminons en montrant comment ces analyses peuvent être mobilisées pour guider les agronomes, tant pour les connaissances à produire sur le fonctionnement de la végétation que pour l'élaboration de modèles de gestion des ressources fourragères

L'apport de la gestion de production aux sciences agronomiques. Le cas des ressources fourragères

Le niveau d'intégration des connaissances des sciences biotechniques (agronomie et zootechnie) ne leur permet pas de définir seules les nouvelles façons de produire imposées par la redéfinition des fonctions de l'agriculture. À partir de la gestion des ressources fourragères, nous montrons l'intérêt des concepts de la gestion de production (gestion de flux, planification et pilotage) pour organiser et orienter la production de connaissances et d'outils devant accompagner ce changement de fonction. Nous proposons ainsi un modèle générique de la gestion des surfaces fourragères qui prend en compte les caractéristiques des surfaces et les objectifs environnementaux. Ce modèle est illustré sur deux exemples de systèmes herbagers en transformation. Nous terminons en montrant comment ces analyses peuvent être mobilisées pour guider les agronomes, tant pour les connaissances à produire sur le fonctionnement de la végétation que pour l'élaboration de modèles de gestion des ressources fourragères.gestion ; agronomie ; utilisation du territoire ; pâturage ; prairie

Michel Griffon - Qu’est ce que l’agriculture écologiquement intensive ? - Édition Quae 2013, Collection : Matière à débattre et décider, 224 pages

Les concepts clés Une première manière de s’approprier le contenu de cet ouvrage est d’examiner le lexique. Les mots clés les plus fréquemment référencés sont : biodiversité, énergie, fertilité, fonctionnalités, paysage. La plupart seront familiers aux lecteurs. Ils fournissent déjà des indications sur ce qu’est l’agriculture écologiquement intensive : elle est basée sur la biodiversité ; elle permet de réduire la consommation d’énergie, notamment en conséquence de l’augmentation de la fertil..

Ebullition en masse dans un milieu poreux modèle

L'ébullition est étudiée expérimentalement dans un réseau de cylindres chauffants répartis aléatoirement entre deux plaques de céramique. Les cylindres chauffants sont des sondes RTD (Resistance Temperature Detector) commandées individuellement via un système d'asservissement, ce qui fournit des mesures en chaque point du milieu poreux modèle bidimensionnel ainsi constitué. Les résultats présentés ici caractérisent seulement l'ébullition autour d'un cylindre unique monté dans la section test ainsi que le renoyage d'une ligne de cylindres surchauffés

Boiling in porous media: toward a local non-equilibrium model

Developing a macro-scale model describing accurately intense boiling in porous media, like in the case of the re-flooding of a debris bed reactor with strong solid heat source terms, is a real challenge. The existence of local non-equilibrium effects has been recognized in the past based on strong experimental and theoretical evidences. Three-temperature models have been proposed coupled with two-phase macro-scale momentum balance equations taking into account inertia effects (see discussion in [1,2]. They seem to offer a good modelling framework, however, many questions remain unresolved. This paper discusses several issues associated to this challenging up-scaling problem. In a first part, experimental results are presented at both the pore-scale (micro-model made of a network of heat resistors) and the macroscopic-scale (porous medium column). Pore-scale results emphasize clearly the existence of various boiling regimes but with significant differences when compared to classical pool boiling or boiling in channels experiments. In particular, classical Nukiyama curves are not strictly recovered, quantitatively and qualitatively, because of the various interactions due to the porous medium structure and tortuosity. This calls for porous medium specific expressions of the heat exchange terms in the macro-scale models, and also suggests that the pore-scale phase repartition is very specific to the boiling regimes and that the resulting macro-scale two-phase flow properties must be adapted. In a second part, the up-scaling problem is reviewed pointing out the many different assumptions that must be made in order to establish a model candidate taking as much as possible the information coming from the experimental program

Kinetic modeling in the context of cerebral blood flow quantification by H215O positron emission tomography: The meaning of the permeability coefficient in Renkin–Crone׳s model revisited at capillary scale.

One the one hand, capillary permeability to water is a well-defined concept in microvascular physiology, and linearly relates the net convective or diffusive mass fluxes (by unit area) to the differences in pressure or concentration, respectively, that drive them through the vessel wall. On the other hand, the permeability coefficient is a central parameter introduced when modeling diffusible tracers transfer from blood vessels to tissue in the framework of compartmental models, in such a way that it is implicitly considered as being identical to the capillary permeability. Despite their simplifying assumptions, such models are at the basis of blood flow quantification by H215O Positron Emission Tomography. In the present paper, we use fluid dynamic modeling to compute the transfers of H215O between the blood and brain parenchyma at capillary scale. The analysis of the so-obtained kinetic data by the Renkin-Crone model, the archetypal compartmental model, demonstrates that, in this framework, the permeability coefficient is highly dependent on both flow rate and capillary radius, contrarily to the central hypothesis of the model which states that it is a physiological constant. Thus, the permeability coefficient in Renkin-Crone's model is not conceptually identical to the physiologic permeability as implicitly stated in the model. If a permeability coefficient is nevertheless arbitrarily chosen in the computed range, the flow rate determined by the Renkin-Crone model can take highly inaccurate quantitative values. The reasons for this failure of compartmental approaches in the framework of brain blood flow quantification are discussed, highlighting the need for a novel approach enabling to fully exploit the wealth of information available from PET data

Experimental and numerical study of two-phase flows in arrays of cylinders

In this paper we study the spreading of a liquid jet in a periodic array of cylinders with a characteristic size of the passages between solid obstacles equal to 1.5 mm, close to the capillary length. An important outcome of our study is to show that this configuration allows most of the two phase flow regimes described in the literature about trickle beds to be observed, even with no gas injection. Different aspects of the flow phenomenology have been studied, such as bubble creation and transport. As direct numerical methods for tracking interfaces would require too much computation time, especially in three-dimensional cases, we propose to simulate the two-phase flows observed experimentally with two-dimensional simulations corresponding to the spreading of a liquid jet in an array of disks. We show that this numerical approach allows the phenomenology observed experimentally to be reproduced satisfactorily. Hence, numerical simulations can be used subsequently to study the effects of specific parameters without setting up a new experimental procedure. As an example, the stabilizing effect of gas injection on the flow pattern is studied numerically

Reflooding with internal boiling of a heating model porous medium with mm-scale pores

This paper presents a pore-scale experimental study of the reflooding of a two-dimensional model porous medium. The objective is to better understand the reflooding mechanisms in play in the context of nuclear reactor safety. The hot debris bed that forms in a nuclear reactor following a loss of coolant accident is comparable to a heat-generating porous medium. Its cooling by water reflooding involves intense boiling mechanisms that must be modeled properly to assess mitigation procedures. The experimental study presented in this paper focuses on the phenomenology of reflooding of a model porous medium composed of a bank of mm-scale heating cylinders placed between two ceramic plates. A Fluorinert™ liquid, HFE-7000, is injected at a temperature close to saturation into the dry and superheated porous medium. Each cylinder of the test section is used both as a heating element and a temperature probe, which enables to track the evolution of the three different macroscopic zones identified during cooling of the system. The reflooding dynamics, in particular the cooling fronts velocities, are thus determined thanks to pore-scale thermal measurements together with direct visualizations. The influence of the injection flow rate and of the heating power are studied in a parametric way