Problème de Graetz généralisé pour un fluide à seuil de contrainte (Fluide plastique de Bingham)(b- température imposée)

Problème de Graetz généralisé pour un fluide à seuil de contrainte (Fluide plastique de Bingham)(b- Température imposée) L'analyse numérique de l'établissement hydrodynamique et thermique de l'écoulement d'un fluide semi- solide de Bingham entre deux plaques planes maintenues à une température constante est réalisée en utilisant un code de calcul basé sur la méthode des éléments finis. Cette étude a permit de déceler l'influence exercée par les caractéristiques rhéologiques du fluide sur la structure de l'écoulement et l'intensité de l'échange thermique fluide- paroi. L'effet de la dissipation visqueuse sur le champ thermique est également analysé. Mots clés : Viscoplastique ; température imposée ; dissipation visqueuse ; fluide de Bingham ; Nusselt

Evidential Clustering: A Review

International audienceIn evidential clustering, uncertainty about the assignment of objects to clusters is represented by Dempster-Shafer mass functions. The resulting clustering structure, called a credal partition, is shown to be more general than hard, fuzzy, possibilistic and rough partitions, which are recovered as special cases. Three algorithms to generate a credal partition are reviewed. Each of these algorithms is shown to implement a decision-directed clustering strategy. Their relative merits are discussed

Biphasic effect of extracellular ATP on human and rat airways is due to multiple P2 purinoceptor activation

BACKGROUND: Extracellular ATP may modulate airway responsiveness. Studies on ATP-induced contraction and [Ca(2+)](i )signalling in airway smooth muscle are rather controversial and discrepancies exist regarding both ATP effects and signalling pathways. We compared the effect of extracellular ATP on rat trachea and extrapulmonary bronchi (EPB) and both human and rat intrapulmonary bronchi (IPB), and investigated the implicated signalling pathways. METHODS: Isometric contraction was measured on rat trachea, EPB and IPB isolated rings and human IPB isolated rings. [Ca(2+)](i )was monitored fluorimetrically using indo 1 in freshly isolated and cultured tracheal myocytes. Statistical comparisons were done with ANOVA or Student's t tests for quantitative variables and χ(2 )tests for qualitative variables. Results were considered significant at P < 0.05. RESULTS: In rat airways, extracellular ATP (10(-6)–10(-3 )M) induced an epithelium-independent and concentration-dependent contraction, which amplitude increased from trachea to IPB. The response was transient and returned to baseline within minutes. Similar responses were obtained with the non-hydrolysable ATP analogous ATP-γ-S. Successive stimulations at 15 min-intervals decreased the contractile response. In human IPB, the contraction was similar to that of rat IPB but the time needed for the return to baseline was longer. In isolated myocytes, ATP induced a concentration-dependent [Ca(2+)](i )response. The contractile response was not reduced by thapsigargin and RB2, a P2Y receptor inhibitor, except in rat and human IPB. By contrast, removal of external Ca(2+), external Na(+ )and treatment with D600 decreased the ATP-induced response. The contraction induced by α-β-methylene ATP, a P2X agonist, was similar to that induced by ATP, except in IPB where it was lower. Indomethacin and H-89, a PKA inhibitor, delayed the return to baseline in extrapulmonary airways. CONCLUSION: Extracellular ATP induces a transient contractile response in human and rat airways, mainly due to P2X receptors and extracellular Ca(2+ )influx in addition with, in IPB, P2Y receptors stimulation and Ca(2+ )release from intracellular Ca(2+ )stores. Extracellular Ca(2+ )influx occurs through L-type voltage-dependent channels activated by external Na(+ )entrance through P2X receptors. The transience of the response cannot be attributed to ATP degradation but to purinoceptor desensitization and, in extrapulmonary airways, prostaglandin-dependent PKA activation

Co-simulation of the Microclimate inside a Greenhouse by Coupling Energy and CFD Models

International audienceEnergy Simulation (ES) is an interesting tool to explore and predict the evolution of the mean climatic parameters inside a greenhouse. A limitation of this tool is that it requires the ventilation rate and the heat transfer coefficients along the walls as inputs. This information may be obtained from Computational Fluid Dynamics (CFD). The present study focuses on the coupling of a CFD code with an ES program. The coupling consists in exchanging data between ES and CFD. ES uses the ventilation rate and the convective heat coefficients provided by CFD. Conversely ES provides the wall temperatures to the CFD code as updated boundary conditions. In a preliminary stage, the greenhouse was represented by a 3D geometry and the calcula¬tion domain reduced to the greenhouse itself i.e., without surroundings. The venti¬lation rate was computed by applying the Mean Age of Air (MAA) technique in the CFD code, which implies the resolution of an additional scalar equation. The internal convective heat coefficients were also computed and transmitted to the ES model via a specific routine written under the TRNSYS environment. The obtained CFD results for the MAA were first compared with experimental data available in the literature. The mean temperature and humidity predicted by the co-simulation ES-CFD were then compared with experimental data recorded inside a plastic tunnel greenhouse located in Angers (France) and the effects of ventilation caused by the opening of the door on the greenhouse microclimate were analysed. One limitation of the approach however, is that the restriction of the CFD calculation domain to the greenhouse does not make it possible yet the CFD calculation of external heat transfers coefficients used in ES, which are still estimated from empirical correlations. This drawback should be overcome in the next developments by extending the CFD-grid to the surroundings of the greenhouse

Coupling Global and Distributed Climate Models to Better Assess Heat Transfers through the Cover of a Greenhouse

International audienceThe determination of the Convective Heat Coefficients (CHC) is a challenge to get a detailed knowledge of the microclimate of greenhouses in order for instance to improve energy management of these buildings. Computational Fluid Dynamics tools (CFD) have been developed to predict the distributed inside climate. However, most of the previous studies focus on steady state cases for which the boundary conditions are established a priori by the modeller. This point is one of the limitations of CFD use. Complementary, Energy Simulation (ES) is based on an energy balance method which takes account of the dynamic variation of the boundary conditions (i.e. meteorological conditions) all day long, meaning that the wall temperatures are calculated at each time step. In the present paper, an ES model predicting the thermal behaviour inside a closed tunnel greenhouse is coupled to a CFD model. The ES model transmits the wall temperatures to the CFD model and conversely, the CFD model provides the ES model with CHC. The comparison between simulation results shows differences between the CHC calculated by ES (from empirical correlations) and those calculated by CFD. These differences are clearly higher for the vertical walls than for the roofs

Intercellular communication: role of gap junctions in establishing the pattern of ATP-elicited Ca2+ oscillations and Ca2+-dependent currents in freshly isolated aortic smooth muscle cells

Cytosolic-free [Ca2+] was evaluated in freshly dissociated smooth muscle cells from mouse thoracic aorta by the ratio of Fura Red and Fluo 4 emitted fluorescence using confocal microscopy. The role of intercellular communication in forming and shaping ATP-elicited responses was demonstrated. Extracellular ATP (250 microM) elicited [Ca2+]i transient responses, sustained [Ca2+]i rise, periodic [Ca2+]i oscillations and aperiodic repetitive [Ca2+]i transients. Quantity of smooth muscle cells in the preparation responding to ATP with periodical [Ca2+]i oscillations depended on the density of isolated cells on the cover slip. ATP-elicited bursts of [Ca2+]i spikes in 66+/-7% of cells in dense and in 33+/-8.5% of cells in non-dense preparations. The number of cells responding to ATP with bursts of [Ca2+]i spikes decreased from 55+/-5% (n=84) to 14+/-3% (n=141) in dense preparations pretreated with carbenoxolone. Simultaneous measurement of [Ca2+]i and ion currents revealed a correlation between [Ca2+]i and current oscillations. ATP-elicited bursts of current spikes in 76% of cells regrouped in small clusters and in 9% of isolated cells. Clustered cells responding to ATP with current oscillations had higher membrane capacity than clustered cells with transient and sustained ATP-elicited responses. Lucifer Yellow (1% in 130 mM KCl) injected into one of clustered cells was transferred to the neighboring cell only when ATP-elicited oscillations. Fast application of carbenoxolone (100 microM) inhibited ATP (250 microM) elicited Ca2+-dependent current oscillations. Taken together these results suggest that the probability of ATP (250 microM) triggered cytosolic [Ca2+]i oscillations accompanied with K+ and Cl- current oscillations increased with the coupling of smooth muscle cells