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Dynamics Modelling (dynamics + modelling)
Selected AbstractsCluster Dynamics Modelling of Precipitation Kinetics in Al(Zr,Sc) AlloysADVANCED ENGINEERING MATERIALS, Issue 12 2006P. Guyot Cluster dynamics simulations in multicomponents alloys emphasize the complexity of the respective roles of atomic mobility and thermodynamics in the precipitation path. Some heuristic assumptions, like the maximum driving force criterion, used in conventional treatments, may fail and need to be carefully revisited at the light of these results. [source] Computational fluid dynamics modelling of boundary roughness in gravel-bed rivers: an investigation of the effects of random variability in bed elevationEARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2001A.P. Nicholas Abstract Results from a series of numerical simulations of two-dimensional open-channel flow, conducted using the computational fluid dynamics (CFD) code FLUENT, are compared with data quantifying the mean and turbulent characteristics of open-channel flow over two contrasting gravel beds. Boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra-grid-scale roughness elements (e.g. particle clusters and small bedforms). Results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed (typically at y/h,=,0·1,0·3). This is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed. Use of the random elevation model to represent supra-grid-scale roughness also allows a reduction in the height of the near-bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness. Despite these benefits, the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution. Increasing the horizontal mesh resolution results in an increase in the near-bed velocity gradient and turbulent kinetic energy, effectively roughening the bed. Varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles, but results in substantial changes to the shape of the turbulent kinetic energy profile. These findings have significant implications for the application of CFD within natural gravel-bed channels, particularly with regard to issues of topographic data collection, roughness parameterization and the derivation of mesh-independent solutions. Copyright © 2001 John Wiley & Sons, Ltd. [source] Implications of different degrees of arytenoid cartilage abduction on equine upper airway characteristicsEQUINE VETERINARY JOURNAL, Issue 7 2008V. RAKESH Summary Reason for performing study: The necessary degree of arytenoid cartilage abduction (ACA) to restore airway patency at maximal exercise has not been determined. Objectives: Use computational fluid dynamics modelling to measure the effects of different degrees of ACA on upper airway characteristics of horses during exercise. Hypothesis: Maximal ACA by laryngoplasty is necessary to restore normal peak airflow and pressure in Thoroughbred racehorses with laryngeal hemiplegia. Methods: The upper airway was modeled with the left arytenoid in 3 different positions: maximal abduction; 88% cross-sectional area of the rima glottis; and 75% cross-sectional area of the rima glottis. The right arytenoid cartilage was maximally abducted. Two models were assumed: Model 1: no compensation of airway pressures; and Model 2: airway pressure compensation occurs to maintain peak airflow. The cross-sectional pressure and velocity distributions for turbulent flow were studied at peak flow and at different positions along the airway. Results: Model 1: In the absence of a change in driving pressure, 12 and 25% reductions in cross-sectional area of the larynx resulted in 4.11 and 5.65% reductions in peak airflow and 3.68 and 5.64% in tidal volume, respectively, with mild changes in wall pressure. Model 2: To maintain peak flow, a 6.27% increase in driving tracheal pressure was required to compensate for a cross-sectional reduction of 12% and a 13.63% increase in driving tracheal pressure was needed for a cross-sectional area reduction of 25%. This increase in negative driving pressure resulted in regions with low intraluminal and wall pressures, depending on the degree of airway diameter reduction. Conclusion: Assuming no increase in driving pressure, the decrease in left ACA reduced airflow and tidal volume. With increasing driving pressure, a decrease in left ACA changed the wall pressure profile, subjecting the submaximally abducted arytenoid cartilage and adjacent areas to airway collapse. Clinical relevance: The surgical target of ACA resulting in 88% of maximal cross-sectional area seems to be appropriate. [source] Quantifying the relationship between soil organic carbon and soil physical properties using shrinkage modellingEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 2 2009P. Boivin Summary Changes in soil organic carbon (SOC) may strongly affect soil structure and soil physical properties, which in turn may have feedback effects on the soil microbial activity and SOC dynamics. Such interactions are still not quantitatively described and accounted for in SOC dynamics modelling. The objective of this study was to test the hypothesis that soil shrinkage curve (ShC) analysis allows the establishment of close relationships between soil physical properties and SOC. We sampled a rice-cropped vertisol, a cambisol under conventional tillage and no-tillage and a restored cambisol. Soil samples were analysed for clay and SOC content, bulk volume, hydro-structural stability and plasma and structural pore volumes changes on the full water content range using ShC analysis. Although the soils behaved differently according to their constituents and history, changes in SOC linearly affected most of the soil physical properties, with stronger effects than changes in clay content. The observed effects of increasing SOC, such as increasing hydro-structural stability, specific bulk volume and water retention, agreed well with previously reported results. However, using ShC measurement and modelling allowed the observation of all these different effects simultaneously for small changes in SOC, and in a single measurement. Moreover, the relation between SOC changes and physical properties could be quantified. ShC analysis may, therefore, be used to account for the effect of changes in SOC on soil physical properties. [source] System dynamics modelling for supply-chain management: A case study on a supermarket chain in the UKINTERNATIONAL TRANSACTIONS IN OPERATIONAL RESEARCH, Issue 5 2004Y. Ge Abstract This paper presents a system dynamics (SD) approach for the analysis of the demand amplification problem, also known as the bullwhip effect, which has been studied fairly extensively in the literature. The construction of an SD model is reported using a part of a supermarket chain system in the UK as an example. Based on the model, the causes of the dynamic behaviour of the system and the sources of amplification from the downstream to the upstream of the chain are investigated. The impact of information delays, demand forecasting and information sharing on the performance of the multi-echelon supply chain is analysed. Some implementation issues are also addressed based on the simulation analysis. [source] Climate mediated exogenous forcing and synchrony in populations of the oak aphid in the UKOIKOS, Issue 2 2009Sergio A. Estay Contemporary population dynamics theory suggests that animal fluctuations in nature are the result of the combined forces of intrinsic and exogenous factors. Weather is the iconic example of an exogenous force. The common approach for analyzing the relationship between population size and climatic variables is by simple correlation or using the climate as an additive covariable in statistical models. Here, we evaluated different functional forms in which climatic variables could influence population dynamics of the oak aphid Tuberculatus annulatus both in each locality and in relation to synchrony between localities. Results indicate that in at least four of eight aphid populations, climate influences population dynamics by modifying the carrying capacity of the system (lateral effect mediated by winter precipitation). Additionally, path analysis showed that synchrony in population dynamics is highly correlated with synchrony in winter precipitation regime, and the spatial scale of both processes is similar, which suggests that this is an example of the Moran effect. Our results show the key effects of precipitation on intra and inter population processes of this aphid. The methods used, mixing population dynamics modelling and test of synchrony, allowed us to connect the direct and indirect effects of exogenous variables into each population with patterns of synchrony inter populations. [source] CFD simulation of gas,solid bubbling fluidized bed: A new method for adjusting drag lawTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2009Farshid Vejahati Abstract In computational fluid dynamics modelling of gas,solid two phase flow, drag force is one of the dominant mechanisms for interphase momentum transfer. Despite the profusion of drag models, none of the available drag functions gives accurate results in their own original form. In this work the drag correlations of Syamlal and O'Brien (Syamlal and O'Brien, Int. J. Multiphase Flow. 1988; 14(4):473,481), Gidaspow (Gidaspow, Appl. Mech. Rev. 1986; 39:1,23), Wen and Yu (Wen and Yu, Chem. Eng. Prog. Symp. Ser. 1966; 62(2):100,111), Arastoopour et al. (Arastoopour et al., Powder Technol. 1990; 62(2): 163,170), Gibilaro et al. (Gibilaro et al., Chem. Eng. Sci. 1985; 40:1817,1823), Di Felice (Di Felice, Int. J. Multiphase Flow. 1994; 20(1):153,159), Zhang-Reese (Zhang and Reese, Chem. Eng. Sci. 2003; 58(8):1641,1644) and Hill et al. (Hill et al., J. Fluid Mech. 2001; 448:243,278) are reviewed using a multi-fluid model of FLUENT V6.3.26 (FLUENT, 2007. Fluent 6.3 User's Guide, 23.5 Eulerian Model, Fluent, Inc.) software with the resulting hydrodynamics parameters being compared with experimental data. The main contribution of this work is to propose an easy to implement and efficient method for adjustment of Di Felice drag law which is more efficient compared to the one proposed by Syamlal-O'Brien. The new method adopted in this work showed a quantitative improvement compared to the adjusted drag model of Syamlal-O'Brien. Prediction of bed expansion and pressure drop showed excellent agreement with results of experiments conducted in a Plexiglas fluidized bed. A mesh size sensitivity analysis with varied interval spacing showed that mesh interval spacing with 18 times the particle diameter and using higher order discretization methods produces acceptable results. Dans la modélisation par la dynamique des fluides par ordinateur de l'écoulement diphasique gaz-solide, la force de traînée est l'un des mécanismes dominants dans le transfert de quantité de mouvement interphase. Malgré la profusion des modèles de traînée, aucune des fonctions de traînée disponibles ne donnent de résultats précis dans leur forme originale. Dans cet article, les corrélations de traînée de Syamlal and O'Brien (Syamlal and O'Brien, Int. J. Multiphase Flow. 1988; 14(4):473,481), Gidaspow (Gidaspow, Appl. Mech. Rev. 1986; 39:1,23), Wen and Yu (Wen and Yu, Chem. Eng. Prog. Symp. Ser. 1966; 62(2):100,111), Arastoopour et al. (Arastoopour et al., Powder Technol. 1990; 62(2):163,170), Gibilaro et al. (Gibilaro et al., Chem. Eng. Sci. 1985; 40:1817,1823), Di Felice (Di Felice, Int. J. Multiphase Flow. 1994; 20(1):153,159), Zhang-Reese (Zhang and Reese, Chem. Eng. Sci. 2003; 58(8):1641,1644) et Hill et al. (Hill et al., J. Fluid Mech. 2001; 448:243,278) sont examinées à l'aide du modèle multi-fluides du logiciel FLUENT V6.3.26 (FLUENT, 2007. Fluent 6.3 User's Guide, 23.5 Eulerian Model, Fluent, Inc.), les paramètres hydrodynamiques résultants étant comparés aux données expérimentales. La principale contribution de ce travail est de proposer une méthode efficace et facile à mettre en ,uvre pour l'ajustement de la loi de traînée de Di Felice qui est plus efficace comparativement à celle proposée par Syamlal-O'Brien. La nouvelle méthode adoptée dans ce travail montre une amélioration quantitative par rapport au modèle de traînée ajusté de Syamlal-O'Brien. La prédiction de l'expansion de lit et de la perte de charge montre un excellent accord avec les résultats des expériences menées dans un lit fluidisé en plexiglass. Une analyse de sensibilité de la taille des mailles avec des mailles de taille variable variés montre qu'une taille de maille égale à 18 fois le diamètre des particules et l'utilisation de méthodes de discrétisation d'ordre supérieur donnent des résultats acceptables. [source] | ||||||||||