First analysis of a numerical benchmark for 2D columnar solidification of binary alloys

International audienceDuring the solidification of metal alloys, chemical heterogeneities at the product scale (macrosegregation) develop. Numerical simulation tools are beginning to appear in the industry, however their predictive capabilities are still limited. We present a numerical benchmark exercise treating the performance of models in the prediction of macrosegregation. In a first stage we defined a "minimal" (i.e. maximally simplified) solidification model, describing the coupling of the solidification of a binary alloy and of the transport phenomena (heat, solute transport and fluid flow) that lead to macrosegregation in a fully columnar ingot with a fixed solid phase. This model is solved by four different numerical codes, employing different numerical methods (FVM and FEM) and various solution schemes. We compare the predictions of the evolution of macrosegregation in a small (10×6 cm) ingot of Sn-10wt%Pb alloys. Further, we present the sensitivities concerning the prediction of instabilities leading to banded channel mesosegregations

A multi-scale 3D model of the vacuum arc remelting process

A multi-scale model of the VAR process was developed to simulate unsteady phenomena within the ingot melt pool due to arc motion and the resulting effects on dendritic microstructure. External magnetic field and surface current measurements were used as boundary conditions, to determine the trajectory of the arcs between electrode and ingot and between ingot and sidewalls. The interactions between magnetic field, turbulent metal flow and heat transfer were modeled using CFD techniques and this "macro" model was linked to a micro model, to resolve the evolving dendritic microstructure, and to establish a relationship between operational parameters and microstructure defects. Arc-driven solute convection in the mushy zone leading to local remelting and changes in local Rayleigh number provided an indicator of when fluid flow channels (freckles) will initiate within the mushy zone. Particle tracking was further used, to characterize the trajectory and dissolution of inclusions entering the melt, causing "white spot" defects

Environmental and Nutritional Factors That Affect Growth and Metabolism of the Pneumococcal Serotype 2 Strain D39 and Its Nonencapsulated Derivative Strain R6

<p>Links between carbohydrate metabolism and virulence in Streptococcus pneumoniae have been recurrently established. To investigate these links further we developed a chemically defined medium (CDM) and standardized growth conditions that allowed for high growth yields of the related pneumococcal strains D39 and R6. The utilization of the defined medium enabled the evaluation of different environmental and nutritional factors on growth and fermentation patterns under controlled conditions of pH, temperature and gas atmosphere. The same growth conditions impacted differently on the nonencapsulated R6, and its encapsulated progenitor D39. A semi-aerobic atmosphere and a raised concentration of uracil, a fundamental component of the D39 capsule, improved considerably D39 growth rate and biomass. In contrast, in strain R6, the growth rate was enhanced by strictly anaerobic conditions and uracil had no effect on biomass. In the presence of oxygen, the difference in the growth rates was mainly attributed to a lower activity of pyruvate oxidase in strain D39. Our data indicate an intricate connection between capsule production in strain D39 and uracil availability. In this study, we have also successfully applied the in vivo NMR technique to study sugar metabolism in S. pneumoniae R6. Glucose consumption, end-products formation and evolution of intracellular metabolite pools were monitored online by C-13-NMR. Additionally, the pools of NTP and inorganic phosphate were followed by P-31-NMR after a pulse of glucose. These results represent the first metabolic profiling data obtained non-invasively for S. pneumoniae, and pave the way to a better understanding of regulation of central metabolism.</p>