Modifications d'une zone alluviale suite à l'endiguement: approche méthodologique.

Les modifications d'une zone alluviale alpine suite a l'endiguement ont ete etudiees en comparant la situation actuelle (sol, vegetation et position geomorphologique) avec differentes situations dans le passe. Pour cette reconstitution historique, deux approches complementaires ont ete employees: la premiere, traditionnelle, basee sur la photo-interpretation, a permis de decrire l'etat du site a differentes dates; la seconde, s'appuie sur un SIG et a permis d'etablir des cartes montrant les changements (qualitatifs et quantitatifs) du mode d'occupation du sol au cours du temps. Ces cartes sont utiles pour identifier les situations-types actuelles et expliquer la distribution spatiel actuelle du sol et de la vegetation

A three-dimensional cellular automation-finite element model for the prediction of solidification grain structures

International audienceA three-dimensional (3-D) model for the prediction of dendritic grain structures formed during solidification is presented. This model is built on the basis of a 3-D cellular automaton (CA) algorithm. The simulation domain is subdivided into a regular lattice of cubic cells. Using physically based rules for the simulation of nucleation and growth phenomena, a state index associated with each cell is switched from zero (liquid state) to a positive value (mushy and solid state) as solidification proceeds. Because these physical phenomena are related to the temperature field, the cell grid is superimposed to a coarser finite element (FE) mesh used for the solution of the heat flow equation. Two coupling modes between the microscopic CA and macroscopic FE calculations have been designed. In a so-called “weak” coupling mode, the temperature of each cell is simply interpolated from the temperature of the FE nodes using a unique solidification path at the macroscopic scale. In a “full” coupling mode, the enthalpy field is also interpolated from the FE nodes to the CA cells and a fraction of solid increment is computed for each mushy cell using a truncated Scheil microsegregation model. These fractions of solid increments are then fed back to the FE nodes in order to update the new temperature field, thus accounting for a more realistic release of the latent heat (i.e., the solidification path is no longer unique). Special dynamic allocation techniques have been designed in order to minimize the computation costs and memory size associated with a very large number of cells (typically 107 to 108). The potentiality of the CAFE model is demonstrated through the predictions of typical grain structures formed during the investment casting and continuous casting processes