Superplasticity and superplastic forming

In both academic and industrial research endeavours, driving forces are essential to keep the interest alive for a specific topic [...

Analysis and Generation of Quality Polytopal Meshes with Applications to the Virtual Element Method

This thesis explores the concept of the quality of a mesh, the latter being intended as the discretization of a two- or three- dimensional domain. The topic is interdisciplinary in nature, as meshes are massively used in several fields from both the geometry processing and the numerical analysis communities. The goal is to produce a mesh with good geometrical properties and the lowest possible number of elements, able to produce results in a target range of accuracy. In other words, a good quality mesh that is also cheap to handle, overcoming the typical trade-off between quality and computational cost. To reach this goal, we first need to answer the question: ''How, and how much, does the accuracy of a numerical simulation or a scientific computation (e.g., rendering, printing, modeling operations) depend on the particular mesh adopted to model the problem? And which geometrical features of the mesh most influence the result?'' We present a comparative study of the different mesh types, mesh generation techniques, and mesh quality measures currently available in the literature related to both engineering and computer graphics applications. This analysis leads to the precise definition of the notion of quality for a mesh, in the particular context of numerical simulations of partial differential equations with the virtual element method, and the consequent construction of criteria to determine and optimize the quality of a given mesh. Our main contribution consists in a new mesh quality indicator for polytopal meshes, able to predict the performance of the virtual element method over a particular mesh before running the simulation. Strictly related to this, we also define a quality agglomeration algorithm that optimizes the quality of a mesh by wisely agglomerating groups of neighboring elements. The accuracy and the reliability of both tools are thoroughly verified in a series of tests in different scenarios

Catheter ablation in atrial fibrillation: a state-of-the-art review

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Don't forget the memory: Contribution of the T wave vector in localizing the site of origin of a monomorphic idiopathic ventricular tachycardia

SummaryWe report a case of cardiac memory following recurrent episodes of monomorphic idiopathic ventricular tachycardia and explain how it could be helpful in localizing the site of origin of the arrhythmia

Investigation of stress-strain behavior of a sheet material using free bulging test

In this work, a new experimental-numerical technique is developed in order to investigate the constitutive behaviour of a sheet material in conditions of superplastic forming. The principal feature of this technique is that unlike classical tensile testing it allows one to obtain stress-strain curves for a material formed in biaxial tension conditions produced by free bulging process. These conditions are much closer to the ones that the material undergoes during the superplastic forming process. Consequently, they give more accurate information about the material behaviour than the ones coming from tensile tests data. The drawback is that the strain (and similarly its time derivative) cannot be directly measured and controlled during free bulging test but its value has to be derived from other macroscopic measurement. Towards this end, a blow forming machine was equipped with a position transducer for the measurement of the dome height during the test. In order to control the stress in the dome apex at a predetermined level the applied pressure was continuously adjusted to current dome height using a special algorithm. After the test, the dome height data were processed to obtain the evolution of stress, strain and strain rate at the dome apex as well as the stress strain curves for constant referenced strain rates. The tests were performed on superplastic aluminium alloy (ALNOVI-U) sheets of 1.35 mm initial thickness at 500°C. Using the data from two tests with different strain rate paths the stress-strain curves and the strain rate sensitivity index evolution were calculated for two constant referenced strain rates. The obtained constitutive data were verified by finite element simulation of a blow forming

Seasonal variability in the Central Mediterranean Sea circulation

International audienceA high resolution eddy-resolving primitive equation numerical model, based on the Princeton Ocean Model (POM), is used to study the seasonal variability of the general circulation in the Central Mediterranean Sea. The model is run on a seasonal cycle, perpetual year simulation for five years, with nesting to the coarser resolution Ocean General Circulation Model (OGCM), covering the whole Mediterranean Sea. The model results are compared to the current knowledge on the hydrography and dynamics of the area, with a special focus on the annual cycle of the Modified Atlantic Water (MAW), on the circulation in the Sardinia Channel, the water exchange across the Strait of Sicily, and on the transition and fate of the Levantine Intermediate Water (LIW). The results show that the adopted coupling techniques between the two models give a proficient downscaling of the large-scale OGCM flow field into the regional scale model. The numerical solution is also used to highlight the seasonal characteristics of important dynamical features in the area, as well as to shed light on the scarcely known circulation regimes along the north African shelf and slope. Key words. Oceanography: general (numerical modelling); Oceanography: physical (currents; general circulation

Trasporto quantistico di spin in eterostrutture magnetiche

2010 - 2011In questa tesi abbiamo studiato il trasporto di spin polarizzato e la torsione dello spin usando l’apporoccio della matrice di scattering secondo la teoria di Buttiker, nel caso di barriere magnetiche finite sotto l'effetto di tensione continua. In questo contesto l'influenza dei parametri geometrici e magnetici (come un campo magnetico esterno o magnetizzazione) sulla conduttanza e sulle correnti di spin sono stati analizzati. Sono stati considerati il caso di una barriera di larghezza finita e due barriere magnetiche a delta. Dallo studio emergono proprietà di interesse per la spintronica (spin transfer torque, filtri di spin) e sono riportati molti confronti con gli esperimenti più recenti. Inoltre si è anche affrontato lo studio della pompa quantistica seguendo l’approccio di Touless e Brouwer. Sono state determinate le proprietà di trasporto quantistico attraverso una giunzione tunnel magnetiche mediante un pompaggio adiabatico usando sempre l’ approccio della matrice di scattering. Effetti quantistici delle dimensione finite sono previsti in presenza di una polarizzazione CC in funzione dello spessore dello strato metallico normale inserito tra due strati magnetici. In presenza di gate , che modulano adiabaticamente due parametri della matrice di scattering fuori fase, si mostra un effetto particolare di magnificazione del torque di spin nella direzione longitudinale al piano delle magnetizzazioni. Infine , con un approccio di scattering generalizzato, includendo la possibilità di spin flip alle interfacce , abbiamo calcolato la conduttanza differenziale e lo spin torque in un sistema ferromagnete / superconduttore / ferromagnete (F / S / F) o metallo / superconduttore / normale eterostrutture (N / S / N) . Si analizzano gli effetti quantistici sulla conduttanza, magnetoresistenza e spin torque in funzione della larghezza dello strato superconduttore, delle altezze di barriera e della lunghezza di coerenza. E’ stato discusso anche il problema del ruolo delle riflessioni Andreev all'interfaccia secondo un approccio esteso delle equazioni di Bogoliubov - De Gennes. I risultati sono discussi in relazione ai recenti esperimenti su valvole di spin superconduttive. [a cura dell'autore]X n.s