The origin of magnetic cycles in the Sun and other cool stars is one of the great theoretical challenge in stellar astrophysics that still resists our understanding. Ab-initio numerical simulations are today required to explore the extreme turbulent regime in which stars operate and sustain their large-scale, cyclic magnetic field. We report in this work on recent progresses made with high performance numerical simulations of global turbulent convective envelopes. We rapidly review previous prominent results from numerical simulations, and present for the first time a series of turbulent, global simulations producing regular magnetic cycles whose period varies systematically with the convective envelope parameters (rotation rate, convective luminosity). We find that the fundamentally non-linear character of the dynamo simulated in this work leads the magnetic cycle period to be inversely proportional to the Rossby number. These results promote an original interpretation of stellar magnetic cycles, and could help reconcile the cyclic behaviour of the Sun and other solar-type stars.

Les opérations d'alésage à outil $\raise.5ex\hbox{$\scriptstyle 3$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $ sont utilisées pour réaliser des alésages pour lesquelles le perçage dans le plein n'est pas capable de procurer la qualité spécifiée et lorsque les contraintes de productivité et d'encombrement n'autorisent pas d'autres procédés et outils. L'article présente une étude de faisabilité réalisée récemment, concernant la possibilité d'utiliser le principe du perçage vibratoire pour les opérations d'alésage à forets $\raise.5ex\hbox{$\scriptstyle 3$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $, afin de favoriser la fragmentation des copeaux et leur évacuation. Les modèles mathématiques du perçage vibratoire sont présentés et une discussion sur la méthodologie employée pour le calcul des paramètres opératoires est donnée. Le phénomène de talonnage est discuté et nous démontrons que son influence, prouvée néfaste pour les opérations de perçage dans le plein, est bien limitée lors des opérations d'alésage. Les résultats expérimentaux obtenus sur un banc d'essai ont validé avec précision les modèles développés et nous permettent d'envisager des essais "in situ", destinés à une industrialisation du procédé.

The author shows that the model of world systems economic theory remains the most probable explanation of the forms of contacts between indigenous peoples and colonists. The six zones where the imported goods from Mediterranean ‘states’ are found to be most numerous are analysed here. They are distributed from east to west across Europe along a discontinuous arc at some distance from the Black Sea and the Mediterranean. They have analogously similar structures, conforming to the principles of world-systems economics. Such similarities are surprising because the a priori probability of finding such similarities is so small given the environmental, historical, and cultural differences, as well as those of the human spirit, imagination and inventiveness. We must suppose that a powerful logic was driving this phenomenon. The criticisms directed at this model are often made by those who are habitually ‘anti-’ … More complex and interesting is the alternative explanation proposed by J. Bintliff and M. Dietler. An analysis of the correlation of the drop in the number of sites with these imported goods and increased distance contradicts their alternative. Moreover, there is not one example of development on the same scale and level of integration as these ‘chiefdoms’ without mediterranean contacts. Finally, the significance of the model may be measured by the frequency that oppontents caricature it in order to criticise it more easily.