Strong Isotopic Effect in Phase II of Dense Solid Hydrogen and Deuterium

Quantum nuclear zero-point motions in solid H$_2$ and D$_2$ under pressure are investigated at 80 K up to 160 GPa by first-principles path-integral molecular dynamics calculations. Molecular orientations are well-defined in phase II of D$_2$, while solid H$_2$ exhibits large and very asymmetric angular quantum fluctuations in this phase, with possible rotation in the (bc) plane, making it difficult to associate a well-identified single classical structure. The mechanism for the transition to phase III is also described. Existing structural data support this microscopic interpretation.Comment: 5 pages, 3 figure

Phase boundary anisotropy and its effects on the maze-to-lamellar transition in a directionally solidified Al-Al2Cu eutectic

Solid-solid phase boundary anisotropy is a key factor controlling the selection and evolution of non-faceted eutectic patterns during directional solidification. This is most remarkably observed during the so-called maze-to-lamellar transition. By using serial sectioning, we followed the spatio-temporal evolution of a maze pattern over long times in a large Al-Al2Cu eutectic grain with known crystal orientation of the Al and Al2Cu phases, hence known crystal orientation relationship (OR). The corresponding phase boundary energy anisotropy ($\gamma$-plot) was also known, as being previously estimated from molecular-dynamics computations. The experimental observations reveal the time-scale of the maze-to-lamellar transition and shed light on the processes involved in the gradual alignment of the phase boundaries to one distinct energy minimum which nearly corresponds to one distinct plane from the family $\{120\}^{\rm{Al}} //\{110\}^{\rm{Al2Cu}}$. This particular plane is selected due to a crystallographic bias induced by a small disorientation of the crystals relative to the perfect OR. The symmetry of the OR is thus slightly broken, which promotes lamellar alignment. Finally, the maze-to-lamellar transition leaves behind a network of fault lines inherited from the phase boundary alignment process. In the maze pattern, the fault lines align along the corners of the Wulff shape, thus allowing us to propose a link between the pattern defects and missing orientations in the Wulff shapeComment: 26 pages, 6 figure

An experimental method for the in-situ observation of eutectic growth patterns in bulk samples of transparent alloys

We present an experimental method for the in-situ observation of directional-solidification fronts in bulk samples of transparent eutectic alloys. The growth front is observed obliquely in dark field through the liquid and a glass wall of the container with a long-distance microscope. We show that a focused image of the whole growth front can be obtained at a certain tilt angle of the microscope. At this tilt angle, eutectic fibers of about 3.5\mic in diameter can be clearly seen over the whole growth front in 400-\mic thick samples

Modeling transitional plane Couette flow

The Galerkin method is used to derive a realistic model of plane Couette flow in terms of partial differential equations governing the space-time dependence of the amplitude of a few cross-stream modes. Numerical simulations show that it reproduces the globally sub-critical behavior typical of this flow. In particular, the statistics of turbulent transients at decay from turbulent to laminar flow displays striking similarities with experimental findings.Comment: 33 pages, 10 figure

Hexagonal eutectic solidification patterns operating near a marginal stability point

We study the long-time dynamics of hexagonal directional-solidification patterns in bulk samples of a transparent eutectic alloy using an optical method which permits real-time observation of the growth front. A slow dilatation of the patterns due to a slight curvature of the isotherms drives the system into a permanent regime, close to the threshold for the rod splitting instability. Thus an apparently minor instrumental imperfection suffices to maintain the system near a marginal stability point. This answers the long-standing question of spacing selection in bulk eutectic growth

Intermittency in a Locally Forced Plane Couette Flow

International audienceThe plane Couette flow displays a globally subcritical transition to turbulence. When forced by a thin wire introduced in the central plane, the basic flow bifurcates towards a new state of streamwise vortices which break down through a complex spatiotemporal regime. When a bead is introduced in the central plane, only a few vortices are generated. Their destabilization occurs via a new type of temporal intermittency hard to understand within the framework of dynamical system

Alone Together

Les bibliothèques – et les Archives – françaises se sont modernisées. Elles ont gagné en efficacité, mais peut-être perdu en « sociabilité », une notion centrale dans la culture anglo-saxonne. L’environnement matériel induit-il certaines pratiques de recherche ? Habituée de la BnF et des recherches en archives, une historienne américaine nous livre ses impressions à coeur ouvert

On the decay of turbulence in plane Couette flow

The decay of turbulent and laminar oblique bands in the lower transitional range of plane Couette flow is studied by means of direct numerical simulations of the Navier--Stokes equations. We consider systems that are extended enough for several bands to exist, thanks to mild wall-normal under-resolution considered as a consistent and well-validated modelling strategy. We point out a two-stage process involving the rupture of a band followed by a slow regression of the fragments left. Previous approaches to turbulence decay in wall-bounded flows making use of the chaotic transient paradigm are reinterpreted within a spatiotemporal perspective in terms of large deviations of an underlying stochastic process.Comment: ETC13 Conference Proceedings, 6 pages, 5 figure