Three-dimensionality in quasi-two dimensional flows: recirculations and barrel effects

A scenario is put forward for the appearance of three-dimensionality both in quasi-2D rotating flows and quasi-2D magnetohydrodynamic (MHD) flows. We show that 3D recirculating flows and currents originate in wall boundary layers and that, unlike in ordinary hydrodynamic flows, they cannot be ignited by confinement alone. They also induce a second form of three-dimensionality with quadratic variations of velocities and current across the channel. This scenario explains both the common tendency of these flows to two-dimensionality and the mechanisms of the recirculations through a single formal analogy covering a wide class of flow including rotating and MHD flows. These trans-disciplinary effects are thus active in atmospheres, oceans or the cooling blankets of nuclear fusion reactors.Comment: 6 pages, 1 Figur

Dynamic Provenance for SPARQL Update

While the Semantic Web currently can exhibit provenance information by using the W3C PROV standards, there is a "missing link" in connecting PROV to storing and querying for dynamic changes to RDF graphs using SPARQL. Solving this problem would be required for such clear use-cases as the creation of version control systems for RDF. While some provenance models and annotation techniques for storing and querying provenance data originally developed with databases or workflows in mind transfer readily to RDF and SPARQL, these techniques do not readily adapt to describing changes in dynamic RDF datasets over time. In this paper we explore how to adapt the dynamic copy-paste provenance model of Buneman et al. [2] to RDF datasets that change over time in response to SPARQL updates, how to represent the resulting provenance records themselves as RDF in a manner compatible with W3C PROV, and how the provenance information can be defined by reinterpreting SPARQL updates. The primary contribution of this paper is a semantic framework that enables the semantics of SPARQL Update to be used as the basis for a 'cut-and-paste' provenance model in a principled manner.Comment: Pre-publication version of ISWC 2014 pape

Force indeterminacy in the jammed state of hard disks

Granular packings of hard discs are investigated by means of contact dynamics which is an appropriate technique to explore the allowed force-realizations in the space of contact forces. Configurations are generated for given values of the friction coefficient, and then an ensemble of equilibrium forces is found for fixed contacts. We study the force fluctuations within this ensemble. In the limit of zero friction the fluctuations vanish in accordance with the isostaticity of the packing. The magnitude of the fluctuations has a non-monotonous friction dependence. The increase for small friction can be attributed to the opening of the angle of the Coulomb cone, while the decrease as friction increases is due to the reduction of connectivity of the contact-network, leading to local, independent clusters of indeterminacy. We discuss the relevance of indeterminacy to packings of deformable particles and to the mechanical response properties.Comment: 4 pages, 3 figures. Minor changes, journal reference adde

Efficiency of a thermodynamic motor at maximum power

Several recent theories address the efficiency of a macroscopic thermodynamic motor at maximum power and question the so-called "Curzon-Ahlborn (CA) efficiency." Considering the entropy exchanges and productions in an n-sources motor, we study the maximization of its power and show that the controversies are partly due to some imprecision in the maximization variables. When power is maximized with respect to the system temperatures, these temperatures are proportional to the square root of the corresponding source temperatures, which leads to the CA formula for a bi-thermal motor. On the other hand, when power is maximized with respect to the transitions durations, the Carnot efficiency of a bi-thermal motor admits the CA efficiency as a lower bound, which is attained if the duration of the adiabatic transitions can be neglected. Additionally, we compute the energetic efficiency, or "sustainable efficiency," which can be defined for n sources, and we show that it has no other universal upper bound than 1, but that in certain situations, favorable for power production, it does not exceed 1/2

Spontaneous Chelation-Driven Reduction of the Neptunyl Cation in Aqueous Solution.

Octadentate hydroxypyridinone (HOPO) and catecholamide (CAM) siderophore analogues are known to be efficacious chelators of the actinide cations, and these ligands are also capable of facilitating both activation and reduction of actinyl species. Utilizing X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, as well as cyclic voltammetry measurements, herein, we elucidate chelation-based mechanisms for driving reactivity and initiating redox processes in a family of neptunyl-HOPO and CAM complexes. Based on the selected chelator, the ability to control the oxidation state of neptunium and the speed of reduction and concurrent oxo group activation was demonstrated. Most notably, reduction kinetics for the NpV O2 +/ /NpIV redox couple upon chelation by the ligands 3,4,3-LI(1,2-HOPO) and 3,4,3-LI(CAM)2 (1,2-HOPO)2 was observed to be faster than ever reported, and in fact quicker than we could measure using either X-ray absorption spectroscopy or electrochemical techniques

An Augmented OxRAM Synapse for Spiking Neural Network (SNN) Circuits

International audienceIn this paper, the conductance modulation of OxRAM memristive devices is evaluated based on experimental data to reveal the memristor inherent analog synaptic behavior. Simulation results are presented to validate the use of OxRAMs as synapses at a circuit level in a spiking neural network context. In the proposed approach, the OxRAM synapse is augmented with a shift register associated with current compliance control transistors to provide an efficient monitoring of the OxRAM conductance

Novel Cascaded Ultra Bright Pulsed Source of Polarization Entangled Photons

A new ultra bright pulsed source of polarization entangled photons has been realized using type-II phase matching in spontaneous parametric down conversion process in two cascaded crystals. The optical axes of the crystals are aligned in such a way that the extraordinarily (ordinarily) polarized cone from one crystal overlaps with the ordinarily (extraordinarily) polarized cone from the second crystal. This spatial overlapping removes the association between the polarization and the output angle of the photons that exist in a single type-II down conversion process. Hence, entanglement of photons originating from any point on the output cones is possible if a suitable optical delay line is used. This delay line is particularly simple and easy to implement.Comment: 8 pages 8 figure

Averaged residence times of stochastic motions in bounded domains

Two years ago, Blanco and Fournier (Blanco S. and Fournier R., Europhys. Lett. 2003) calculated the mean first exit time of a domain of a particle undergoing a randomly reoriented ballistic motion which starts from the boundary. They showed that it is simply related to the ratio of the volume's domain over its surface. This work was extended by Mazzolo (Mazzolo A., Europhys. Lett. 2004) who studied the case of trajectories which start inside the volume. In this letter, we propose an alternative formulation of the problem which allows us to calculate not only the mean exit time, but also the mean residence time inside a sub-domain. The cases of any combinations of reflecting and absorbing boundary conditions are considered. Lastly, we generalize our results for a wide class of stochastic motions.Comment: 7 pages, 3 figure

Cascade time-scales for energy and helicity in homogeneous isotropic turbulence

We extend the Kolmogorov phenomenology for the scaling of energy spectra in high-Reynolds number turbulence, to explicitly include the effect of helicity. There exists a time-scale $\tau_H$ for helicity transfer in homogeneous, isotropic turbulence with helicity. We arrive at this timescale using the phenomenological arguments used by Kraichnan to derive the timescale $\tau_E$ for energy transfer (J. Fluid Mech. {\bf 47}, 525--535 (1971)). We show that in general $\tau_H$ may not be neglected compared to $\tau_E$, even for rather low relative helicity. We then deduce an inertial range joint cascade of energy and helicity in which the dynamics are dominated by $\tau_E$ in the low wavenumbers with both energy and helicity spectra scaling as $k^{-5/3}$; and by $\tau_H$ at larger wavenumbers with spectra scaling as $k^{-4/3}$. We demonstrate how, within this phenomenology, the commonly observed ``bottleneck'' in the energy spectrum might be explained. We derive a wavenumber $k_h$ which is less than the Kolmogorov dissipation wavenumber, at which both energy and helicity cascades terminate due to dissipation effects. Data from direct numerical simulations are used to check our predictions.Comment: 14 pages, 5 figures, accepted to Physical Review