Internal waves and boundary layer for an oscillating disc in a stratified fluid

International audienceInternal or baroclinic tides, namely internal waves generated in the ocean by the oscillation of the barotropic tide over bottom topography, exhibit a complex pattern of primary wave beams and secondary beams resulting from the interaction of the primary beams with themselves and with the boundary layer at the topography. In this context, the problem of an oscillating horizontal disc, however remote it may be from oceanic configurations, recently gained visibility as the only problem for which a full analytical solution may be found including both waves and the boundary layer (Davis & Llewellyn Smith 2010). To date, all available approaches used an approximate free-slip condition at the topography instead of the actual no-slip condition, thus eliminating the boundary layer. In this communication, we examine the relation between those approaches and compare them with original high-resolution experimental measurements. Specifically, we consider the vertical heaving oscillations of a horizontal circular disc and compare fully inviscid investigations where both the generation and propagation of the waves are inviscid – using either orthogonal curvilinear coordinates (Sarma & Krishna 1972), boundary integrals (Gabov & Pletner 1988) or eigenfunction expansions (Martin & Llewellyn Smith 2011, 2012) – with fully viscous investigations where both generation and propagation are viscous – using either the actual disc oscillating in free space (Davis & Llewellyn Smith 2010) or a fictitious baffled disc oscillating through an aperture in a horizontal plane (Chashechkin, Vasil'ev & Bardakov 2004; Bardakov, Vasil'ev & Chashechkin 2007). We discuss the relevance of an intermediate model where propagation is viscous but generation inviscid (as used recently for a sphere by Voisin, Ermanyuk & Flór 2011), as a function of the Reynolds-Stokes number. At all but very high values of this number (of order $10^5$ or more say), it appears that the presence of the boundary layer must be taken into account for accurate prediction of the waves

Internal waves and boundary layers in a density-stratified fluid

International audienceThe topic of internal gravity wave generation by oscillating bodies in density-stratified fluids has recently gained prominence owing to the importance of oceanic internal tides, generated by the oscillation of the barotropic tide over bottom topography, in the dynamics of the Earth-Moon system. To assess the role of the boundary condition at the body (in the ocean at the topography), we consider the only problem for which a full viscous solution is known: the oscillating circular disk. The inviscid theory, the full viscous theory (i.e. with viscous effects on both wave propagation and generation) and the partial viscous theory (i.e. with viscous effects on propagation alone) are compared with low-resolution conductimetric measurements from the literature and with new and original high-resolution PIV measurements. Viscous effects on propagation are required for prediction of the wave profiles; generation becomes free-slip at Stokes number over a million, and remains no-slip otherwise

Internal wave-vorticity coupling for an oscillating disk

International audienceIn a density-stratified fluid, viscosity couples internal waves with vertical vorticity. So far this coupling used to be neglected in analytical studies and only the viscous attenuation and spreading of the waves was taken into account, except in a very recent study of the oscillations of a horizontal circular disk (Davis & Llewellyn Smith, JFM 2010). We investigate the relations between the previous analytical approaches of the disk, considering either inviscid or viscous propagation of the waves and either free- or no-slip conditions at the disk, and compare their output with an original approach based on the boundary integral method. In particular, the role of the Stokes number is clarified. The analytical predictions are compared with contact measurements for vertical oscillations (Bardakov, Vasil'ev & Chashechkin, Fluid Dynamics 2007) and with original PIV measurements and visualizations for both vertical and horizontal oscillations

Nuclear spin physics in quantum dots: an optical investigation

The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic,coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.Comment: 61 pages, 45 figures, updated reference list, corrected typographical error

Dynamic nuclear polarization of a single charge-tunable InAs/GaAs quantum dot

We report on the dynamic nuclear polarization of a single charge-tunable self-assembled InAs/GaAs quantum dot in a longitudinal magnetic field of $\sim$0.2T. The hyperfine interaction between the optically oriented electron and nuclei spins leads to the polarization of the quantum dot nuclei measured by the Overhauser-shift of the singly-charged excitons ($X^{+}$ and $X^{-}$). When going from $X^{+}$ to $X^{-}$, we observe a reversal of this shift which reflects the average electron spin optically written down in the quantum dot either in the $X^{+}$ state or in the final state of $X^{-}$ recombination. We discuss a theoretical model which indicates an efficient depolarization mechanism for the nuclei limiting their polarization to ~10%.Comment: 4+ pages, 3 figure

Activité physique chez les adultes ayant subi un traumatisme craniocérébral présentant des symptômes dépressifs : recension des écrits et vision de professionnels

Le traumatisme craniocérébral (TCC) constitue une préoccupation importante pour le système de santé étant donné les nombreuses difficultés fonctionnelles associées, telles que des difficultés physiques et cognitives, mais aussi des symptômes dépressifs et des troubles de l’humeur. Pour un ergothérapeute travaillant avec une clientèle TCC, le lien entre ces dimensions physiques et affectives est particulièrement intrigant. Dans le présent article, nous documenterons tout d’abord, par une recension des écrits, l’effet de l’activité physique sur les symptômes dépressifs chez les clients adultes ayant subi un TCC. Dans un second temps, nous vérifierons l’applicabilité de ces résultats en clinique auprès de professionnels de la santé. Pour ce faire, une recension des écrits à partir de CINAHL et PubMed a été effectuée, en utilisant le MCREO comme cadre conceptuel pour analyser les résultats. Les résultats ont ensuite été présentés à un groupe de trois intervenants de la santé afin de connaître leur opinion quant aux facteurs favorisant et limitant l’implantation de l’intervention par l’activité physique dans leur pratique. Bien que le niveau de preuve soit modeste, les résultats de la recension des écrits suggèrent que l’activité physique a des effets bénéfiques quant à la réduction des symptômes dépressifs. En regard de ces conclusions, la principale recommandation est que les ergothérapeutes incluent l’activité physique en tant que facteur préventif des symptômes dépressifs chez les clients TCC

The Orexin receptors: Structural and anti-tumoral properties

At the end of the 20th century, two new neuropeptides (Orexin-A/hypocretin-1 and Orexin-B/hypocretins-2) expressed in hypothalamus as a prepro-orexins precursor, were discovered. These two neuropeptides interacted with two G protein-coupled receptor isoforms named OX1R and OX2R. The orexins/OX receptors system play an important role in the central and peripheral nervous system where it controls wakefulness, addiction, reward seeking, stress, motivation, memory, energy homeostasis, food intake, blood pressure, hormone secretions, reproduction, gut motility and lipolysis. Orexins and their receptors are involved in pathologies including narcolepsy type I, neuro- and chronic inflammation, neurodegenerative diseases, metabolic syndrome, and cancers. Associated with these physiopathological roles, the extensive development of pharmacological molecules including OXR antagonists, has emerged in association with the determination of the structural properties of orexins and their receptors. Moreover, the identification of OX1R expression in digestive cancers encompassing colon, pancreas and liver cancers and its ability to trigger mitochondrial apoptosis in tumoral cells, indicate a new putative therapeutical action of orexins and paradoxically OXR antagonists. The present review focuses on structural and anti-tumoral aspects of orexins and their receptors

Raman probing of uniaxial strain in individual single-wall carbon nanotubes in a composite material

International audienceThe temperature dependence of the Raman spectrum of a gelatine-based composite material doped with single-walled carbon nanotubes (SWNT@gelatin) is reported. A significant up-shift of the G-mode frequency is observed when the temperature is decreased from room temperature to 20~K. This frequency shift is significantly stronger than the one found for pure thermal effects. In contrast, the features of the radial breathing modes (frequencies and width) display no significant change in the same temperature range. These results are well understood by considering a uniaxial strain on the nanotube induced by the thermal expansitivity mismatch between the nanotube and the surrounding matrix

L’épilepsie dans les démences du sujet âgé

Neurodegenerative diseases are an important cause of seizure in the elderly. Alzheimer’s disease and other dementias are associated with an increased risk of unprovoked seizure. This increased risk is present for patients with generalized- and for patients with partialonset seizures. Alzheimer’s disease is associated with seizures, typically generalized tonic-clonic but also partial, in approximately 10% to 16% of cases, and 12% for myoclonus, although even higher prevalence have also been reported. Convulsions are more likely to occur late in the course of Alzheimer’s disease. The prevalence and the incidence of epilepsy in the non AD-dementia in the elderly are not sufficiently studied in literature. EEG abnormalities are always not specific in these dementias. Anti epileptic treatment should take into account the peculiarities of elderly patients with cognitive impairment.Les maladies neurodégénératives sont une cause importante de crise d’épilepsie chez le sujet âgé. La maladie d’Alzheimer et les autres pathologies démentielles multiplient le risque de crise d’épilepsie que ce soit de crises d’épilepsie généralisées ou de crises partielles. Les crises d’épilepsie, typiquement tonico-cloniques, sont rapportées approximativement chez 10 à 16% des patients porteurs d’une maladie d’Alzheimer et 12% pour les myoclonies, mais des prévalences plus élevées ont également été rapportées. Les crises d’épilepsie surviennent surtout aux stades avancés. La prévalence et l’incidence de l’épilepsie au cours des autres types de démences du sujet âgé ont fait rarement l’objet d’études spécifiquement dédiées dans la littérature. Les anomalies EEG restent non spécifiques de ces démences. La prise en charge thérapeutique antiépileptique doit prendre en compte les particularités de ces patients âgés et porteurs de déficits cognitifs