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Three-dimensional instabilities and optimal perturbations of a counter-rotating vortex pair in stratified flows
No full textInternational audienceThis paper investigates the three-dimensional instabilities and the optimal perturbations on a pair of horizontal counter-rotating Lamb-Oseen vortices in a vertically stably stratified flow. Two-dimensional (2D) simulations are first performed, showing that while the dipole moves vertically against the stratification the vortex parameters: the radius a ∗, the separation distance b ∗, and the circulation Γ∗ are solely function of the time rescaled by the Brunt-Väisälä frequency N, independently of the Froude number, when the Reynolds number is large enough. Here, the Froude number is Fr = W 0/Nb 0 with W 0 the initial advection velocity of the dipole and b 0 the initial separation distance between the two vortices. For weak and moderate stratifications (large Fr), the stratification acts on a long time scale compared to the advection time of the dipole implying that the 2D flow can be considered as quasi-steady. In that case, when three dimensional instabilities are added, a linear stability analysis of this 2D flow at different instants retrieves the instability peaks corresponding to the Crow instability for the long wavelengths and to the elliptic instability for the short wavelengths showing that the dynamics is almost unaffected by buoyancy effects. The Crow and elliptic instabilities scale with the instantaneous dipole parameters showing in particular that stratification promotes instability by reducing the distance b ∗ between vortices [K. K. Nomura et al., “Short-wavelength instability and decay of a vortex pair in stratified fluid,” J. Fluid Mech. 553, 283-322 (2006)]. For strong stratifications (Froude numbers of order unity or smaller), the quasi-steady approximation is not valid, and the question of stability should be formulated in a different way, by, for example, searching for the transient growth of the energy of perturbation that may be computed for steady or unsteady base flow. Then, for each time horizon τ, we should determine the critical perturbation leading to the largest energy growth by the time τ. Presently, we compute the optimal perturbations at two time horizons τ = 4 and τ = 10 dimensionalized by with a direct-adjoint technique which takes into account the evolution of the base flow. In the homogeneous case, this technique allows to investigate the effect of the weak unsteadiness of the flow due to viscous diffusion which induces a growth of the vortex core radius a ∗. Both Crow and elliptic instabilities are retrieved in the optimal response and in the energy gain curves. Even if very slow, the viscous diffusion is found to increase the gain of the antisymmetric elliptic perturbation compared to the symmetric one. When the fluid is stratified, peaks at small wavenumber and at wavenumber of the order of the vortex core size are found for all Froude numbers with optimal responses strongly resembling, respectively, the Crow and the elliptic modes with optimal gains corresponding to mean growth rates larger than in the homogeneous case for both modes. However, as the strength of stratification increases (Froude numbers smaller than 2), optimal perturbations start departing from their homogeneous counterpart with large perturbation in the wake of the dipole associated with density effects.2πb20/Γ0© 2015 AIP Publishing LL
Piezoresistivity of thin film semiconductors with application to thin film silicon solar cells
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Bi-Sn alloy catalyst for simultaneous morphology and doping control of silicon nanowires in radial junction solar cells
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High structural quality InGaN/GaN multiple quantum well solar cells
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Lifetime assessment in crystalline silicon: From nanopatterned wafer to ultra-thin crystalline films for solar cells
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Tissue Tomographic Phase Image Contrast Improvement with Adaptive Optics
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A Quantum Electrodynamics Kondo Circuit with Orbital and Spin Entanglement
No full textRecent progress in nanotechnology allows to engineer hybrid mesoscopic devices comprising on chip an artificial atom or quantum dot, capacitively coupled to a microwave (superconducting) resonator. These systems can then contribute to explore non-equilibrium quantum impurity physics with light and matter, by increasing the input power on the cavity and the bias voltage across the mesoscopic system. Here, we build such a prototype system where the artificial atom is a graphene double quantum dot (DQD). Controlling the coupling of the photon field and the charge states of the DQD, we measure the microwave reflection spectrum of the resonator. When the DQD is at the charge degeneracy points, experimental results are consistent with a Kondo impurity model entangling charge, spin and orbital degrees of freedom. The light reveals the formation of the Kondo or Abrikosov-Suhl resonance at low temperatures. We then study the complete phase diagram as a function of gate voltages, bias voltage and microwave input power
Ice water content vertical profiles of high-level clouds: classification and impact on radiative fluxes
No full textInternational audienceIn this work, we discuss the shape of ice water content (IWC) vertical profiles in high ice clouds and its effect on radiative properties of these clouds, both in short- and in long-wave bands (SW and LW). Based on the analysis of colocated satellite data, we suggest a minimal set of primitive shapes (rectangular, isosceles trapezoid, lower and upper triangle), which sufficiently well represents the IWC profiles. About 75% of all high-level ice clouds (P 300 W m−2
MANIPULATION ÉLECTRIQUE ET CHIMIQUE DE L’AIMANTATION DES COUCHES ULTRA MINCES
No full textSpintronics is primarily based on the precise control of a nanostructure magnetization direction, are becoming increasingly important issues in device miniaturization. One line of research is tailoring the magnetic anisotropy energy (MAE) of the nanostructure. A second line aims at manipulating the magnetization through the magneto-electric coupling effect (MEC). In this work, influences of surface chemistry and influences of electric field on the MAE of the ferromagnetic thin films (cobalt) have been studied separately by an all electrochemical approach, compared to solid state devices. The electrolyte environments allow us to deposit ultraflat 2D epitaxial thin films; modify the surface chemistry easily; and apply a relative large and uniform electric field through a naturally formed defects-free ionic dielectric double layer. Oxidation of the Co was achieved in an alkaline electrolyte. The structures of different oxide layers were investigated via DFT calculations and electrochemical charge. Their influences on the MAE were studied in real time by in-situ polar magneto-optic Kerr effect (MOKE) measurements. Influences of other molecular layers linked by –S, -C, -N were also investigated. MEC effects only due to the electric field of Co thin films covered by different molecular layers were characterized quantitatively, and dependence of MEC effect with the surface chemistry was observed. Furthermore, modifications of MAE of Pd-Co systems induced by H ad/absorption were analyzed as well. In most cases, theoretic hypothesis were suggested to explain the experimental results.Le développement de composants actifs en électronique de spin bénéficierait de la > électrique de la direction de l’aimantation de nanostructures pour réduire la consommation électrique des composants, qui reposerait sur la modification de l’énergie d’anisotropie magnétique (MAE) par un stimulus électrique. La quasi-totalité des travaux concernent des structures solides obtenues par un empilement de couches minces. Dans le contexte de ma thèse, on a développé une approche toute en électrochimie qui apporte des éléments quantitatifs nouveaux sur l’origine des effets chimiques et magnétoélectriques (l’effet MEC) :i) Des couches magnétiques déposées sont bien définies à l’échelle atomique. ii) l’interface solide/liquide permet d’appliquer un très fort champ électrique uniforme. iii) L’environnement liquide apporte des flexibilités pour modifier la chimie de surface facilement. iv) Les propriétés magnétiques pourront être couplées à des caractérisations structurales obtenues dans les mêmes conditions. Des couches d’oxydes ont été formées et leurs structures ont été clarifiées via des calculs DFT et des determinations électrochimiques. Ensuite leurs influences sur l’anisotropie magnétique ont été mesurées et quantifiées. Des autres couches moléculaires accrochées par –C, -S et –N ont été adsorbées sur la surface. Leurs impacts sur la MAE ont présentés par la dépendence de Ks à la chimie de surface. De puis, des effets MECs liées directement au champ électrique ont été observés. Une dépendence du ΔKs/ΔU à la chimie de surface s’est manifestée. Finalement des mofications remarquables sur la MAE dans le système du Pd-Co induites par l’H2 ont été mesurées
The Arp2/3 inhibitory protein arpin induces cell turning by pausing cell migration
No full textInternational audienceBranched actin networks generated by the Arp2/3 complex provide the driving force for leading edge protrusion in migrating cells. We recently identified Arpin, a protein that inhibits the Arp2/3 complex in lamellipodia. Arpin is activated by the small GTPase Rac, which triggers lamellipodium formation, and thus Arpin renders protrusions unstable. A conserved role of Arpin is to induce migrating cells to turn in different migration models. Here we investigated the mechanism by which Arpin controls directional persistence. For this analysis, we segmented migration trajectories into alternating phases of active migration and pauses, based on a speed threshold. Regardless of the threshold value, Arpin induced more frequent pausing, during which the cell was more likely to change the direction of its migration. Arpin simultaneously acts on cell speed and directional persistence, which are strongly coupled parameters. Induction of frequent pausing by Arpin is consistent with Arpin circuitry: by inhibiting the Arp2/3 complex as a response to Rac activation, Arpin antagonizes a positive feedback loop that sustains protrusions at the leading edge and maintains active migration. We propose the duration of active migration' as a useful proxy to measure feedbacks associated with cell migration. (c) 2015 Wiley Periodicals, Inc