Analytic curves in algebraic varieties over number fields

We establish algebraicity criteria for formal germs of curves in algebraic varieties over number fields and apply them to derive a rationality criterion for formal germs of functions, which extends the classical rationality theorems of Borel-Dwork and P\'olya-Bertrandias valid over the projective line to arbitrary algebraic curves over a number field. The formulation and the proof of these criteria involve some basic notions in Arakelov geometry, combined with complex and rigid analytic geometry (notably, potential theory over complex and $p$-adic curves). We also discuss geometric analogues, pertaining to the algebraic geometry of projective surfaces, of these arithmetic criteria.Comment: 55 pages. To appear in "Algebra, Arithmetic, and Geometry: In Honor of Y.i. Manin", Y. Tschinkel & Yu. Manin editors, Birkh\"auser, 200

Madagascar Étude économique et sectorielle (ESW) : Marchés agricoles à Madagascar : contraintes et opportunités. Rapport de synthèse, No. 66028

Ce rapport présente les principaux résultats et les principales conclusions de l'étude sur les marchés agricoles à Madagascar menée conjointement par la Banque Mondiale et le Centre de coopération internationale en recherche agronomique pour le développement (CIRAD). L'étude poursuivait quatre objectifs : (i) Pour les produits agricoles de base et groupes de produits sélectionnés, décrire la structure de la production et de la consommation, les échanges commerciaux sur les marchés nationaux et internationaux et les prix ; (ii) Pour les produits agricoles de base et groupes de produits sélectionnés, décrire les principales filières existantes en mettant en exergue la structure des marchés et leurs degrés de concentration, les agents concernés, les activités réalisées, les mécanismes de coordination et les principales sources de risques et d'incertitude ; (iii) Pour chaque grande filière identifiée, estimer les coûts et marges pour évaluer sa performance et identifier les goulots d'étranglement qui entravent sa compétitivité ; (iv) Identifier les mesures politiques et institutionnelles, concernant les différents domaines d'intervention, pour améliorer la performance des filières et accroître la compétitivité des produits cibles. (Résumé d'auteur

\'Etude du cas rationnel de la th\'eorie des formes lin\'eaires de logarithmes. (French) [Study of the rational case of the theory of linear forms in logarithms]

We establish new measures of linear independence of logarithms on commutative algebraic groups in the so-called \emph{rational case}. More precisely, let k be a number field and v_{0} be an arbitrary place of k. Let G be a commutative algebraic group defined over k and H be a connected algebraic subgroup of G. Denote by Lie(H) its Lie algebra at the origin. Let u\in Lie(G(C_{v_{0}})) a logarithm of a point p\in G(k). Assuming (essentially) that p is not a torsion point modulo proper connected algebraic subgroups of G, we obtain lower bounds for the distance from u to Lie(H)\otimes_{k} C_{v_{0}}. For the most part, they generalize the measures already known when G is a linear group. The main feature of these results is to provide a better dependence in the height Log a of p, removing a polynomial term in LogLog a. The proof relies on sharp estimates of sizes of formal subschemes associated to H (in the sense of J.-B. Bost) obtained from a lemma by M. Raynaud as well as an absolute Siegel lemma and, in the ultrametric case, a recent interpolation lemma by D. Roy.Comment: Version d\'efinitiv

On the jet formation through a leaky glottis

The present experimental study aims at analyzing the jet formation of the glottal jet flow using a model of a leaky glottis. Experiments were performed in a flow channel with dynamic models of the vocal folds in order to measure the glottal waveform and the velocity distribution in the supraglottal region using High Speed Particle Image Velocimetry (HSPIV). Proper Orthogonal Decomposition (POD) of the vortex Q-criterion was performed in order to detect the energetically most significant large-scale vortex structures and their appearance in the jet flow. The spectral analysis of the glottal waveforms results in an increased spectral decay with more prominent peaks at higher frequencies in the case of a leaky glottis compared to a completely closing reference case. Vortex induced fluctuation frequencies have similar spectral content in both cases as they appear as trains of vortex packets in a regular manner over the glottal cycle. However, when removing the false vocal folds in the leaky glottis model, coherence of vortex generation is lost over the motion cycle. Thus, the presence of the false vocal folds retains most of the vortex induced characteristics in the source spectrum even when the vocal folds do not close fully

Automated swirl detection algorithm (ASDA) and its application to simulation and observational data

Swirling motions in the solar atmosphere have been widely observed in recent years and suggested to play a key role in channeling energy from the photosphere into the corona. Here, we present a newly developed Automated Swirl Detection Algorithm (ASDA) and discuss its applications. ASDA is found to be very proficient at detecting swirls in a variety of synthetic data with various levels of noise, implying our subsequent scientific results are astute. Applying ASDA to photospheric observations with a pixel size of 39.2 km sampled by the Solar Optical Telescope on board Hinode suggests a total number of 1.62 × 105 swirls in the photosphere, with an average radius and rotating speed of ~290 km and <1.0 km s−1, respectively. Comparisons between swirls detected in Bifrost numerical MHD simulations and both ground-based and space-borne observations suggest that (1) the spatial resolution of data plays a vital role in the total number and radii of swirls detected, and (2) noise introduced by seeing effects could decrease the detection rate of swirls, but has no significant influences in determining their inferred properties. All results have shown that there is no significant difference in the analyzed properties between counterclockwise or clockwise rotating swirls. About 70% of swirls are located in intergranular lanes. Most of the swirls have lifetimes of less than twice the cadences, meaning future research should aim to use data with much higher cadences than 6 s. In the conclusions, we propose some promising future research applications where ASDA may provide useful insight

A combined numerical and experimental study of the 3D tumble structure and piston boundary layer development during the intake stroke of a gasoline engine

Due to its positive effect on flame propagation in the case of a well-defined breakdown, the formation of a large-scale tumble motion is an important goal in engine development. Cycle-to-cycle variations (CCV) in the tumble position and strength however lead to a fluctuating tumble breakdown in space and time and therefore to combustion variations, indicated by CCV of the peak pressure. This work aims at a detailed investigation of the large-scale tumble motion and its interaction with the piston boundary layer during the intake stroke in a state-of-the-art gasoline engine. To allow the validation of the flow near the piston surface obtained by simulation, a new measurement technique called “Flying PIV” is applied. A detailed comparison between experimental and simulation results is carried out as well as an analysis of the obtained flow field. The large-scale tumble motion is investigated based on numerical data of multiple highly resolved intake strokes obtained using scale-resolving simulations. A method to detect the tumble center position within a 3D flow field, as an extension of previously developed 2D and 3D algorithms, is presented and applied. It is then used to investigate the phase-averaged tumble structure, its characteristics in terms of angular velocity and the CCV between the individual intake strokes. Finally, an analysis is presented of the piston boundary layer and how it is influenced by the tumble motion during the final phase of the intake stroke

Development and application of bivariate 2D-EMD for the analysis of instantaneous flow structures and cycle-to-cycle variations of in-cylinder flow

International audienceThe bivariate two dimensional empirical mode decomposition (Bivariate 2D-EMD) is extended to estimate the turbulent fluctuations and to identify cycle-to-cycle variations (CCV) of in-cylinder flow. The Bivariate 2D-EMD is an adaptive approach that is not restricted by statistical convergence criterion, hence it can be used for analyzing the nonlinear and non-stationary phenomena. The methodology is applied to a high-speed PIV dataset that measures the velocity field within the tumble symmetry plane of an optically accessible engine. The instantaneous velocity field is decomposed into a finite number of 2D spatial modes. Based on energy considerations, the in-cylinder flow large-scale organized motion is separated from turbulent fluctuations. This study is focused on the second half of the compression stroke. For most of the cycles, the maximum of turbulent fluctuations is located between 50 and 30 crank angle degrees before top dead center (TDC). In regards to the phase-averaged velocity field, the contribution of CCV to the fluctuating kinetic energy is approximately 55% near TDC

Turbulent separated shear flow control by surface plasma actuator: experimental optimization by genetic algorithm approach

The final publication is available at Springer via http://dx.doi.org/10.1007/s00348-015-2107-3The potential benefits of active flow control are no more debated. Among many others applications, flow control provides an effective mean for manipulating turbulent separated flows. Here, a nonthermal surface plasma discharge (dielectric barrier discharge) is installed at the step corner of a backward-facing step (U0 = 15 m/s, Reh = 30,000, Re¿ = 1650). Wall pressure sensors are used to estimate the reattaching location downstream of the step (objective function #1) and also to measure the wall pressure fluctuation coefficients (objective function #2). An autonomous multi-variable optimization by genetic algorithm is implemented in an experiment for optimizing simultaneously the voltage amplitude, the burst frequency and the duty cycle of the high-voltage signal producing the surface plasma discharge. The single-objective optimization problems concern alternatively the minimization of the objective function #1 and the maximization of the objective function #2. The present paper demonstrates that when coupled with the plasma actuator and the wall pressure sensors, the genetic algorithm can find the optimum forcing conditions in only a few generations. At the end of the iterative search process, the minimum reattaching position is achieved by forcing the flow at the shear layer mode where a large spreading rate is obtained by increasing the periodicity of the vortex street and by enhancing the vortex pairing process. The objective function #2 is maximized for an actuation at half the shear layer mode. In this specific forcing mode, time-resolved PIV shows that the vortex pairing is reduced and that the strong fluctuations of the wall pressure coefficients result from the periodic passages of flow structures whose size corresponds to the height of the step model.Peer ReviewedPostprint (author's final draft