Dynamic Behaviour of a Flexible Yacht Sail Plan

• Dynamic fluid structure interaction of a sail plan is modeled in harmonic pitching • Aerodynamic forces oscillations show hysteresis phenomena • Neglecting the structural deformation underestimates the forces oscillations • Both aerodynamic and structure inertia affect loads in the rig.A numerical investigation of the dynamic Fluid Structure Interaction (FSI) of a yacht sail plan submitted to harmonic pitching is presented to address both issues of aerodynamic unsteadiness and structural deformation. The FSI model | Vortex Lattice Method uid model and Finite Element structure model | has been validated with full-scale measurements. It is shown that the dynamic behaviour of a sail plan subject to yacht motion clearly deviates from the quasi-steady theory. The aerodynamic forces presented as a function of the instantaneous apparent wind angle show hysteresis loops, suggesting that some energy is exchanged by the system. The area included in the hysteresis loop increases with the motion reduced frequency and amplitude. Comparison of rigid versus soft structures shows that FSI increases the energy exchanged by the system and that the oscillations of aerodynamic forces are underestimated when the structure deformation is not considered. Dynamic loads in the fore and aft rigging wires are dominated by structural and inertial effects. This FSI model and the obtained results may be useful firstly for yacht design, and also in the field of auxiliary wind assisted ship propulsion, or to investigate other marine soft structures.This work was supported by the French Naval Academy

Mechanically Compliant Grating Reflectors for Optomechanics

We demonstrate micromechanical reflectors with a reflectivity as large as 99.4% and a mechanical quality factor Q as large as 7.8*10^5 for optomechanical applications. The reflectors are silicon nitride membranes patterned with sub-wavelength grating structures, obviating the need for the many dielectric layers used in conventional mirrors. We have employed the reflectors in the construction of a Fabry-Perot cavity with a finesse as high as F=1200, and used the optical response to probe the mechanical properties of the membrane. By driving the cavity with light detuned to the high-frequency side of a cavity resonance, we create an optical antidamping force that causes the reflector to self-oscillate at 211 kHz

Hydrothermal synthesis of nanosized BaTiO3 powders and dielectric properties of corresponding ceramics

BaTiO3 fine powders were synthesized by hydrothermal method at 150 °C or 250 °C for 7 h, starting from a mixture of TiCl3 + BaCl2 or TiO2 + BaCl2. The size of the crystallites is close to 20 nm whatever the starting mixture and the reaction temperature. These powders are well crystallized and constituted of a mixture of the metastable cubic and stable tetragonal phases. The ceramics obtained after uniaxial pressing and sintering at 1250 °C for 10 h or 20 h present high densification (up to 99.8%). The Curie temperature (Tc) and the electrical permittivity ( r) of the ceramics strongly depend on the type of titanium source that has been used for preparing the powder and on the sintering dwell time. Particularly, Tc is shifted towards lower temperature when TiCl3 is used. The permittivity value at Tc of BaTiO3 sintered at 1250 °C for 10 h reaches 7000 and 11,000 with respectively TiCl3 and TiO2 used as titanium source

Aggregating Correlated Estimations with (Almost) no Training

Many decision problems cannot be solved exactly and use several estimation algorithms that assign scores to the different available options. The estimation errors can have various correlations, from low (e.g. between two very different approaches) to high (e.g. when using a given algorithm with different hyperparameters). Most aggregation rules would suffer from this diversity of correlations. In this article, we propose different aggregation rules that take correlations into account, and we compare them to naive rules in various experiments based on synthetic data. Our results show that when sufficient information is known about the correlations between errors, a maximum likelihood aggregation should be preferred. Otherwise, typically with limited training data, we recommend a method that we call Embedded Voting (EV)

Les zones humides urbaines à Bogota, conflits d’usage et patrimonialisation

La mise en valeur de la zone humide de Cordoba, au cœur des quartiers aisés de Bogota, a fait l’objet d’un conflit, opposant les autorités municipales et une organisation de quartier, associée aux mouvements de protection de l’environnement. Au-delà de ce conflit, il s’agit d’une opposition entre deux visions de l’aménagement des paysages naturels urbains. Si la nécessité de mettre en valeur cet espace est une évidence pour tous, des projets différents co-existent. Le District Capital de Bogota souhaite aménager cette zone humide afin d’en avoir un usage simplement social (création d’un parc récréatif), tandis que l’autre partie veut préserver l’espace naturel et ses écosystèmes hors de la vie urbaine. Pour faire annuler le projet municipal devant un tribunal, les riverains ont construit leur argumentation autour de la notion de patrimoine. Grâce à la représentation différente qu’ont une partie des riverains de la zone humide de Cordoba, les espaces naturels urbains ne sont plus de simples espaces verts récréatifs, mais ont une réelle fonction environnementale.The development of the wetland of Cordoba, in the middle of the easy districts of Bogota, was the subject of a conflict, opposing the municipal authorities and a district organization, associated with the movements of environmental protection. Beyond this conflict, it is about an opposition between two visions on the urban natural landscaping. If the need for emphasizing this space is obvious for all, different projects coexist. The Capital District of Bogota wishes to develop this wetland in order to make a simply social use of it (creation of an entertaining park), while the other part wants to preserve natural space and its ecosystems out of the urban life. With the aim of making cancel the municipal project in front of a court, the residents built a sales leaflet around the concept of inheritance. Thanks to the fact that some residents have different representations of the Cordoba’s wetland, the urban natural spaces are not any more simple entertaining spaces, but have a real environmental function

Interaction fluide-structure souple et legere, application aux voiliers

This thesis, devoted to simulations of sailboat sail, was initiated by K-Epsilon, acompany specialized in numerical computations for naval hydrodynamics, IRENav, the Frenchnaval academy laboratory and LHEEA from Ecole Centrale Nantes. In this context a finiteelement program was developed dedicated to computing sail membranes and sailboat structures.The program was coupled with an inviscid fluid solver. A more detailed modeling of the flow andinteraction was realized by implementing a coupling with a fluid solver code which solves theReynolds Averaged Navier-Stokes Equations, developed by the DSPM team from LHEEA. Forthe coupling it was necessary to look at the interface over which a transfer of variables betweenthe fluid and structure occurs. Another key consideration was the deformation of the fluid solversmesh. The part has been revisited and extended to reach the development of a fast, robust, andparallelized method to treat the considered deformations. For good solution convergence andstability properties an iterative, partitioned algorithm that relies on an approximation of theinterface’s Jacobian evaluated by the inviscid code and integrated in the structure’s equationswas used. Finally, applications employing these methodologies are presented. Comparisons weremade with an instrumented sailboat. A second experiment of an oscillating cloth was developedto validate the case of interaction of a fluid with a light and flexible structure. Results were usedto validate the RANSE solver coupling. A more realistic calculation was also conducted on anunsteady sailing spinnaker with an automatic trimming algorithm, showing the potential of thepresent coupling.Cette th`ese CIFRE qui s’int´eresse `a la simulation des voiles de bateaux, a ´et´einiti´ee par la soci´et´e k-Epsilon, entreprise sp´ecialis´ee dans le calcul num´erique en hydrodynamiquenavale, l’IRENav, laboratoire de l’Ecole Navale et le LHEEA de l’Ecole Centrale Nantes.Dans ce contexte, un logiciel d´edi´e au calcul structure a ´et´e d´evelopp´e afin de simuler au moyend’´el´ements finis les membranes des voiles et les structures des voiliers. Ce code a ensuite ´et´ecoupl´e `a un solveur fluide parfait, puis `a un code fluide r´esolvant les ´equations de Navier-Stokesen moyenne de Reynolds d´evelopp´e par l’´equipe DSPM du LHEEA. Pour cela, il a ´et´e n´ecessairede s’int´eresser `a l’interface, o`u s’op`ere le transfert de variables entre le fluide et la structure.Autre point-cl´e, la partie d´eformation de maillage fluide a aussi ´et´e revisit´ee et ´etendue pouraboutir au d´eveloppement d’une m´ethode rapide, robuste et parall`ele permettant de traiter lesd´eformations envisag´ees. Pour obtenir de bonnes propri´et´es de convergence et de stabilit´e, l’algorithmepartitionn´e et it´eratif s’appuie de plus sur une approximation du Jacobien de l’interface´evalu´e par l’approche fluide parfait et int´egr´e au code structure. Enfin, des applications mettanten oeuvre ces m´ethodologies sont pr´esent´ees. Des comparaisons sont effectu´ees avec un voilierinstrument´e. Une seconde exp´erience, la voile oscillante, est mise au point pour valider les casd’interaction fluide-structure souple et l´eg`ere. Les r´esultats ont permis de valider le couplage avecun solveur RANSE. Un calcul plus r´ealiste a aussi ´et´e men´e en instationnaire sur un spinnakerde voilier avec un algorithme de r´eglage

Building possibility distribution based on confidence intervals of parameters of Gaussian mixtures

International audienceIn parametric methods, building a probability distribution from data requires an a priori knowledge about the shape of the distribution. Once the shape is known, we can estimate the optimal parameters value from the data set. However, there is always a gap between the estimated parameters from the sample sets and true parameters, and this gap depends on the number of observations. Even if an exact estimation of parameters values might not be performed, confidence intervals for these parameters can be built. One interpretation of the quantitative possibility theory is in terms of families of probabilities that are upper and lower bounded by the associated possibility and necessity measure. In this paper, we assume that the data follow a Gaussian distribution, or a mixture of Gaussian distributions. We propose to use confidence interval parameters (computed from a sample set of data) in order to build a possibility distribution that upper approximate the family of probability distributions whose parameters are in the confidence intervals. Starting from the case of a single Gaussian distribution, we extend our approach to the case of Gaussian mixture models