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Performance prediction of a hydrofoil near the free surface using low (BEM) and high (RANS) fidelity methods
No full textInternational audienceAs a first step toward a multi-fidelity optimization tool for hydrofoils, the present work assesses the ability of the in-house code PUFFIn to be used as a “low-fidelity” solver within the multi-fidelity framework. The code, based on the Boundary Element Method (BEM) and the potential flow theory, is used to study the performance of a typical windsurf hydrofoil operating near the free surface. The hydrofoil is composed of a front wing and a rear stabilizer in a plane-like configuration. Computations are performed for single body configurations (only one wing) and two-body configurations (front wing and stabilizer). First, three linearized models of the free surface are compared for the single front wing configuration with several values of the Froude number: the symmetry, anti-symmetry and Neumann-Kelvin conditions. The results show that for relatively high Froude number, the anti-symmetry and the Neumann-Kelvin conditions provide very similar forces. Then, the predictions of the BEM solver are compared with “high-fidelity” RANS computations, in terms of pressure drag and lift, pressure distribution on the hydrofoil and free surface elevation. Several Froude numbers and submergence depths are studied. The global lift and drag variations predicted by the BEM with the anti-symmetry and Neumann-Kelvin conditions on the single-body configurations are similar to the RANS predictions. For the two-body configurations, the Neumann-Kelvin condition outperforms the anti-symmetry condition. Based on the BEM/RANS comparison, the potential flow solver reveals to be a relevant tool for multi-fidelity optimization
Flow topology changes with bubbly flow around a circular cylinder
No full textInternational audienceVortex induced vibration (VIV) experienced during flow past a cylinder can reduce equipment performance and in some cases lead to failure. Previous studies have shown that the injection of bubbles in the flow over a cylinder typically leads to a monotonic increase in shedding frequency with void fraction, however, a satisfactory explanation for this phenomenon has not been proposed. Unexplained scatter in the data exists, including that the increase in shedding frequency is not universal. More research is needed to characterize the influence of bubbles on the wake structure, and subsequent shift in shedding frequency. To this aim, the effect of bubbles on the structure of the wake and VIV was examined over two values of Reynolds number, and 160,000. Time-resolved particle image velocimetry (TR-PIV), proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) of the wake structures, vibration of the cylinder, and bubble image velocimetry (BIV) were used to assess the flow topology changes under the influence of gas injection. Using SPOD/POD analysis in the near wake, it was found that the primary Karman shedding frequency decreased with the injection of gas, from a Strouhal number of St = 0.2 to St = ; the width of the spectral peak was found to increase with void fraction. Notably, the vibration of the cylinder at the primary Karman shedding frequency was suppressed following the injection of gas, even at spanwise-averaged volumetric qualities below 0.01%. This suppression occurred regardless of if gas was concentrated locally near the centerline of the channel, or along the span. BIV data suggests that gas accumulation in the near wake, driven by the high velocity vertical motion of gas, serves to uncouple the cylinder motion from the formation of the vortex street downstream while promoting faster wake recovery
Simulation et analyse des équations de Maxwell avec changement de signe dans un plasma froid
No full textNowadays, plasmas are mainly used for industrial purpose. One of the most frequently cited examples of industrial use is electric energy production via fusion nuclear reactors. Then, in order to contain plasma properly inside the reactor, a background magnetic field is imposed, and the density and temperature of the plasma must be precisely controlled. This is done by sending electromagnetic waves at specific frequencies and directions depending on the characteristics of the plasma.The first part of this PhD thesis consists in the study of the model of plasma in a strong background magnetic field, which corresponds to a hyperbolic metamaterial. The objective is to extend the existing results in 2D to the 3D-case and to derive a radiation condition. We introduce a splitting of the electric and magnetic fields resembling the usual TE and TM decomposition, then, it gives some results on the two resulting problems. The results are in a very partial state, and constitute a rough draft on the subject.The second part consists in the study of the degenerate PDE associated to the lower-hybrid resonant waves in plasma. The associated boundary-value problem is well-posed within a ``natural'' variational framework. However, this framework does not include the singular behavior presented by the physical solutions obtained via the limiting absorption principle. Notice that this singular behavior is important from the physical point of view since it induces the plasma heating mentioned before. One of the key results of this second part is the definition of a notion of weak jump through the interface inside the domain, which allows to characterize the decomposition of the limiting absorption solution into a regular and a singular parts.De nos jours, les plasmas sont principalement utilisés à des fins industrielles. L'un des exemples les plus fréquemment cités d'utilisation industrielle est la production d'énergie électrique via des réacteurs nucléaires à fusion. Pour contenir le plasma correctement à l'intérieur du réacteur, un champ magnétique est imposé en arrière-plan, et la densité et la température du plasma doivent être précisément contrôlées. Cela est effectué en envoyant des ondes électromagnétiques à des fréquences et dans des directions spécifiques en fonction des caractéristiques du plasma.La première partie de cette thèse de doctorat est consacrée à l'étude du modèle du plasma avec un fort champ magnétique en arrière-plan, ce qui correspond à un métamatériau hyperbolique. L'objectif est d'étendre les résultats existant en 2D au cas 3D et de dériver une condition de radiation. Nous introduisons une séparation des champs électriques et magnétiques ressemblant à la décomposition TE et TM habituelle, puis nous présentons quelques résultats sur les deux problèmes résultants. Les résultats sont dans un état très partiel et constituent un brouillon approximatif sur le sujet.La deuxième partie étudie l'EDP dégénérée associée aux ondes résonantes « lower-hybrid » dans le plasma. Le problème aux limites associé est bien posé dans un cadre variationnel « naturel ». Cependant, ce cadre n'inclut pas le comportement singulier présenté par les solutions physiques obtenues via le principe d'absorption limite. Ce comportement singulier est important du point de vue physique car il induit le chauffage du plasma mentionné précédemment. Un des résultats clés de cette deuxième partie est la définition d'une notion de saut à travers l'interface à l'intérieur du domaine, ce qui permet de caractériser la décomposition de la solution d'absorption limite en parties régulière et singulière
Bark cloth structure and properties: A naturally occurring fabric and ancestral textile craft from Uganda
No full textInternational audienceNatural fabrics from two bark trees of Uganda (“Kilundu”: Antiaris toxicaria and “Mutuba”: Ficus natalensis) are being considered today for the local footwear industry. Here, we thoroughly examined to determine their structural, biochemical and mechanical properties of the two bark cloths, in order to ascertain their potential use as locally-available, low-cost composite reinforcements and eventually for a use in textile industry as well. Fibres in both fabrics are rich in highly crystalline cellulose (76.6 ± 4.9% and 66.3 ± 0.5%) and have very cohesive cell walls with small lumen size and stiff middle lamellae. However, their low indentation modulus is due to a high microfibrillar angle, penalizing their longitudinal stiffness but making them interesting for impact or deformation performance. The two fabrics exhibit layers of preferred fibres orientations, quasi similar to unidirectional or +/-45° preforms, and hence may be used “as produced” as polymer reinforcements in technical composites but also in textile sector, in place of leather. Finally, their water uptake behaviour (21.2%-wt for Mutuba and 13.8%-wt for Kilundu) indicates opportunities for a range of environmentally-sensitive applicationsNatural fabrics from two bark trees of Uganda (“Kilundu”: Antiaris toxicaria and “Mutuba”: Ficus natalensis) are being considered today for the local footwear industry. Here, we thoroughly examined to determine their structural, biochemical and mechanical properties of the two bark cloths, in order to ascertain their potential use as locally-available, low-cost composite reinforcements and eventually for a use in textile industry as well. Fibres in both fabrics are rich in highly crystalline cellulose (76.6 ± 4.9% and 66.3 ± 0.5%) and have very cohesive cell walls with small lumen size and stiff middle lamellae. However, their low indentation modulus is due to a high microfibrillar angle, penalizing their longitudinal stiffness but making them interesting for impact or deformation performance. The two fabrics exhibit layers of preferred fibres orientations, quasi similar to unidirectional or +/-45° preforms, and hence may be used “as produced” as polymer reinforcements in technical composites but also in textile sector, in place of leather. Finally, their water uptake behaviour (21.2%-wt for Mutuba and 13.8%-wt for Kilundu) indicates opportunities for a range of environmentally-sensitive applications
A Deep Split-Step Wavelet Model for the Long-Range Propagation
No full textInternational audienceThis article presents a new approach based on a deep-learning method using a U-Net architecture to generate electromagnetic propagation over a specific terrain. For this purpose, the learning dataset is constructed artificially using a fast split-step wavelet (SSW) method. For this phase, the synthetic 1D profiles are randomly generated from rectangle and triangle shapes. This latter allows for conveying the “staircase” model used in SSW. To ensure a precise sampling of the underlying manifold, the study employs Latin Hypercube Sampling. To achieve robust and precise predictions, a specific loss function is proposed. To evaluate this approach, numerical tests are realized. These tests demonstrate the effectiveness of the proposed method in realistic terrain
Extending femtosecond laser superfilamentation in air with a multifocal phase mask
No full textInternational audienceLaser filamentation is a spectacular phenomenon where the self-focusing of the laser pulse generates ionizing light channels. Many applications of filamentation, such as the laser lightning rod, require the generation of superfilaments, long plasma channels of higher electron density than normal filaments. Using a multifocal phase mask, we demonstrate an extension of the superfilamentation length of a focused terawatt laser beam. Optimized superfilaments show increased energy deposition compared to a normal gaussian beam and an extension of their length by at least a factor two. When put in contact with a high voltage electrode, the guiding of a single plasma column with a length of ∼1 m is observed. The length of an air waveguide generated by a vortex laser pulse is also increased by a factor 2 in the presence of the phase mask
Crouzeix-Raviart elements on simplicial meshes in dimensions
No full textIn this paper we introduce Crouzeix-Raviart elements of general polynomialorder and spatial dimension for simplicial finite element meshes.We give explicit representations of the non-conforming basis functions andprove that the conforming companion space, i.e., the conforming finite elementspace of polynomial order is contained in the Crouzeix-Raviart space. Weprove a direct sum decomposition of the Crouzeix-Raviart space into (asubspace of) the conforming companion space and the span of the non-conformingbasis functions.Degrees of freedom are introduced which are bidual to the basis functions andgive rise to the definition of a local approximation/interpolation operator.In two dimensions or for , these freedoms can be split into simplex and dimensional facet integrals in such a way that, in abasis representation of Crouzeix-Raviart functions, all coefficients whichbelong to basis functions related to lower-dimensional faces in the mesh aredetermined by these facet integrals. It will also be shown that such a set ofdegrees of freedom does not exist in higher space dimension and k>1
Réseau d'Antennes à Polarisation Circulaire et à Forte Isolation Pour Liaisons "In-Band Full-Duplex" Pour CubeSats en Bande Ka
No full textInternational audienc
Security Contracts a Property-Based Approach to Support Security Patterns
No full textInternational audienceSecurity patterns represent reusable solutions and best practices intended to avoid security-related flaws in software and system designs. Unfortunately, the implementation and enforcement of these patterns remains a complex and error-prone task. As a consequence, and besides implementing a given security pattern, applications often remain insecure w.r.t. the security risk they intended to tackle. This is so for two main reasons: 1) patterns are rarely re-usable without adaptation, and thus concrete implementations may fail to deal with a number of (often implicit) properties, which must hold in order for the pattern to be effective; 2) patterns are deployed in environments with uncertainties that can only be known at runtime. In order to deal with this problem, we propose here Security Contracts, a framework that permits the specification and runtime monitoring of security patterns and related properties (including temporal ones) in both new and existing applications. It is based on an extension of the Design-by-Contract paradigm to enable the specification of security patterns and the runtime adaptation of applications. We demonstrate the feasibility of our approach with an implementation and its evaluation on a framework used worldwide in web technologies, Spring
ABOUT SEMILINEAR LOW DIMENSION BESSEL PDEs
No full textWe prove existence and uniqueness of solutions of a semilinear PDE driven by a Bessel type generator with low dimension . is a local operator, whose drift is thederivative of :in particular it is a Schwartz distribution, whichis not the derivative of a continuous function.The solutions are intended in a duality ("weak") sensewith respect to state space being an invariant measure for the Bessel semigroup