Portail HAL Ensta
Not a member yet
11080 research outputs found
Sort by
Tracking the changes into mechanical properties and ultrastructure of flax cell walls during a dynamic heating treatment
No full textInternational audienceThe impact of temperature on plant fibres is key point when processing plant fibre composite with thermoplastic resin. Structural and biochemical evolution of plant cell walls with temperature may affect their final mechanical properties. These evolutions have been studied in-situ on flax fibres during a dynamic heating treatment, representative of conventional polymer processing temperatures. Originals results were observed regarding the reversibility of the indentation modulus, hardness and cellulose crystalline structure
Modélisation, simulation et synthèse sonore de sources de bruit d'origine aérodynamique et vibratoire
No full textValidity of the effective sound speed approximation in parabolic equation models for wind turbine noise propagation
No full textInternational audienceParabolic equation (PE) based methods are widely used in outdoor acoustics because they can solve acoustic propagation problems above a mixed ground in a refractive and scattering atmosphere. However, recent research has shown phase error due to the effective sound speed approximation (ESSA). To overcome these limitations, a new PE formulation derived without the ESSA has been proposed recently. We investigate the impact of such phase error on wind turbine noise modeling, as the classical wide-angle parabolic equation (WAPE) with ESSA is widely used in the research community. We propose a comparison between the classical WAPE with ESSA and the new WAPE derived without the ESSA in the context of wind turbine noise. We highlight large phase error (several dB) on monochromatic calculations with a point source. Using an extended sound source representative of a wind turbine, we show small phase error (<1 dB) in a wind turbine noise context where sound level variability far from the source is of several dB. The validity of previous works using WAPE with ESSA is, thus, not questioned, although we do recommend the use of the new WAPE derived without the ESSA to accurately model the effect of wind speed on sound propagation
HeROfake: Heterogeneous Resources Orchestration in a Serverless Cloud – An Application to Deepfake Detection
No full textInternational audienceServerless is a trending service model for cloud computing. It shifts a lot of the complexity from customers to service providers. However, current serverless platforms mostly consider the provider's infrastructure as homogeneous, as well as the users' requests. This limits possibilities for the provider to leverage heterogeneity in their infrastructure to improve function response time and reduce energy consumption. We propose a heterogeneity-aware serverless orchestrator for private clouds that consists of two components: the autoscaler allocates heterogeneous hardware resources (CPUs, GPUs, FPGAs) for function replicas, while the scheduler maps function executions to these replicas. Our objective is to guarantee function response time, while enabling the provider to reduce resource usage and energy consumption. This work considers a case study for a deepfake detection application relying on CNN inference. We devised a simulation environment that implements our model and a baseline Knative orchestrator, and evaluated both policies with regard to consolidation of tasks, energy consumption and SLA penalties. Experimental results show that our platform yields substantial gains for all those metrics, with an average of 35% less energy consumed for function executions while consolidating tasks on less than 40% of the infrastructure's nodes, and more than 60% less SLA violations
Gas gun driven dynamic expansion of 3D-printed AlSi10Mg rings
No full textInternational audienceThis paper presents a new experimental setup to study dynamic fragmentation of metallic rings manufactured by 3D-printing technology. The ring is inserted over a ductile thin-walled tube which is impacted axially by a conical-nosed cylindrical projectile fired by a single-stage light–gas gun. The diameter of the projectile is larger than the internal diameter of the thin-walled tube, which expands as the projectile advances, pushing radially outwards the metallic ring, that eventually breaks into multiple fragments. The impact velocities considered in the present experimental campaign range from 197 m/s to 385 m/s. The AlSi10Mg ring specimens are printed by Selective Laser Melting technique, with an inner diameter of 14 mm and square cross section of 2 × 2 mm2. To obtain time-resolved information on the mechanics of fragmentation, the number of fractures, and the size of the fragments, the tests have been recorded with two-high speed cameras. A tunnel-shaped soft casing made of polymer foam is placed around the specimen to softly recover the ejected fragments, that have been weighted and sized to determine the statistics of the fragments size distribution. In addition, the predictions of the fragmentation theory of Kipp and Grady (1985) for the evolution of the number of fragments with the impact velocity have been compared with the experimental evidence, and an excellent quantitative agreement has been found within the whole range of loading rates investigated. Compared to other available techniques for dynamic fragmentation of rings, in which the expansion of the specimen is driven by electromagnetic or explosive loading, this experimental setup stands out for its simplicity, fast operation, quick assembly, and flexibility to test different engineering materials, which facilitates performing extensive experimental campaigns (34 successful tests have been carried out in this research). To the authors’ knowledge, this paper presents the most comprehensive data set to date on the fragmentation of dynamically expanded printed rings, including the first high-resolution video recordings of the formation of multiple cracks throughout the circumference of the samples, and scanning electron microscopy images of the fractures showing the porous microstructure on the cracks surface
Weak Dirichlet processes and generalized martingale problems
No full textInternational audienceIn this paper we explain how the notion of "weak Dirichlet process" is the suitable generalization of the one of semimartingale with jumps. For such a process we provide a unique decomposition: in particular we introduce "characteristics" for weak Dirichlet processes. We also introduce a weak concept (in law) of finite quadratic variation. We investigate a set of new useful chain rules and we discuss a general framework of (possibly path-dependent with jumps) martingale problems with a set of examples of SDEs with jumps driven by a distributional drift
Hydrofoil Optimization via Automated Multi-Fidelity Surrogate Models
No full textInternational audienceLifting hydrofoils are gaining importance, since they drastically reduce the wetted surface area of a ship hull, thus decreasing resistance. To attain efficient hydrofoils, the geometries can be obtained from an automated optimization process, based on simulations. However, hydrofoil high-fidelity simulations are computationally demanding, since fine meshes are needed to accurately capture the pressure field and the boundary layer on the hydrofoil. Simulation-based optimization can therefore be very expensive. Automated surrogate models, trained by a limited number of simulations, can reduce the required computational demand for the optimization by performing simulations where these are more informative. Furthermore, if an efficient low-fidelity hydrofoil performance prediction tool (with a low computational cost) is available, using surrogates in a multi-fidelity framework can provide a further reduction in required simulations, by combining the accuracy of a few high-fidelity with an exploration process based on a larger number of low-fidelity computations. In this study, we propose a hydrofoil optimization procedure based on two simulation codes, a dedicated hydrofoil potential flow solver for low-fidelity simulations and a RANS solver for both medium-and high-fidelity simulations. The RANS solver uses adaptive grid refinement to attain high accuracy with a limited computational budget. Two different multi-fidelity frameworks are compared for a parameterized geometric model of a realistic hydrofoil: only RANS based and potential-RANS based. The effect of different combinations of fidelity levels on the efficiency of the optimization and the performance of the kitefoil-type hydrofoil is investigated and discussed
Experimental Characterization of Internal Structure and Physical Properties of Unidirectional Ply‐Level Hybrid Carbon Composite Material
No full textInternational audienceThe complexities associated with the response of carbon fiber composites to different loading cases and methods of improving it without increasing their cost are still being researched. Herein, by an experimental approach, the ply-level hybridization technique for tailoring the composite response is explored, and two different types of unidirectional prepregs with the same fiber type but different quality and prices are mixed. The purpose of this study is to analyze the influence of this type of hybridization on the structure and mechanical response of carbon fiber composites and to assess the compromise between manufacturing cost reduction and mechanical response. Hybridization is obtained in both layer thickness and material quality and reference materials are also manufactured. The properties of the unidirectional hybrid and nonhybrid composites are evaluated by density and fiber volume fraction measurements. The in-plane mechanical properties are determined using quasistatic tensile tests, in 0° and 90° directions. No thickness effects are noticed on the stress–strain response under both longitudinal and transverse directions and hybrid composites exhibit an improved 0° failure stress
Some aspects of the pressure field preceding the onset of critical jets in a breaking wave
No full textInternational audienceWe consider in this article two-dimensional nonlinear free surface motions in potential theory. There are situations where the onset of critical jet along a free surface can be predicted by analyzing the temporal and spatial variation of the pressure itself and its successive derivatives. In particular, when a local maximum of pressure appears close enough to a free surface where the pressure vanishes, it is expected a great pressure gradient at the free surface and consequently a large Lagrangian acceleration. The present analysis examines this phenomenon. This analysis is facilitated after identifying a line along which the pressure gradient is parallel to one of the eigenvectors of the Hessian matrix of the pressure. In a standard overturning crest, this line connects the tip of the crest to the bottom of the tank in which the flow is simulated. The analysis of its time evolution gives much information into the onset of critical jets. Other types of critical jet can also appear when a region of positive Gaussian curvature of the pressure suddenly grows in the vicinity of the free surface
Spectral splitting of the lasing emission of nitrogen ions pumped by 800-nm femtosecond laser pulses
No full textInternational audienceWe report on a spectral splitting effect of the cavity-less lasing emission of nitrogen ions at 391.4 nm pumped by 800-nm femtosecond laser pulses. It was found that with the increase of the nitrogen gas pressure and pump pulse energy, both R and P branches experience spectral splitting. With an external injected seeding pulse, a similar split spectral line is observed for the amplified emission. In contrast, for the fluorescence radiation, no such spectral splitting phenomenon is observed with much more abundant R branch structures. Our theoretical model considers gas ionization by the pump pulse, the competition of excitation of all relevant electronic and vibrational states, and an amplification of the seeding pulse in the plasma with a population inversion. Our simulation reproduces this spectral splitting effect, which is attributed to the gain saturation resulting in the oscillation of the amplitude of the amplified signal