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Symmetric and asymmetric shocked gas jets for laser-plasma experiments
No full textInternational audienceShocks in supersonic flows offer both high density and sharp density gradients that are used, for instance, for gradient injection in laser-plasma accelerators. We report on a parametric study of oblique shocks created by inserting a straight axisymmetric section at the end of a supersonic “de Laval” nozzle. The effect of different parameters, such as the throat diameter and straight section length on the shock position and density, is studied through computational fluid dynamics (CFD) simulations. Experimental characterizations of a shocked nozzle are compared to CFD simulations and found to be in good agreement. We then introduce a newly designed asymmetric shocked gas jet, where the straight section is only present on one lateral side of the nozzle, thus providing a gas profile well adapted for density transition injection. In this case, full-3D fluid simulations and experimental measurements are compared and show excellent agreement
A stable, unified model for resonant Faraday cages
No full textInternational audienceWe study some effective transmission conditions able to reproduce the effect of a periodic array of Dirichlet wires on wave propagation, in particular when the array delimits an acoustic Faraday cage able to resonate. In the study of Hewett & Hewitt (2016 Proc. R. Soc. A 472 , 20160062 ( doi:10.1098/rspa.2016.0062 )) different transmission conditions emerge from the asymptotic analysis whose validity depends on the frequency, specifically the distance to a resonance frequency of the cage. In practice, dealing with such conditions is difficult, especially if the problem is set in the time domain. In the present study, we demonstrate the validity of a simpler unified model derived in Marigo & Maurel (2016 Proc. R. Soc. A 472 , 20160068 ( doi:10.1098/rspa.2016.0068 )), where unified means valid whatever the distance to the resonance frequencies. The effectiveness of the model is discussed in the harmonic regime owing to explicit solutions. It is also exemplified in the time domain, where a formulation guaranteeing the stability of the numerical scheme has been implemented
Simultaneous up- and down-frequency mixing based on a cascaded SOA-MZIs link
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1D/3D Finite-Volume coupling in conjunction with beam/shell elements coupling for fast transients in pipelines with fluid–structure interaction
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Mixed Integer Nonlinear Approaches for the Satellite Constellation Design Problem
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Revisiting imperfect interface laws for two-dimensional elastodynamics
No full textInternational audienceWe study the interaction of in-plane elastic waves with imperfect interfaces composed of a periodic array of voids or cracks. An effective model is derived from high-order asymptotic analysis based on two-scale homogenization and matched asymptotic technique. In two-dimensional elasticity, we obtain jump conditions set on the in-plane displacements and normal stresses; the jumps involve in addition effective parameters provided by static, elementary problems being the equivalents of the cell problems in classical two-scale homogenization. The derivation of the model is conducted in the transient regime and its stability is guarantied by the positiveness of the effective interfacial energy. Spring models are envisioned as particular cases. It is shown that massless-spring models are recovered in the limit of small void thicknesses and collinear cracks. By contrast, the use of mass-spring model is justified at normal incidence, otherwise unjustified. We provide quantitative validations of our model and comparison with spring models by means of comparison with direct numerical calculations in the harmonic regime
Outer Space Doesn't Exist: On the Theory and Practice of Studying Astroculture
No full textInternational audienceThe central thesis of this paper is that outer space doesn’t exist. This assertion-- mostly articulated in terms borrowed from Markus Gabriel's New Realism--may seem crazy or absurd, but it is rather less radical than it sounds, in no way involving the denial of the reality of the Moon, Mars, and the stars, and only insisting that the real location of outer space is in culture. The obvious importance of this claim for the study of astroculture is clear, if at first it may seem tantamount to denying the possibility or the legitimacy of the study of astroculture as it has been carried out up to the present. Yet recognizing that outer space doesn’t exist both legitimates the study of astroculture and sheds light on the nature of this endeavor. It also helps to see the limitations of outer space as a concept for thinking critically about the social, economic and ecological implications of the new space economy, and so clarifies why we may need to develop alternative and resolutely realist approach to the study of astroculture
EpidemiOptim: a Toolbox for the Optimization of Control Policies in Epidemiological Models
No full textInternational audienceModelling the dynamics of epidemics helps proposing control strategies based on phar-maceutical and non-pharmaceutical interventions (contact limitation, lock down, vaccina-tion, etc). Hand-designing such strategies is not trivial because of the number of pos-sible interventions and the difficulty to predict long-term effects. This task can be castas an optimization problem where state-of-the-art machine learning algorithms such asdeep reinforcement learning, might bring significant value. However, the specificity ofeach domain – epidemic modelling or solving optimization problem – requires strong col-laborations between researchers from different fields of expertise. This is why we intro-duce EpidemiOptim, a Python toolbox that facilitates collaborations between researchersin epidemiology and optimization. EpidemiOptim turns epidemiological models and costfunctions into optimization problems via a standard interface commonly used by optimiza-tion practitioners (OpenAI Gym). Reinforcement learning algorithms based on Q-Learningwith deep neural networks (dqn) and evolutionary algorithms (nsga-ii) are already im-plemented. We illustrate the use of EpidemiOptim to find optimal policies for dynamicalon-off lock-down control under the optimization of death toll and economic recess using aSusceptible-Exposed-Infectious-Removed (seir) model for COVID-19. Using EpidemiOp-tim and its interactive visualization platform in Jupyter notebooks, epidemiologists, op-timization practitioners and others (e.g. economists) can easily compare epidemiologicalmodels, costs functions and optimization algorithms to address important choices to bemade by health decision-makers. Trained models can be explored by experts and non-experts via a web interface
Direct-Sequence Spread Spectrum with Signal Space Diversity for High Resistance to Jamming
No full textInternational audienceJamming attacks can severely limit wireless networks availability and can cause serious damage, in particular for tactical applications. Over the past decades, Direct-Sequence Spread Spectrum (DSSS) has been used to enhance resistance to jamming. In this paper, we first analyze the performance of the DSSS modulation in the presence of malicious jamming; we take into account by considering different physical phenomena such as a large Doppler shift and we use at the receiver side robust synchronization algorithms. We then propose to consider jointly rotated constellations and the DSSS technique in order to enhance robustness against jamming, while keeping reasonable complexity. Simulations results underline the good performance of our proposal as it shows a gain of several dBs compared to the DSSS technique with conventional constellations