Supersonic turbulence in 3D isothermal flow collision

Colliding supersonic bulk flows shape observable properties and internal physics of various astrophysical objects, like O-star winds, molecular clouds, galactic sheets, binaries, or gamma-ray bursts. Using numerical simulations, we show that the bulk flows leave a clear imprint on the collision zone, its mean properties and the turbulence it naturally develops. Our model setup consists of 3D head-on colliding isothermal hydrodynamical flows with Mach numbers between 2 and 43. Simulation results are in line with expectations from self-similarity: root mean square Mach numbers (Mrms) scale linearly with upstream Mach numbers, mean densities remain limited to a few times the upstream density. The density PDF is not log-normal. The turbulence is inhomogeneous: weaker in the zone center than close to the confining shocks. It is anisotropic: while Mrms is generally supersonic, Mrms transverse to the upstream flow is always subsonic. We argue that uniform, isothermal, head-on colliding flows generally disfavor isotropic, supersonic turbulence. The anisotropy carries over to other quantities like the density variance - Mach number relation. Structure functions differ depending on whether they are computed along a line-of-sight perpendicular or parallel to the upstream flow. We suggest that such line-of-sight effects should be kept in mind when interpreting turbulence characteristics derived from observations.Comment: 20 pages, 14 figures, 4 tables, accepted by Astronomy and Astrophysic

Properties of Cosmological Filaments extracted from Eulerian Simulations

Using a new parallel algorithm implemented within the VisIt framework, we analysed large cosmological grid simulations to study the properties of baryons in filaments. The procedure allows us to build large catalogues with up to $\sim 3 \cdot 10^4$ filaments per simulated volume and to investigate the properties of cosmic filaments for very large volumes at high resolution (up to $300^3 ~\rm Mpc^3$ simulated with $2048^3$ cells). We determined scaling relations for the mass, volume, length and temperature of filaments and compared them to those of galaxy clusters. The longest filaments have a total length of about $200 ~\rm Mpc$ with a mass of several $10^{15} M_{\odot}$. We also investigated the effects of different gas physics. Radiative cooling significantly modifies the thermal properties of the warm-hot-intergalactic medium of filaments, mainly by lowering their mean temperature via line cooling. On the other hand, powerful feedback from active galactic nuclei in surrounding halos can heat up the gas in filaments. The impact of shock-accelerated cosmic rays from diffusive shock acceleration on filaments is small and the ratio of between cosmic ray and gas pressure within filaments is of the order of $\sim 10-20$ percent.Comment: 27 pages, 24 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journa

Megamodelling and Etymology

Is a model of a model, a metamodel? Is the relational model a metamodel? Is it a model? What is a component metamodel? Is it a model of a component model? The word MODEL is subject to a lot of debates in Model Driven Engineering. Add the notion of metamodel on top of it and you will just enter what some people call the Meta-muddle. Recently megamodels have been proposed to avoid the meta-muddle. This approach is very promising but it does not solve however the primary problem. That is, even a simple use of the word Model could lead to misunderstanding and confusion. This paper tackles this problem from its very source: the polysemic nature of the word MODEL. The evolution and semantic variations of the word MODEL are modelled from many different perspectives. This papers tells how the prefix MED in indo-european has lead, five millenniums after, to the acronym MDE, and this via the word MODEL. Based on an extensive study of encyclopedias, dictionaries, thesauri, and etymological sources, it is shown that the many senses of the word MODEL can be clustered into four groups, namely model-as-representation, model-as-example, model-as-type, and model-as-mold. All these groups are fundamental to understand the real nature of Model Driven Engineering. Megamodels and Etymology are indeed keys to avoid the Meta-muddle.on

Real 3D-packaging: a breakthrough for high power electronics for automotive applications

Power electronics stands at the crossroad of highly convoluted disciplines of physics: thermal science, mechanics and electronics. In order to respond to the growing and demanding automotive market, designers have no choice but to assemble off-the-shelf parts, keeping the onus of managing themselves thermal losses and stray inductances at interfaces. Power mechatronics is the buzz word for this. It has for long envisioned double-sided cooling and wire-bondless interconnections as ways to go. Merging those impediments and opportunities into a global solution, aPSI3D has developed an integrated approach for multi-physical co-design with its customers, offering both plug-in cooled power modules and maximum value transfer to the inverter. Eventually, a 50 power module with imbedded cold plate size reduction for a 100kW inverter is obtained vs. state-of-the art pin fin standard products. With a 10nH stray inductance, this solution is paving the way to more than 100A wide bandgap semiconductor die based highly efficient power modules

Relativistic magnetic reconnection in collisionless ion-electron plasmas explored with particle-in-cell simulations

Magnetic reconnection is a leading mechanism for magnetic energy conversion and high-energy non-thermal particle production in a variety of high-energy astrophysical objects, including ones with relativistic ion-electron plasmas (e.g., microquasars or AGNs) - a regime where first principle studies are scarce. We present 2D particle-in-cell (PIC) simulations of low $\beta$ ion-electron plasmas under relativistic conditions, i.e., with inflow magnetic energy exceeding the plasma rest-mass energy. We identify outstanding properties: (i) For relativistic inflow magnetizations (here $10 < \sigma_e < 360$), the reconnection outflows are dominated by thermal agitation instead of bulk kinetic energy. (ii) At large inflow electron magnetization ($\sigma_e > 80$), the reconnection electric field is sustained more by bulk inertia than by thermal inertia. It challenges the thermal-inertia-paradigm and its implications. (iii) The inflows feature sharp transitions at the entrance of the diffusion zones. These are not shocks but results from particle ballistic motions, all bouncing at the same location, provided that the thermal velocity in the inflow is far smaller than the inflow E cross B bulk velocity. (iv) Island centers are magnetically isolated from the rest of the flow, and can present a density depletion at their center. (v) The reconnection rates are slightly larger than in non-relativistic studies. They are best normalized by the inflow relativistic Alfv\'en speed projected in the outflow direction, which then leads to rates in a close range (0.14-0.25) thus allowing for an easy estimation of the reconnection electric field.Comment: Submitted to A&

The energetics of relativistic magnetic reconnection: ion-electron repartition and particle distribution hardness

Collisionless magnetic reconnection is a prime candidate to account for flare-like or steady emission, outflow launching, or plasma heating, in a variety of high-energy astrophysical objects, including ones with relativistic ion-electron plasmas. But the fate of the initial magnetic energy in a reconnection event remains poorly known: what is the amount given to kinetic energy, the ion/electron repartition, and the hardness of the particle distributions? We explore these questions with 2D particle-in-cell simulations of ion-electron plasmas. We find that 45 to 75% of the total initial magnetic energy ends up in kinetic energy, this fraction increasing with the inflow magnetization. Depending on the guide field strength, ions get from 30 to 60% of the total kinetic energy. Particles can be separated into two populations that only weakly mix: (i) particles initially in the current sheet, heated by its initial tearing and subsequent contraction of the islands; and (ii) particles from the background plasma that primarily gain energy via the reconnection electric field when passing near the X-point. Particles (ii) tend to form a power-law with an index $p=-d\log n(\gamma)/d\log\gamma$, that depends mostly on the inflow Alfv\'en speed $V_A$ and magnetization $\sigma_s$ of species $s$, with for electrons $p=5$ to $1.2$ for increasing $\sigma_e$. The highest particle Lorentz factor, for ions or electrons, increases roughly linearly with time for all the relativistic simulations. This is faster, and the spectra can be harder, than for collisionless shock acceleration. We discuss applications to microquasar and AGN coronae, to extragalactic jets, and to radio lobes. We point out situations where effects such as Compton drag or pair creation are important.Comment: 15 pages, submitted to A&

Intégration des élèves avec autisme en classe ordinaire :: que se passe-t-il en réalité ?

Cette recherche étudie l’intégration des enfants atteints d’autisme dans le cadre scolaire ordinaire. Cette intégration leur permet de bénéficier de nombreux avantages que ce soit au niveau des interactions sociales et des apprentissages dans de multiples activités. Mais c’est également mettre un enfant atteint d’autisme dans un environnement empli de stimulations et à l’intérieur duquel il aura de la difficulté à prendre les informations, à les traiter, à décoder des regards et à comprendre les règles de la communauté. Au travers de ce travail, je cherche à savoir si le cadre que l’on offre, actuellement, aux élèves atteints d’autisme répond à leurs besoins (aménagements, soutiens, formations des enseignants ordinaires). Pour y parvenir j’ai effectué des observations de deux périodes de 45 minutes dans chacune des trois classes cibles et trois entretiens d’une trentaine de minutes avec chaque enseignante pour pouvoir les analyser. Ces trois classes proviennent de l’ensemble du canton de Neuchâtel, représentant chacune un cycle différent (1, 2 et 3) et accueillant un élève atteint d’autisme. Après le remplissage des grilles d’observation en fonction d’items (aménagements, interactions) et de l’enregistrement de mes entretiens par rapport à divers thèmes (représentation de l’intégration, les pratiques et partenaires externes), j’ai pu analyser ces résultats. L’analyse consiste en une comparaison entre les différentes enseignantes et le cadre théorique qui m’a, ensuite, permis de prendre position quant à l’intégration des enfants atteints d’autisme