Normal mode analysis of fluid discontinuities: numerical method and application to magnetohydrodynamics

Fluid discontinuities, such as shock fronts and vortex sheets, can reflect waves and become unstable to corrugation. Analytical calculations of these phenomena are tractable in the simplest cases only, while their numerical simulations are biased by truncation errors inherent to discretization schemes. The author lays down a computational framework to study the coupling of normal modes (plane linear waves) through discontinuities satisfying arbitrary conservation laws, as is relevant to a variety of fluid mechanical problems. A systematic method is provided to solve these problems numerically, along with a series of validation cases. As a demonstration, it is applied to magnetohydrodynamic shocks and shear layers to exactly recover their linear stability properties. The straightforward inclusion of nonideal (dispersive, dissipative) effects notably opens a route to investigate how these phenomena are altered in weakly ionized plasmas.Comment: 7+3 pages, 6+2 figures, accepted for publication in Physics of Fluids (post language editing

Envelopes of embedded super-Earths - I. Two-dimensional simulations

Measurements of exoplanetary masses and radii have revealed a population of massive super-Earths --- planets sufficiently large that, according to one dimensional models, they should have turned into gas giants. To better understand the origin of these objects, we carry out hydrodynamical simulations of planetary cores embedded in a nascent protoplanetary disk. In this first paper of a series, to gain intuition as well as to develop useful diagnostics, we focus on two-dimensional simulations of the flow around protoplanetary cores. We use the pluto code to study isothermal and adiabatic envelopes around cores of sub- to super-thermal masses, fully resolving the envelope properties down to the core surface. Owing to the conservation of vortensity, envelopes acquire a substantial degree of rotational support when the core mass increases beyond the thermal mass, suggesting a limited applicability of one-dimensional models for describing the envelope structure. The finite size of the core (relatively large for super-Earths) also controls the amount of rotational support in the entire envelope. Steady non-axisymmetric shocks develop in the supersonic envelopes of high-mass cores, triggering mass accretion and turbulent mixing in their interiors. We also examine the influence of the gas self-gravity on the envelope structure. Although it only weakly alters the properties of the envelopes, the gas gravity has significant effect on the properties of the density waves triggered by the core in the protoplanetary disk

Envelopes of embedded super-Earths – II. Three-dimensional isothermal simulations

Massive planetary cores embedded in protoplanetary discs are believed to accrete extended atmospheres, providing a pathway to forming gas giants and gas-rich super-Earths. The properties of these atmospheres strongly depend on the nature of the coupling between the atmosphere and the surrounding disc. We examine the formation of gaseous envelopes around massive planetary cores via three-dimensional inviscid and isothermal hydrodynamic simulations. We focus the changes in the envelope properties as the core mass varies from low (sub-thermal) to high (super-thermal) values, a regime relevant to close-in super-Earths. We show that global envelope properties such as the amount of rotational support or turbulent mixing are mostly sensitive to the ratio of the Bondi radius of the core to its physical size. High-mass cores are fed by supersonic inflows arriving along the polar axis and shocking on the densest parts of the envelope, driving turbulence and mass accretion. Gas flows out of the core's Hill sphere in the equatorial plane, describing a global mass circulation through the envelope. The shell of shocked gas atop the core surface delimits regions of slow (inside) and fast (outside) material recycling by gas from the surrounding disc. While recycling hinders the runaway growth towards gas giants, the inner regions of protoplanetary atmospheres, more immune to mixing, may remain bound to the planet

Spiral structures in gravito-turbulent gaseous disks

CONTEXT. Gravitational instabilities can drive small-scale turbulence and large-scale spiral arms in massive gaseous disks under conditions of slow radiative cooling. These motions affect the observed disk morphology, its mass accretion rate and variability, and could control the process of planet formation via dust grain concentration, processing, and collisional fragmentation. AIMS. We study gravito-turbulence and its associated spiral structure in thin gaseous disks subject to a prescribed cooling law. We characterize the morphology, coherence, and propagation of the spirals and examine when the flow deviates from viscous disk models. METHODS. We used the finite-volume code PLUTO to integrate the equations of self-gravitating hydrodynamics in three-dimensional spherical geometry. The gas was cooled over longer-than-orbital timescales to trigger the gravitational instability and sustain turbulence. We ran models for various disk masses and cooling rates. RESULTS. In all cases considered, the turbulent gravitational stress transports angular momentum outward at a rate compatible with viscous disk theory. The dissipation of orbital energy happens via shocks in spiral density wakes, heating the disk back to a marginally stable thermal equilibrium. These wakes drive vertical motions and contribute to mix material from the disk with its corona. They are formed and destroyed intermittently, and they nearly corotate with the gas at every radius. As a consequence, large-scale spiral arms exhibit no long-term global coherence, and energy thermalization is an essentially local process. CONCLUSIONS. In the absence of radial substructures or tidal forcing, and provided a local cooling law, gravito-turbulence reduces to a local phenomenon in thin gaseous disks

Le hip hop : une expression mineure

Saisissant l’occasion ouverte par Deleuze et Guattari à propos du concept de « littérature mineure », l’article questionne le hip hop à l’aune de la perspective Guattaro-deleuzienne : déterritorialisation linguistique, immédiat politique, énonciation collective, tous les ingrédients d’une expression mineure semblent effectivement à l’œuvre dans la culture hip hop. Ce qui ne veut bien sûr pas dire qu’il s’agit d’une expression de second ordre, bien au contraire, mais d’une expression radicalement hors des normes établies, qui vient questionner les arts majeurs de façon « révolutionnaire »

Delphine Peiretti-Courtis, Corps noirs et médecins blancs. La fabrique du préjugé racial au xixe-xxe siècles

La collusion entre la science médicale et les idéologies raciales est une longue histoire, elle est à la mesure à la fois de l’incapacité d’admettre l’autre comme membre à part entière du genre humain du fait de ses apparentes différences, mais également de la volonté de s’approprier au meilleur compte, richesses et force de travail au nom d’une suprématie naturelle, pratiquement posée comme de droit divin. Dans la mesure où le racisme n’est ni moralement ou religieusement justifiable, ni hu..

Le franchissement de l’Atlantique

Il s’agit dans cette note de recherche de baliser un terrain encore en friche, celui du mode de passage des musiques afro-américaines — et plus particulièrement du rap — sur la rive orientale de l’Atlantique. Pour le rap ce voyage s’insère dans une perspective doublement originale. Contrairement à ce qui s’est passé pour le jazz, ce n’est pas une intelligentsia qui en a pris en charge le contenu mais une classe de laissés-pour-compte du système scolaire et de la culture. En outre, les éventuelles racines africaines dont se prévaut fantasmatiquement la culture afro-américaine se trouvent effectivement à l’œuvre chez ceux qui sur notre sol ont choisi d’assumer la culture hip-hop. Ces particularités donnent au franchissement de l’Atlantique par le rap une teneur bien particulière dont la présente note s’efforce de poser les linéaments