195 research outputs found
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
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
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 ), the reconnection outflows are dominated by thermal agitation instead of
bulk kinetic energy. (ii) At large inflow electron magnetization (), 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 , that
depends mostly on the inflow Alfv\'en speed and magnetization
of species , with for electrons to for increasing .
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&
Apar-T: code, validation, and physical interpretation of particle-in-cell results
We present the parallel particle-in-cell (PIC) code Apar-T and, more
importantly, address the fundamental question of the relations between the PIC
model, the Vlasov-Maxwell theory, and real plasmas.
First, we present four validation tests: spectra from simulations of thermal
plasmas, linear growth rates of the relativistic tearing instability and of the
filamentation instability, and non-linear filamentation merging phase. For the
filamentation instability we show that the effective growth rates measured on
the total energy can differ by more than 50% from the linear cold predictions
and from the fastest modes of the simulation.
Second, we detail a new method for initial loading of Maxwell-J\"uttner
particle distributions with relativistic bulk velocity and relativistic
temperature, and explain why the traditional method with individual particle
boosting fails.
Third, we scrutinize the question of what description of physical plasmas is
obtained by PIC models. These models rely on two building blocks:
coarse-graining, i.e., grouping of the order of p~10^10 real particles into a
single computer superparticle, and field storage on a grid with its subsequent
finite superparticle size. We introduce the notion of coarse-graining dependent
quantities, i.e., quantities depending on p. They derive from the PIC plasma
parameter Lambda^{PIC}, which we show to scale as 1/p. We explore two
implications. One is that PIC collision- and fluctuation-induced thermalization
times are expected to scale with the number of superparticles per grid cell,
and thus to be a factor p~10^10 smaller than in real plasmas. The other is that
the level of electric field fluctuations scales as 1/Lambda^{PIC} ~ p. We
provide a corresponding exact expression.
Fourth, we compare the Vlasov-Maxwell theory, which describes a phase-space
fluid with infinite Lambda, to the PIC model and its relatively small Lambda.Comment: 24 pages, 14 figures, accepted in Astronomy & Astrophysic
Complement depletion during haemofiltration with polyacrilonitrile membranes
Background Polyacrylonitrile (PAN, AN69®) dialysis membranes have been shown to improve the outcome of critically ill patients. Factor D is an essential enzyme of the alternative pathway of complement and is increased during renal failure. On the other hand the contact of blood with biomaterials activates the complement cascade through the alternative pathway. PAN filters adsorb factor D which looses its enzymatic activity whilst bound to the membrane [1] the complement alternative pathway function of serum exposed to PAN filters is greatly diminished and restored after addition of purified factor D [1]. The aim of our study was to measure the time course of factor D adsorption and its blood concentration during CVVH in critically ill patients with acute renal failure. Methods We studied seven critically ill patients with ARF before, during and after continuous veno-venous haemofiltration (CVVH) with AN69. Results There was a rapid decrease of factor D levels to 62(±6%) of the pre-CVVH value during the first 2 h, which continued to 51(±7.3%) after 12 h; at 24 h there was a slight rise to 62±12%. Sequential use of Polyacrylonitrile (AN69®) filters lowered factor D levels below the normal plasma concentration in three patients, thus producing a state of factor D depletion. Conclusions The significant reduction of factor D levels during CVVH with PAN filters suggests that frequent changes of PAN filters may reduce alternative pathway function by lowering factor D levels. CVVH (as opposed to intermittent dialysis) with PAN membranes may further improve the outcome of critically ill patient
Advanced visualization of large datasets for Discrete Element Method simulations
State-of-the-art Discrete Element Method (DEM) simulations of granular flows produce large datasets that contain a wealth of information describing the time-dependent physical state of the particulate medium. To extract this information, both comprehensive and efficient post-processing methods are essential. Special attention must be paid to the interactive visualization of these large hybrid datasets containing both particle-based and surface-based data. In this paper, we report the use of the open-source visualization package ParaView, which we have customized specifically to perform advanced techniques for the post-treatment of large DEM datasets. Particular attention is given to the method used to render the individual particles, based either on triangulation of glyphs or using GPU-accelerated primitives. A demonstration of these techniques, and their relative merits when applied to the visualization of DEM datasets, is presented via their application to real industrial examples
GMF: A Model Migration Case for the Transformation Tool Contest
Using a real-life evolution taken from the Graphical Modeling Framework, we
invite submissions to explore ways in which model transformation and migration
tools can be used to migrate models in response to metamodel adaptation.Comment: In Proceedings TTC 2011, arXiv:1111.440
Simulation des écoulements à surface libre dans les turbines Pelton par une méthode hybride SPH-ALE
International audienceAn Arbitrary Lagrange Euler (ALE) description of fluid flows is used together with the meshless numerical method Smoothed Particle Hydrodynamics (SPH) to simulate free surface flows. The ALE description leads to an hybrid method that can be closely connected to the finite volume approach. It is then possible to adapt some common techniques like upwind schemes and preconditioning to remedy some of the well known drawbacks of SPH like stability and accuracy. An efficient boundary treatment based on a proper upwinding of fluid information at the boundary surface is settled. The resulting SPH-ALE numerical method is applied to simulate free surface flows encountered in Pelton turbines.La méthode numérique sans maillage Smoothed Particle Hydrodynamics (SPH) est modifiée par l'adoption d'une description Arbitrary Lagrange Euler (ALE) des écoulements fluides, dans le but de simuler des écoulements à surface libre. Le formalisme ALE conduit à une méthode numérique hybride s'apparentant sur de nombreux points à une approche volumes finis. Il est alors possible d'adapter des techniques numériques courantes comme les schémas décentrés et le préconditionnement pour résoudre certains défauts majeurs de la méthode SPH, comme la stabilité numérique ou le manque de précision. Par ailleurs, le traitement des conditions limites est réalisé par un décentrement approprié des informations fluides sur les surfaces frontières. La méthode numérique SPH-ALE résultante est appliquée à la simulation d'écoulements à surface libre tels que ceux rencontrés dans les turbines Pelton
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