32,398 research outputs found
Simulation of an Axial Vircator
An algorithm of particle-in-cell simulations is described and tested to aid
further the actual design of simple vircators working on axially symmetric
modes. The methods of correction of the numerical solution, have been chosen
and jointly tested, allow the stable simulation of the fast nonlinear multiflow
dynamics of virtual cathode formation and evolution, as well as the fields
generated by the virtual cathode. The selected combination of the correction
methods can be straightforwardly generalized to the case of axially
nonsymmetric modes, while the parameters of these correction methods can be
widely used to improve an agreement between the simulation predictions and the
experimental data.Comment: 9 pages, 3 figure
Particle-Gas Dynamics with Athena: Method and Convergence
The Athena MHD code has been extended to integrates the motion of particles
coupled with the gas via aerodynamic drag, in order to study the dynamics of
gas and solids in protoplanetary disks and the formation of planetesimals. Our
particle-gas hybrid scheme is based on a second order predictor-corrector
method. Careful treatment of the momentum feedback on the gas guarantees exact
conservation. The hybrid scheme is stable and convergent in most regimes
relevant to protoplanetary disks. We describe a semi-implicit integrator
generalized from the leap-frog approach. In the absence of drag force, it
preserves the geometric properties of a particle orbit. We also present a
fully-implicit integrator that is unconditionally stable for all regimes of
particle-gas coupling. Using our hybrid code, we study the numerical
convergence of the non-linear saturated state of the streaming instability. We
find that gas flow properties are well converged with modest grid resolution
(128 cells per pressure length \eta r for dimensionless stopping time
tau_s=0.1), and equal number of particles and grid cells. On the other hand,
particle clumping properties converge only at higher resolutions, and finer
resolution leads to stronger clumping before convergence is reached. Finally,
we find that measurement of particle transport properties resulted from the
streaming instability may be subject to error of about 20%.Comment: 33 pages, accepted to ApJ
ORB5: a global electromagnetic gyrokinetic code using the PIC approach in toroidal geometry
This paper presents the current state of the global gyrokinetic code ORB5 as
an update of the previous reference [Jolliet et al., Comp. Phys. Commun. 177
409 (2007)]. The ORB5 code solves the electromagnetic Vlasov-Maxwell system of
equations using a PIC scheme and also includes collisions and strong flows. The
code assumes multiple gyrokinetic ion species at all wavelengths for the
polarization density and drift-kinetic electrons. Variants of the physical
model can be selected for electrons such as assuming an adiabatic response or a
``hybrid'' model in which passing electrons are assumed adiabatic and trapped
electrons are drift-kinetic. A Fourier filter as well as various control
variates and noise reduction techniques enable simulations with good
signal-to-noise ratios at a limited numerical cost. They are completed with
different momentum and zonal flow-conserving heat sources allowing for
temperature-gradient and flux-driven simulations. The code, which runs on both
CPUs and GPUs, is well benchmarked against other similar codes and analytical
predictions, and shows good scalability up to thousands of nodes
High-Temperature Processing of Solids Through Solar Nebular Bow Shocks: 3D Radiation Hydrodynamics Simulations with Particles
A fundamental, unsolved problem in Solar System formation is explaining the
melting and crystallization of chondrules found in chondritic meteorites.
Theoretical models of chondrule melting in nebular shocks has been shown to be
consistent with many aspects of thermal histories inferred for chondrules from
laboratory experiments; but, the mechanism driving these shocks is unknown.
Planetesimals and planetary embryos on eccentric orbits can produce bow shocks
as they move supersonically through the disk gas, and are one possible source
of chondrule-melting shocks. We investigate chondrule formation in bow shocks
around planetoids through 3D radiation hydrodynamics simulations. A new
radiation transport algorithm that combines elements of flux-limited diffusion
and Monte Carlo methods is used to capture the complexity of radiative
transport around bow shocks. An equation of state that includes the rotational,
vibrational, and dissociation modes of H is also used. Solids are followed
directly in the simulations and their thermal histories are recorded. Adiabatic
expansion creates rapid cooling of the gas, and tail shocks behind the embryo
can cause secondary heating events. Radiative transport is efficient, and bow
shocks around planetoids can have luminosities few
L. While barred and radial chondrule textures could be produced in
the radiative shocks explored here, porphyritic chondrules may only be possible
in the adiabatic limit. We present a series of predicted cooling curves that
merit investigation in laboratory experiments to determine whether the solids
produced by bow shocks are represented in the meteoritic record by chondrules
or other solids.Comment: Accepted for publication in ApJ. Images have been resized to conform
to arXiv limits, but are all readable upon adjusting the zoom. Changes from
v1: Corrected typos discovered in proofs. Most changes are in the appendi
A Unified Gas-kinetic Scheme for Continuum and Rarefied Flows IV: full Boltzmann and Model Equations
Fluid dynamic equations are valid in their respective modeling scales. With a
variation of the modeling scales, theoretically there should have a continuous
spectrum of fluid dynamic equations. In order to study multiscale flow
evolution efficiently, the dynamics in the computational fluid has to be
changed with the scales. A direct modeling of flow physics with a changeable
scale may become an appropriate approach. The unified gas-kinetic scheme (UGKS)
is a direct modeling method in the mesh size scale, and its underlying flow
physics depends on the resolution of the cell size relative to the particle
mean free path. The cell size of UGKS is not limited by the particle mean free
path. With the variation of the ratio between the numerical cell size and local
particle mean free path, the UGKS recovers the flow dynamics from the particle
transport and collision in the kinetic scale to the wave propagation in the
hydrodynamic scale.
The previous UGKS is mostly constructed from the evolution solution of
kinetic model equations. This work is about the further development of the UGKS
with the implementation of the full Boltzmann collision term in the region
where it is needed. The central ingredient of the UGKS is the coupled treatment
of particle transport and collision in the flux evaluation across a cell
interface, where a continuous flow dynamics from kinetic to hydrodynamic scales
is modeled. The newly developed UGKS has the asymptotic preserving (AP)
property of recovering the NS solutions in the continuum flow regime, and the
full Boltzmann solution in the rarefied regime. In the mostly unexplored
transition regime, the UGKS itself provides a valuable tool for the flow study
in this regime. The mathematical properties of the scheme, such as stability,
accuracy, and the asymptotic preserving, will be analyzed in this paper as
well
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