18 research outputs found
Realistic Simulation of Local Solar Supergranulation
I represent results three-dimensional numerical simulation of solar surface
convection on scales local supergranulation with realistic model physics. I
study thermal structure of convective motions in photosphere, the range of
convection cell sizes and the penetration depths of convection. A portion of
the solar photosphere extending 100 x 100 Mm horizontally and from 0 Mm down to
20 Mm below the visible surface is considered. I take equation of state and
opacities of stellar matter and distribution with radius of all physical
variables from Solar Standard Model. The equations of fully compressible
radiation hydrodynamics with dynamical viscosity and gravity are solved. The
high order conservative PPML difference scheme for the hydrodynamics, the
method of characteristic for the radiative transfer and dynamical viscosity
from subgrid scale modeling are applied. The simulations are conducted on a
uniform horizontal grid of 1000 x 1000, with 168 nonuniformly spaced vertical
grid points, on 256 processors with distributed memory multiprocessors on
supercomputer MVS5000 in Computational Centre of Russian Academy of Sciences.Comment: 4 pages, 3 figures. To appear in AIP Conference Proceedings,
"Exploring the Solar System and the Universe", Apr 8-12 2008, Bucharest,
Romania, eds. Vasile Mioc, Cristiana Dumitrache & Nedelia A. Popesc
Realistic Magnetohydrodynamical Simulation of Solar Local Supergranulation
Three-dimensional numerical simulations of solar surface magnetoconvection
using realistic model physics are conducted. The thermal structure of
convective motions into the upper radiative layers of the photosphere, the main
scales of convective cells and the penetration depths of convection are
investigated. We take part of the solar photosphere with size of 60x60 Mm in
horizontal direction and by depth 20 Mm from level of the visible solar
surface. We use a realistic initial model of the Sun and apply equation of
state and opacities of stellar matter. The equations of fully compressible
radiation magnetohydrodynamics with dynamical viscosity and gravity are solved.
We apply: 1) conservative TVD difference scheme for the magnetohydrodynamics,
2) the diffusion approximation for the radiative transfer, 3) dynamical
viscosity from subgrid scale modeling. In simulation we take uniform
two-dimesional grid in gorizontal plane and nonuniform grid in vertical
direction with number of cells 600x600x204. We use 512 processors with
distributed memory multiprocessors on supercomputer MVS-100k in the Joint
Computational Centre of the Russian Academy of Sciences.Comment: 6 pages, 5 figures, submitted to the proceedings of the GONG 2008 /
SOHO XXI conferenc
Interstellar Turbulence and Star Formation
We provide a brief overview of recent advances and outstanding issues in
simulations of interstellar turbulence, including isothermal models for
interior structure of molecular clouds and larger-scale multiphase models
designed to simulate the formation of molecular clouds. We show how
self-organization in highly compressible magnetized turbulence in the
multiphase ISM can be exploited in simple numerical models to generate
realistic initial conditions for star formation.Comment: 8 pages, 5 color figures; submitted to Proceedings of IAU Symposium
270 "Computational Star Formation" held in Barcelona, May 31 - June 4, 201
Simulating Supersonic Turbulence in Magnetized Molecular Clouds
We present results of large-scale three-dimensional simulations of weakly
magnetized supersonic turbulence at grid resolutions up to 1024^3 cells. Our
numerical experiments are carried out with the Piecewise Parabolic Method on a
Local Stencil and assume an isothermal equation of state. The turbulence is
driven by a large-scale isotropic solenoidal force in a periodic computational
domain and fully develops in a few flow crossing times. We then evolve the flow
for a number of flow crossing times and analyze various statistical properties
of the saturated turbulent state. We show that the energy transfer rate in the
inertial range of scales is surprisingly close to a constant, indicating that
Kolmogorov's phenomenology for incompressible turbulence can be extended to
magnetized supersonic flows. We also discuss numerical dissipation effects and
convergence of different turbulence diagnostics as grid resolution refines from
256^3 to 1024^3 cells.Comment: 10 pages, 3 figures, to appear in the proceedings of the DOE/SciDAC
2009 conferenc
Piecewise Parabolic Method on a Local Stencil for Magnetized Supersonic Turbulence Simulation
Stable, accurate, divergence-free simulation of magnetized supersonic
turbulence is a severe test of numerical MHD schemes and has been surprisingly
difficult to achieve due to the range of flow conditions present. Here we
present a new, higher order-accurate, low dissipation numerical method which
requires no additional dissipation or local "fixes" for stable execution. We
describe PPML, a local stencil variant of the popular PPM algorithm for solving
the equations of compressible ideal magnetohydrodynamics. The principal
difference between PPML and PPM is that cell interface states are evolved
rather that reconstructed at every timestep, resulting in a compact stencil.
Interface states are evolved using Riemann invariants containing all transverse
derivative information. The conservation laws are updated in an unsplit
fashion, making the scheme fully multidimensional. Divergence-free evolution of
the magnetic field is maintained using the higher order-accurate constrained
transport technique of Gardiner and Stone. The accuracy and stability of the
scheme is documented against a bank of standard test problems drawn from the
literature. The method is applied to numerical simulation of supersonic MHD
turbulence, which is important for many problems in astrophysics, including
star formation in dark molecular clouds. PPML accurately reproduces in
three-dimensions a transition to turbulence in highly compressible isothermal
gas in a molecular cloud model. The low dissipation and wide spectral bandwidth
of this method make it an ideal candidate for direct turbulence simulations.Comment: 28 pages, 18 figure
The Two States of Star Forming Clouds
We examine the effects of self-gravity and magnetic fields on supersonic
turbulence in isothermal molecular clouds with high resolution simulations and
adaptive mesh refinement. These simulations use large root grids (512^3) to
capture turbulence and four levels of refinement to capture high density, for
an effective resolution of 8,196^3. Three Mach 9 simulations are performed, two
super-Alfv\'enic and one trans-Alfv\'enic. We find that gravity splits the
clouds into two populations, one low density turbulent state and one high
density collapsing state. The low density state exhibits properties similar to
non-self-gravitating in this regime, and we examine the effects of varied
magnetic field strength on statistical properties: the density probability
distribution function is approximately lognormal; velocity power spectral
slopes decrease with field strength; alignment between velocity and magnetic
field increases with field; the magnetic field probability distribution can be
fit to a stretched exponential. The high density state is characterized by
self-similar spheres; the density PDF is a power-law; collapse rate decreases
with increasing mean field; density power spectra have positive slopes,
P({\rho},k) \propto k; thermal-to-magnetic pressure ratios are unity for all
simulations; dynamic-to-magnetic pressure ratios are larger than unity for all
simulations; magnetic field distribution is a power-law. The high Alfv\'en Mach
numbers in collapsing regions explain recent observations of magnetic influence
decreasing with density. We also find that the high density state is found in
filaments formed by converging flows, consistent with recent Herschel
observations. Possible modifications to existing star formation theories are
explored.Comment: 19 pages, 20 figure
Comparing Numerical Methods for Isothermal Magnetized Supersonic Turbulence
We employ simulations of supersonic super-Alfvenic turbulence decay as a
benchmark test problem to assess and compare the performance of nine
astrophysical MHD methods actively used to model star formation. The set of
nine codes includes: ENZO, FLASH, KT-MHD, LL-MHD, PLUTO, PPML, RAMSES, STAGGER,
and ZEUS. We present a comprehensive set of statistical measures designed to
quantify the effects of numerical dissipation in these MHD solvers. We compare
power spectra for basic fields to determine the effective spectral bandwidth of
the methods and rank them based on their relative effective Reynolds numbers.
We also compare numerical dissipation for solenoidal and dilatational velocity
components to check for possible impacts of the numerics on small-scale density
statistics. Finally, we discuss convergence of various characteristics for the
turbulence decay test and impacts of various components of numerical schemes on
the accuracy of solutions. We show that the best performing codes employ a
consistently high order of accuracy for spatial reconstruction of the evolved
fields, transverse gradient interpolation, conservation law update step, and
Lorentz force computation. The best results are achieved with divergence-free
evolution of the magnetic field using the constrained transport method, and
using little to no explicit artificial viscosity. Codes which fall short in one
or more of these areas are still useful, but they must compensate higher
numerical dissipation with higher numerical resolution. This paper is the
largest, most comprehensive MHD code comparison on an application-like test
problem to date. We hope this work will help developers improve their numerical
algorithms while helping users to make informed choices in picking optimal
applications for their specific astrophysical problems.Comment: 17 pages, 5 color figures, revised version to appear in ApJ, 735,
July 201
Low level of expression of C-terminally truncated human FUS causes extensive changes in the spinal cord transcriptome of asymptomatic transgenic mice
A number of mutations in a gene encoding RNA-binding protein FUS have been linked to the development of a familial form of amyotrophic lateral sclerosis known as FUS-ALS. C-terminal truncations of FUS by either nonsense or frameshift mutations lead to the development of FUS-ALS with a particularly early onset and fast progression. However, even in patients bearing these highly pathogenic mutations the function of motor neurons is not noticeably compromised for at least a couple of decades, suggesting that until cytoplasmic levels of FUS lacking its C-terminal nuclear localisation signal reaches a critical threshold, motor neurons are able to tolerate its permanent production.In order to identify how the nervous system responds to low levels of pathogenic variants of FUS we produced and characterised a mouse line, L-FUS[1-359], with a low neuronal expression level of a highly aggregation-prone and pathogenic form of C-terminally truncated FUS. In contrast to mice that express substantially higher level of the same FUS variant and develop severe early onset motor neuron pathology, L-FUS[1-359] mice do not develop any clinical or histopathological signs of motor neuron deficiency even at old age. Nevertheless, we detected substantial changes in the spinal cord transcriptome of these mice compared to their wild type littermates. We suggest that at least some of these changes reflect activation of cellular mechanisms compensating for the potentially damaging effect of pathogenic FUS production. Further studies of these mechanism might reveal effective targets for therapy of FUS-ALS and possibly, other forms of ALS