36,578 research outputs found
The cosmological simulation code GADGET-2
We discuss the cosmological simulation code GADGET-2, a new massively
parallel TreeSPH code, capable of following a collisionless fluid with the
N-body method, and an ideal gas by means of smoothed particle hydrodynamics
(SPH). Our implementation of SPH manifestly conserves energy and entropy in
regions free of dissipation, while allowing for fully adaptive smoothing
lengths. Gravitational forces are computed with a hierarchical multipole
expansion, which can optionally be applied in the form of a TreePM algorithm,
where only short-range forces are computed with the `tree'-method while
long-range forces are determined with Fourier techniques. Time integration is
based on a quasi-symplectic scheme where long-range and short-range forces can
be integrated with different timesteps. Individual and adaptive short-range
timesteps may also be employed. The domain decomposition used in the
parallelisation algorithm is based on a space-filling curve, resulting in high
flexibility and tree force errors that do not depend on the way the domains are
cut. The code is efficient in terms of memory consumption and required
communication bandwidth. It has been used to compute the first cosmological
N-body simulation with more than 10^10 dark matter particles, reaching a
homogeneous spatial dynamic range of 10^5 per dimension in a 3D box. It has
also been used to carry out very large cosmological SPH simulations that
account for radiative cooling and star formation, reaching total particle
numbers of more than 250 million. We present the algorithms used by the code
and discuss their accuracy and performance using a number of test problems.
GADGET-2 is publicly released to the research community.Comment: submitted to MNRAS, 31 pages, 20 figures (reduced resolution), code
available at http://www.mpa-garching.mpg.de/gadge
Convolutional Codes in Rank Metric with Application to Random Network Coding
Random network coding recently attracts attention as a technique to
disseminate information in a network. This paper considers a non-coherent
multi-shot network, where the unknown and time-variant network is used several
times. In order to create dependencies between the different shots, particular
convolutional codes in rank metric are used. These codes are so-called
(partial) unit memory ((P)UM) codes, i.e., convolutional codes with memory one.
First, distance measures for convolutional codes in rank metric are shown and
two constructions of (P)UM codes in rank metric based on the generator matrices
of maximum rank distance codes are presented. Second, an efficient
error-erasure decoding algorithm for these codes is presented. Its guaranteed
decoding radius is derived and its complexity is bounded. Finally, it is shown
how to apply these codes for error correction in random linear and affine
network coding.Comment: presented in part at Netcod 2012, submitted to IEEE Transactions on
Information Theor
GADGET: A code for collisionless and gasdynamical cosmological simulations
We describe the newly written code GADGET which is suitable both for
cosmological simulations of structure formation and for the simulation of
interacting galaxies. GADGET evolves self-gravitating collisionless fluids with
the traditional N-body approach, and a collisional gas by smoothed particle
hydrodynamics. Along with the serial version of the code, we discuss a parallel
version that has been designed to run on massively parallel supercomputers with
distributed memory. While both versions use a tree algorithm to compute
gravitational forces, the serial version of GADGET can optionally employ the
special-purpose hardware GRAPE instead of the tree. Periodic boundary
conditions are supported by means of an Ewald summation technique. The code
uses individual and adaptive timesteps for all particles, and it combines this
with a scheme for dynamic tree updates. Due to its Lagrangian nature, GADGET
thus allows a very large dynamic range to be bridged, both in space and time.
So far, GADGET has been successfully used to run simulations with up to 7.5e7
particles, including cosmological studies of large-scale structure formation,
high-resolution simulations of the formation of clusters of galaxies, as well
as workstation-sized problems of interacting galaxies. In this study, we detail
the numerical algorithms employed, and show various tests of the code. We
publically release both the serial and the massively parallel version of the
code.Comment: 32 pages, 14 figures, replaced to match published version in New
Astronomy. For download of the code, see
http://www.mpa-garching.mpg.de/gadget (new version 1.1 available
The stellar atmosphere simulation code Bifrost
Context: Numerical simulations of stellar convection and photospheres have
been developed to the point where detailed shapes of observed spectral lines
can be explained. Stellar atmospheres are very complex, and very different
physical regimes are present in the convection zone, photosphere, chromosphere,
transition region and corona. To understand the details of the atmosphere it is
necessary to simulate the whole atmosphere since the different layers interact
strongly. These physical regimes are very diverse and it takes a highly
efficient massively parallel numerical code to solve the associated equations.
Aims: The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the convection
zone to the corona.
Methods: The code is subjected to a number of validation tests, among them
the Sod shock tube test, the Orzag-Tang colliding shock test, boundary
condition tests and tests of how the code treats magnetic field advection,
chromospheric radiation, radiative transfer in an isothermal scattering
atmosphere, hydrogen ionization and thermal conduction.
Results: Bifrost completes the tests with good results and shows near linear
efficiency scaling to thousands of computing cores
Stable quantum memories with limited measurement
We demonstrate the existence of a finite temperature threshold for a 1D
stabilizer code under an error correcting protocol that requires only a
fraction of the syndrome measurements. Below the threshold temperature, encoded
states have exponentially long lifetimes, as demonstrated by numerical and
analytical arguments. We sketch how this algorithm generalizes to higher
dimensional stabilizer codes with string-like excitations, like the toric code.Comment: 11 Pages, 7 Figure
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