60 research outputs found
The Living Application: a Self-Organising System for Complex Grid Tasks
We present the living application, a method to autonomously manage
applications on the grid. During its execution on the grid, the living
application makes choices on the resources to use in order to complete its
tasks. These choices can be based on the internal state, or on autonomously
acquired knowledge from external sensors. By giving limited user capabilities
to a living application, the living application is able to port itself from one
resource topology to another. The application performs these actions at
run-time without depending on users or external workflow tools. We demonstrate
this new concept in a special case of a living application: the living
simulation. Today, many simulations require a wide range of numerical solvers
and run most efficiently if specialized nodes are matched to the solvers. The
idea of the living simulation is that it decides itself which grid machines to
use based on the numerical solver currently in use. In this paper we apply the
living simulation to modelling the collision between two galaxies in a test
setup with two specialized computers. This simulation switces at run-time
between a GPU-enabled computer in the Netherlands and a GRAPE-enabled machine
that resides in the United States, using an oct-tree N-body code whenever it
runs in the Netherlands and a direct N-body solver in the United States.Comment: 26 pages, 3 figures, accepted by IJHPC
Direct -body code on low-power embedded ARM GPUs
This work arises on the environment of the ExaNeSt project aiming at design
and development of an exascale ready supercomputer with low energy consumption
profile but able to support the most demanding scientific and technical
applications. The ExaNeSt compute unit consists of densely-packed low-power
64-bit ARM processors, embedded within Xilinx FPGA SoCs. SoC boards are
heterogeneous architecture where computing power is supplied both by CPUs and
GPUs, and are emerging as a possible low-power and low-cost alternative to
clusters based on traditional CPUs. A state-of-the-art direct -body code
suitable for astrophysical simulations has been re-engineered in order to
exploit SoC heterogeneous platforms based on ARM CPUs and embedded GPUs.
Performance tests show that embedded GPUs can be effectively used to accelerate
real-life scientific calculations, and that are promising also because of their
energy efficiency, which is a crucial design in future exascale platforms.Comment: 16 pages, 7 figures, 1 table, accepted for publication in the
Computing Conference 2019 proceeding
How well do STARLAB and NBODY compare? II: Hardware and accuracy
Most recent progress in understanding the dynamical evolution of star
clusters relies on direct N-body simulations. Owing to the computational
demands, and the desire to model more complex and more massive star clusters,
hardware calculational accelerators, such as GRAPE special-purpose hardware or,
more recently, GPUs (i.e. graphics cards), are generally utilised. In addition,
simulations can be accelerated by adjusting parameters determining the
calculation accuracy (i.e. changing the internal simulation time step used for
each star).
We extend our previous thorough comparison (Anders et al. 2009) of basic
quantities as derived from simulations performed either with STARLAB/KIRA or
NBODY6. Here we focus on differences arising from using different hardware
accelerations (including the increasingly popular graphic card
accelerations/GPUs) and different calculation accuracy settings.
We use the large number of star cluster models (for a fixed stellar mass
function, without stellar/binary evolution, primordial binaries, external tidal
fields etc) already used in the previous paper, evolve them with STARLAB/KIRA
(and NBODY6, where required), analyse them in a consistent way and compare the
averaged results quantitatively. For this quantitative comparison, we apply the
bootstrap algorithm for functional dependencies developed in our previous
study.
In general we find very high comparability of the simulation results,
independent of the used computer hardware (including the hardware accelerators)
and the used N-body code. For the tested accuracy settings we find that for
reduced accuracy (i.e. time step at least a factor 2.5 larger than the standard
setting) most simulation results deviate significantly from the results using
standard settings. The remaining deviations are comprehensible and explicable.Comment: 14 pages incl. 3 pages with figures and 4 pages of tables (analysis
results), MNRAS in pres
Star cluster evolution in barred disc galaxies. I. Planar periodic orbits
The dynamical evolution of stellar clusters is driven to a large extent by
their environment. Several studies so far have considered the effect of tidal
fields and their variations, such as, e.g., from giant molecular clouds,
galactic discs, or spiral arms. In this paper we will concentrate on a tidal
field whose effects on star clusters have not yet been studied, namely that of
bars. We present a set of direct N-body simulations of star clusters moving in
an analytic potential representing a barred galaxy. We compare the evolution of
the clusters moving both on different planar periodic orbits in the barred
potential and on circular orbits in a potential obtained by axisymmetrising its
mass distribution. We show that both the shape of the underlying orbit and its
stability have strong impact on the cluster evolution as well as the morphology
and orientation of the tidal tails and the sub-structures therein. We find that
the dissolution time-scale of the cluster in our simulations is mainly
determined by the tidal forcing along the orbit and, for a given tidal forcing,
only very little by the exact shape of the gravitational potential in which the
cluster is moving.Comment: 15 pages, 17 figures, 5 tables; accepted for publication in MNRAS.
Complementary movies can be be found at this http URL
http://lam.oamp.fr/research/dynamique-des-galaxies/scientific-results/star-cluster-evolution
Shape parameters of Galactic open clusters
(abridged) In this paper we derive observed and modelled shape parameters
(apparent ellipticity and orientation of the ellipse) of 650 Galactic open
clusters identified in the ASCC-2.5 catalogue. We provide the observed shape
parameters of Galactic open clusters, computed with the help of a
multi-component analysis. For the vast majority of clusters these parameters
are determined for the first time. High resolution ("star by star") N-body
simulations are carried out with the specially developed GRAPE code
providing models of clusters of different initial masses, Galactocentric
distances and rotation velocities. The comparison of models and observations of
about 150 clusters reveals ellipticities of observed clusters which are too low
(0.2 vs. 0.3), and offers the basis to find the main reason for this
discrepancy. The models predict that after Myr clusters reach an
oblate shape with an axes ratio of , and with the major axis
tilted by an angle of with respect to the
Galactocentric radius due to differential rotation of the Galaxy. Unbiased
estimates of cluster shape parameters require reliable membership determination
in large cluster areas up to 2-3 tidal radii where the density of cluster stars
is considerably lower than the background. Although dynamically bound stars
outside the tidal radius contribute insignificantly to the cluster mass, their
distribution is essential for a correct determination of cluster shape
parameters. In contrast, a restricted mass range of cluster stars does not play
such a dramatic role, though deep surveys allow to identify more cluster
members and, therefore, to increase the accuracy of the observed shape
parameters.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
The Origin of the Mass-Metallicity relation: an analytical approach
The existence of a mass-metallicity (MZ) relation in star forming galaxies at
all redshift has been recently established. We aim at studying some possible
physical mechanisms contributing to the MZ relation by adopting analytical
solutions of chemical evolution models including infall and outflow. We explore
the hypotheses of a variable galactic wind rate, infall rate and yield per
stellar generation (i.e. a variation in the IMF), as possible causes for the MZ
relation. By means of analytical models we compute the expected O abundance for
galaxies of a given total baryonic mass and gas mass.The stellar mass is
derived observationally and the gas mass is derived by inverting the Kennicutt
law of star formation, once the star formation rate is known. Then we test how
the parameters describing the outflow, infall and IMF should vary to reproduce
the MZ relation, and we exclude the cases where such a variation leads to
unrealistic situations. We find that a galactic wind rate increasing with
decreasing galactic mass or a variable IMF are both viable solutions for the MZ
relation. A variable infall rate instead is not acceptable. It is difficult to
disentangle among the outflow and IMF solutions only by considering the MZ
relation, and other observational constraints should be taken into account to
select a specific solution. For example, a variable efficiency of star
formation increasing with galactic mass can also reproduce the MZ relation and
explain the downsizing in star formation suggested for ellipticals. The best
solution could be a variable efficiency of star formation coupled with galactic
winds, which are indeed observed in low mass galaxies.Comment: Accepted by A&
Simulating the formation and evolution of galaxies: Multi-phase description of the interstellar medium, star formation, and energy feedback
We present a multi-phase representation of the ISM in NB-TSPH simulations of
galaxy formation and evolution with particular attention to the case of
early-type galaxies. Cold gas clouds are described by the so-called sticky
particles algorithm. They can freely move throughout the hot ISM medium; stars
form within these clouds and the mass exchange among the three baryonic phases
(hot gas, cold clouds, stars) is governed by radiative and Compton cooling and
energy feedback by supernova (SN) explosions, stellar winds, and UV radiation.
We also consider thermal conduction, cloud-cloud collisions, and chemical
enrichment. Our model agrees with and improves upon previous studies on the
same subject. The results for the star formation rate are very promising and
agree with recent observational data on early-type galaxies. These models lend
further support to the revised monolithic scheme of galaxy formation, which has
recently been also strengthened by high redshift data leading to the so-called
downsizing and top-down scenarios.Comment: 17 pages, 17 figure
A pilgrimage to gravity on GPUs
In this short review we present the developments over the last 5 decades that
have led to the use of Graphics Processing Units (GPUs) for astrophysical
simulations. Since the introduction of NVIDIA's Compute Unified Device
Architecture (CUDA) in 2007 the GPU has become a valuable tool for N-body
simulations and is so popular these days that almost all papers about high
precision N-body simulations use methods that are accelerated by GPUs. With the
GPU hardware becoming more advanced and being used for more advanced algorithms
like gravitational tree-codes we see a bright future for GPU like hardware in
computational astrophysics.Comment: To appear in: European Physical Journal "Special Topics" : "Computer
Simulations on Graphics Processing Units" . 18 pages, 8 figure
EvoL: The new Padova T-SPH parallel code for cosmological simulations - I. Basic code: gravity and hydrodynamics
We present EvoL, the new release of the Padova N-body code for cosmological
simulations of galaxy formation and evolution. In this paper, the basic Tree +
SPH code is presented and analysed, together with an overview on the software
architectures. EvoL is a flexible parallel Fortran95 code, specifically
designed for simulations of cosmological structure formation on cluster,
galactic and sub-galactic scales. EvoL is a fully Lagrangian self-adaptive
code, based on the classical Oct-tree and on the Smoothed Particle
Hydrodynamics algorithm. It includes special features such as adaptive
softening lengths with correcting extra-terms, and modern formulations of SPH
and artificial viscosity. It is designed to be run in parallel on multiple CPUs
to optimize the performance and save computational time. We describe the code
in detail, and present the results of a number of standard hydrodynamical
tests.Comment: 33 pages, 49 figures, accepted on A&
N-body simulations of gravitational dynamics
We describe the astrophysical and numerical basis of N-body simulations, both
of collisional stellar systems (dense star clusters and galactic centres) and
collisionless stellar dynamics (galaxies and large-scale structure). We explain
and discuss the state-of-the-art algorithms used for these quite different
regimes, attempt to give a fair critique, and point out possible directions of
future improvement and development. We briefly touch upon the history of N-body
simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu
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