184 research outputs found
The turbulent pressure support in galaxy clusters revisited
Due to their late formation in cosmic history, clusters of galaxies are not
fully in hydrostatic equilibrium and the gravitational pull of their mass at a
given radius is expected not to be entirely balanced by the thermal gas
pressure. Turbulence may supply additional pressure, and recent (X-ray and SZ)
hydrostatic mass reconstructions claim a pressure support of of
the total pressure at . In this work we show that, after carefully
disentangling bulk from small-scale turbulent motions in high-resolution
simulations of galaxy clusters, we can constrain which fraction of the gas
kinetic energy effectively provides pressure support in the cluster's
gravitational potential. While the ubiquitous presence of radial inflows in the
cluster can lead to significant bias in the estimate of the non-thermal
pressure support, we report that only a part of this energy effectively acts as
a source of pressure, providing a support of the order of of the
total pressure at .Comment: 5 pages, 5 pages, accepted, to appear in MNRAS Letter
Efficiency of Turbulent Reacceleration by Solenoidal Turbulence and Its Application to the Origin of Radio Megahalos in Cluster Outskirts
Recent radio observations with the Low Frequency Array (LOFAR) discovered diffuse emission extending beyond the scale of classical radio halos. The presence of such megahalos indicates that the amplification of the magnetic field and acceleration of relativistic particles are working in the cluster outskirts, presumably due to the combination of shocks and turbulence that dissipate energy in these regions. Cosmological magnetohydrodynamical (MHD) simulations of galaxy clusters suggest that solenoidal turbulence has a significant energy budget in the outskirts of galaxy clusters. In this paper, we explore the possibility that this turbulence contributes to the emission observed in megahalos through second-order Fermi acceleration of relativistic particles and magnetic field amplification by the dynamo. We focus on the case of A2255 and find that this scenario can explain the basic properties of the diffuse emission component that is observed under assumptions that are used in previous literature. More specifically, we conduct a numerical follow-up, solving the Fokker-Planck equation by using a snapshot of an MHD simulation and deducing the synchrotron brightness integrated along the lines of sight. We find that a volume-filling emission, ranging between 30% and almost 100% of the projected area, depending on our assumptions on the particle diffusion and transport, can be detected at LOFAR sensitivities. Assuming a magnetic field B ∼ 0.2 μG, as derived from a dynamo model applied to the emitting region, we find that the observed brightness can be matched when ∼1% of the solenoidal turbulent energy flux is channeled into particle acceleration
A portable platform for accelerated PIC codes and its application to GPUs using OpenACC
We present a portable platform, called PIC_ENGINE, for accelerating
Particle-In-Cell (PIC) codes on heterogeneous many-core architectures such as
Graphic Processing Units (GPUs). The aim of this development is efficient
simulations on future exascale systems by allowing different parallelization
strategies depending on the application problem and the specific architecture.
To this end, this platform contains the basic steps of the PIC algorithm and
has been designed as a test bed for different algorithmic options and data
structures. Among the architectures that this engine can explore, particular
attention is given here to systems equipped with GPUs. The study demonstrates
that our portable PIC implementation based on the OpenACC programming model can
achieve performance closely matching theoretical predictions. Using the Cray
XC30 system, Piz Daint, at the Swiss National Supercomputing Centre (CSCS), we
show that PIC_ENGINE running on an NVIDIA Kepler K20X GPU can outperform the
one on an Intel Sandybridge 8-core CPU by a factor of 3.4
A Map-Making Algorithm for the Planck Surveyor
We present a parallel implementation of a map-making algorithm for CMB
anisotropy experiments which is both fast and efficient. We show for the first
time a Maximum Likelihood, minimum variance map obtained by processing the
entire data stream expected from the Planck Surveyor, under the assumption of a
symmetric beam profile. Here we restrict ourselves to the case of the 30 GHz
channel of the Planck Low Frequency Instrument. The extension to Planck higher
frequency channels is straightforward. If the satellite pointing periodicity is
good enough to average data that belong to the same sky circle, then the code
runs very efficiently on workstations. The serial version of our code also runs
on very competitive time-scales the map-making pipeline for current and
forthcoming balloon borne experiments.Comment: 11 pages, 10 figures. Reflects accepted A&A versio
VisIVO - Integrated Tools and Services for Large-Scale Astrophysical Visualization
VisIVO is an integrated suite of tools and services specifically designed for
the Virtual Observatory. This suite constitutes a software framework for
effective visual discovery in currently available (and next-generation) very
large-scale astrophysical datasets. VisIVO consists of VisiVO Desktop - a stand
alone application for interactive visualization on standard PCs, VisIVO Server
- a grid-enabled platform for high performance visualization and VisIVO Web - a
custom designed web portal supporting services based on the VisIVO Server
functionality. The main characteristic of VisIVO is support for
high-performance, multidimensional visualization of very large-scale
astrophysical datasets. Users can obtain meaningful visualizations rapidly
while preserving full and intuitive control of the relevant visualization
parameters. This paper focuses on newly developed integrated tools in VisIVO
Server allowing intuitive visual discovery with 3D views being created from
data tables. VisIVO Server can be installed easily on any web server with a
database repository. We discuss briefly aspects of our implementation of VisiVO
Server on a computational grid and also outline the functionality of the
services offered by VisIVO Web. Finally we conclude with a summary of our work
and pointers to future developments
VisIVOWeb: A WWW Environment for Large-Scale Astrophysical Visualization
This article presents a newly developed Web portal called VisIVOWeb that aims
to provide the astrophysical community with powerful visualization tools for
large-scale data sets in the context of Web 2.0. VisIVOWeb can effectively
handle modern numerical simulations and real-world observations. Our
open-source software is based on established visualization toolkits offering
high-quality rendering algorithms. The underlying data management is discussed
with the supported visualization interfaces and movie-making functionality. We
introduce VisIVOWeb Network, a robust network of customized Web portals for
visual discovery, and VisIVOWeb Connect, a lightweight and efficient solution
for seamlessly connecting to existing astrophysical archives. A significant
effort has been devoted for ensuring interoperability with existing tools by
adhering to IVOA standards. We conclude with a summary of our work and a
discussion on future developments
Artificial neural network for tilting pad journal bearing characterization
Tilting pad journal bearings (TPJBs) are modeled with Reynold-based models or computational fluid dynamics (CFD) approach. In both cases, the estimation of the dynamic coefficients of the oil-film forces and the static characteristic, can be computationally expensive and time consuming. Artificial Intelligence (AI) is assuming a key role in engineering but is rarely applied in fluid film bearing analysis. A properly trained Deep Learning (DL) model can perform very fast predictions of TPJB behavior with accuracy comparable to more time-consuming
models. In this case, the main drawback is the time required to build the training dataset. In this work, an Artificial Neural Network (ANN) is trained to predict the dynamic stiffness and damping coefficients along with the main static quantities of TPJBs, such as minimum oil-film thickness and inlet flowrate. At first, a design of experiment is performed to build an appropriate training dataset. Secondly, a Reynolds-based thermo-hydrodynamic
(THD) model is used to populate the training dataset and an appropriate test dataset. Then, a feedforward ANN is trained with Levenberg–Marquardt backpropagation and its architecture is optimized to increase accuracy. Finally, the accuracy of the ANN is tested using the test dataset and experimental data. The time and computational effort required by the ANN regression are much less than those required by the THD model.
Therefore, the trained ANN is an effective and efficient tool for the characterization of TPJBs
A comparison of cosmological codes: properties of thermal gas and shock waves in large-scale structures
Cosmological hydrodynamical simulations are a valuable tool for understanding the growth of large-scale structure and the observables connected with this. Yet, comparably little attention has been given to validation studies of the properties of shocks and of the resulting thermal gas between different numerical methods something of immediate importance as gravitational shocks are responsible for generating most of the entropy of the large-scale structure in the Universe. Here, we present results for the statistics of thermal gas and the shock wave properties for a large volume simulated with three different cosmological numerical codes: the Eulerian total variations diminishing (TVD) code, the Eulerian piecewise parabolic method based code enzo and the Lagrangian smoothed particle hydrodynamics (SPH) code gadget. Starting from a shared set of initial conditions, we present convergence tests for a cosmological volume of side-length 100 Mpc h-1, studying in detail the morphological and statistical properties of the thermal gas as a function of mass and spatial resolution in all codes. By applying shock-finding methods to each code, we measure the statistics of shock waves and the related cosmic ray acceleration efficiencies, within the sample of simulations and for the results of the different approaches. We discuss the regimes of uncertainties and disagreement among codes, with a particular focus on the results at the scale of galaxy clusters. Even if the bulk of thermal and shock properties is reasonably in agreement among the three codes, yet some significant differences exist (especially between Eulerian methods and SPH). In particular, we report (a) differences of huge factors (similar to 10100) in the values of average gas density, temperature, entropy, Mach number and shock thermal energy flux in the most rarefied regions of the simulations (?/?cr < 1) between grid and SPH methods; (b) the hint of an entropy core inside clusters simulated in grid codes; (c) significantly different phase diagrams of shocked cells in grid codes compared to SPH and (d) sizable differences in the morphologies of accretion shocks between grid and SPH methodsopen393
COSMOS: A Hybrid N-Body/Hydrodynamics Code for Cosmological Problems
We describe a new hybrid N-body/hydrodynamical code based on the
particle-mesh (PM) method and the piecewise-parabolic method (PPM) for use in
solving problems related to the evolution of large-scale structure, galaxy
clusters, and individual galaxies. The code, named COSMOS, possesses several
new features which distinguish it from other PM-PPM codes. In particular, to
solve the Poisson equation we have written a new multigrid solver which can
determine the gravitational potential of isolated matter distributions and
which properly takes into account the finite-volume discretization required by
PPM. All components of the code are constructed to work with a nonuniform mesh,
preserving second-order spatial differences. The PPM code uses vacuum boundary
conditions for isolated problems, preventing inflows when appropriate. The PM
code uses a second-order variable-timestep time integration scheme. Radiative
cooling and cosmological expansion terms are included. COSMOS has been
implemented for parallel computers using the Parallel Virtual Machine (PVM)
library, and it features a modular design which simplifies the addition of new
physics and the configuration of the code for different types of problems. We
discuss the equations solved by COSMOS and describe the algorithms used, with
emphasis on these features. We also discuss the results of tests we have
performed to establish that COSMOS works and to determine its range of
validity.Comment: 43 pages, 14 figures, submitted to ApJS and revised according to
referee's comment
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