5,707 research outputs found
A Parallel Adaptive P3M code with Hierarchical Particle Reordering
We discuss the design and implementation of HYDRA_OMP a parallel
implementation of the Smoothed Particle Hydrodynamics-Adaptive P3M (SPH-AP3M)
code HYDRA. The code is designed primarily for conducting cosmological
hydrodynamic simulations and is written in Fortran77+OpenMP. A number of
optimizations for RISC processors and SMP-NUMA architectures have been
implemented, the most important optimization being hierarchical reordering of
particles within chaining cells, which greatly improves data locality thereby
removing the cache misses typically associated with linked lists. Parallel
scaling is good, with a minimum parallel scaling of 73% achieved on 32 nodes
for a variety of modern SMP architectures. We give performance data in terms of
the number of particle updates per second, which is a more useful performance
metric than raw MFlops. A basic version of the code will be made available to
the community in the near future.Comment: 34 pages, 12 figures, accepted for publication in Computer Physics
Communication
Global Optimization for Future Gravitational Wave Detectors' Sites
We consider the optimal site selection of future generations of gravitational
wave detectors. Previously, Raffai et al. optimized a 2-detector network with a
combined figure of merit. This optimization was extended to networks with more
than two detectors in a limited way by first fixing the parameters of all other
component detectors. In this work we now present a more general optimization
that allows the locations of all detectors to be simultaneously chosen. We
follow the definition of Raffai et al. on the metric that defines the
suitability of a certain detector network. Given the locations of the component
detectors in the network, we compute a measure of the network's ability to
distinguish the polarization, constrain the sky localization and reconstruct
the parameters of a gravitational wave source. We further define the
`flexibility index' for a possible site location, by counting the number of
multi-detector networks with a sufficiently high Figure of Merit that include
that site location. We confirm the conclusion of Raffai et al., that in terms
of flexibility index as defined in this work, Australia hosts the best
candidate site to build a future generation gravitational wave detector. This
conclusion is valid for either a 3-detector network or a 5-detector network.
For a 3-detector network site locations in Northern Europe display a comparable
flexibility index to sites in Australia. However for a 5-detector network,
Australia is found to be a clearly better candidate than any other location.Comment: 30 pages, 23 figures, 2 table
Strong Lensing Reconstruction
We present a general linear algorithm for measuring the surface mass density
1-\kappa from the observable reduced shear g=\gamma/(1-\kappa) in the strong
lensing regime. We show that in general, the observed polarization field can be
decomposed into ``electric'' and ``magnetic'' components, which have
independent and redundant solutions, but perfectly orthogonal noise properties.
By combining these solutions, one can increase the signal-to-noise ratio by
\sqrt{2}. The solutions allow dynamic optimization of signal and noise, both in
real and Fourier space (using arbitrary smoothing windows). Boundary conditions
have no effect on the reconstructions, apart from its effect on the
signal-to-noise. Many existing reconstruction techniques are recovered as
special cases of this framework. The magnetic solution has the added benefit of
yielding the global and local parity of the reconstruction in a single step.Comment: final accepted version for ApJ
GRAPESPH with Fully Periodic Boundary Conditions: Fragmentation of Molecular Clouds
A method of adapting smoothed particle hydrodynamics (SPH) with periodic
boundary conditions for use with the special purpose device GRAPE is presented.
GRAPE (GRAvity PipE) solves the Poisson and force equations for an N-body
system by direct summation on a specially designed chip and in addition returns
the neighbour list for each particle. Due to its design, GRAPE cannot treat
periodic particle distributions directly. This limitation of GRAPESPH can be
overcome by computing a correction force for each particle due to periodicity
(Ewald correction) on the host computer using a PM-like method.
This scheme is applied to study the fragmentation process in giant molecular
clouds. Assuming a pure isothermal model, we follow the dynamical evolution in
the interior of a molecular cloud starting from an Gaussian initial density
distribution to the formation of selfgravitating clumps until most of the gas
is consumed in these dense cores. Despite its simplicity, this model can
reproduce some fundamental properties of observed molecular clouds, like a
clump mass distribution of the form , with .Comment: 8 pages; LaTeX + 7 PS figures; accepted for publication in MNRAS;
also available at
http://www.mpia-hd.mpg.de/MPIA/Projects/THEORY/klessen/Preprints/p5.p
A New Estimate of the Hubble Time with Improved Modeling of Gravitational Lenses
This paper examines free-form modeling of gravitational lenses using Bayesian
ensembles of pixelated mass maps. The priors and algorithms from previous work
are clarified and significant technical improvements are made. Lens
reconstruction and Hubble Time recovery are tested using mock data from simple
analytic models and recent galaxy-formation simulations. Finally, using
published data, the Hubble Time is inferred through the simultaneous
reconstruction of eleven time-delay lenses. The result is
H_0^{-1}=13.7^{+1.8}_{-1.0} Gyr.Comment: 24 pages, 9 figures. Accepted to Ap
A simple multigrid scheme for solving the Poisson equation with arbitrary domain boundaries
We present a new multigrid scheme for solving the Poisson equation with
Dirichlet boundary conditions on a Cartesian grid with irregular domain
boundaries. This scheme was developed in the context of the Adaptive Mesh
Refinement (AMR) schemes based on a graded-octree data structure. The Poisson
equation is solved on a level-by-level basis, using a "one-way interface"
scheme in which boundary conditions are interpolated from the previous coarser
level solution. Such a scheme is particularly well suited for self-gravitating
astrophysical flows requiring an adaptive time stepping strategy. By
constructing a multigrid hierarchy covering the active cells of each AMR level,
we have designed a memory-efficient algorithm that can benefit fully from the
multigrid acceleration. We present a simple method for capturing the boundary
conditions across the multigrid hierarchy, based on a second-order accurate
reconstruction of the boundaries of the multigrid levels. In case of very
complex boundaries, small scale features become smaller than the discretization
cell size of coarse multigrid levels and convergence problems arise. We propose
a simple solution to address these issues. Using our scheme, the convergence
rate usually depends on the grid size for complex grids, but good linear
convergence is maintained. The proposed method was successfully implemented on
distributed memory architectures in the RAMSES code, for which we present and
discuss convergence and accuracy properties as well as timing performances.Comment: 33 pages, 15 figures, accepted for publication in Journal of
Computational Physic
Altimetry, gravimetry, GPS and viscoelastic modeling data for the joint inversion for glacial isostatic adjustment in Antarctica (ESA STSE Project REGINA)
The poorly known correction for the ongoing deformation of the solid Earth caused by glacial isostatic adjustment (GIA) is a major uncertainty in determining the mass balance of the Antarctic ice sheet from measurements of satellite gravimetry and to a lesser extent satellite altimetry. In the past decade, much progress has been made in consistently modeling ice sheet and solid Earth interactions; however, forward-modeling solutions of GIA in Antarctica remain uncertain due to the sparsity of constraints on the ice sheet evolution, as well as the Earth's rheological properties. An alternative approach towards estimating GIA is the joint inversion of multiple satellite data – namely, satellite gravimetry, satellite altimetry and GPS, which reflect, with different sensitivities, trends in recent glacial changes and GIA. Crucial to the success of this approach is the accuracy of the space-geodetic data sets. Here, we present reprocessed rates of surface-ice elevation change (Envisat/Ice, Cloud,and land Elevation Satellite, ICESat; 2003–2009), gravity field change (Gravity Recovery and Climate Experiment, GRACE; 2003–2009) and bedrock uplift (GPS; 1995–2013). The data analysis is complemented by the forward modeling of viscoelastic response functions to disc load forcing, allowing us to relate GIA-induced surface displacements with gravity changes for different rheological parameters of the solid Earth. The data and modeling results presented here are available in the PANGAEA database (https://doi.org/10.1594/PANGAEA.875745). The data sets are the input streams for the joint inversion estimate of present-day ice-mass change and GIA, focusing on Antarctica. However, the methods, code and data provided in this paper can be used to solve other problems, such as volume balances of the Antarctic ice sheet, or can be applied to other geographical regions in the case of the viscoelastic response functions. This paper presents the first of two contributions summarizing the work carried out within a European Space Agency funded study: Regional glacial isostatic adjustment and CryoSat elevation rate corrections in Antarctica (REGINA)
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