4,871 research outputs found
Hydra: An Adaptive--Mesh Implementation of PPPM--SPH
We present an implementation of Smoothed Particle Hydrodynamics (SPH) in an
adaptive-mesh PPPM algorithm. The code evolves a mixture of purely
gravitational particles and gas particles. The code retains the desirable
properties of previous PPPM--SPH implementations; speed under light clustering,
naturally periodic boundary conditions and accurate pairwise forces. Under
heavy clustering the cycle time of the new code is only 2--3 times slower than
for a uniform particle distribution, overcoming the principal disadvantage of
previous implementations\dash a dramatic loss of efficiency as clustering
develops. A 1000 step simulation with 65,536 particles (half dark, half gas)
runs in one day on a Sun Sparc10 workstation. The choice of time integration
scheme is investigated in detail. A simple single-step Predictor--Corrector
type integrator is most efficient. A method for generating an initial
distribution of particles by allowing a a uniform temperature gas of SPH
particles to relax within a periodic box is presented. The average SPH density
that results varies by \%. We present a modified form of the
Layzer--Irvine equation which includes the thermal contribution of the gas
together with radiative cooling. Tests of sound waves, shocks, spherical infall
and collapse are presented. Appropriate timestep constraints sufficient to
ensure both energy and entropy conservation are discussed. A cluster
simulation, repeating Thomas andComment: 29 pp, uuencoded Postscrip
Developing modern multifunctional agroforestry systems for sustainable intensification
Agroforestry is a land-use system that integrates trees and shrubs with crops and/or livestock production. It has been identified by the International Assessment of Agricultural Knowledge, Science and Technology for Development (IAASTD, 2008) as a âwin-winâ approach that balances the production of commodities (food, feed, fuel, fibre, etc.) with non-commodity outputs such as environmental protection and cultural and landscape amenities. This paper will review the potential of agroforestry as part of a multifunctional working landscape in temperate regions, and will consider management and policy implications of widespread adoption of this form of land-use
The effect of radiative cooling on scaling laws of X-ray groups and clusters
We have performed cosmological simulations in a ÎCDM cosmology with and without radiative cooling in order to study the effect of cooling on the cluster scaling laws. Our simulations consist of 4.1 million particles each of gas and dark matter within a box size of 100 h-1 Mpc, and the run with cooling is the largest of its kind to have been evolved to z = 0. Our cluster catalogs both consist of over 400 objects and are complete in mass down to ~1013 h-1 Mâ. We contrast the emission-weighted temperature-mass (Tew-M) and bolometric luminosity-temperature (Lbol-Tew) relations for the simulations at z = 0. We find that radiative cooling increases the temperature of intracluster gas and decreases its total luminosity, in agreement with the results of Pearce et al. Furthermore, the temperature dependence of these effects flattens the slope of the Tew-M relation and steepens the slope of the Lbol-Tew relation. Inclusion of radiative cooling in the simulations is sufficient to reproduce the observed X-ray scaling relations without requiring excessive nongravitational energy injection
Smoothed Particle Hydrodynamics in cosmology: a comparative study of implementations
We analyse the performance of twelve different implementations of Smoothed
Particle Hydrodynamics (SPH) using seven tests designed to isolate key
hydrodynamic elements of cosmological simulations which are known to cause the
SPH algorithm problems. In order, we consider a shock tube, spherical adiabatic
collapse, cooling flow model, drag, a cosmological simulation, rotating
cloud-collapse and disc stability. In the implementations special attention is
given to the way in which force symmetry is enforced in the equations of
motion. We study in detail how the hydrodynamics are affected by different
implementations of the artificial viscosity including those with a
shear-correction modification. We present an improved first-order
smoothing-length update algorithm that is designed to remove instabilities that
are present in the Hernquist and Katz (1989) algorithm.
For all tests we find that the artificial viscosity is the most important
factor distinguishing the results from the various implementations. The second
most important factor is the way force symmetry is achieved in the equation of
motion. Most results favour a kernel symmetrization approach. The exact method
by which SPH pressure forces are included has comparatively little effect on
the results. Combining the equation of motion presented in Thomas and Couchman
(1992) with a modification of the Monaghan and Gingold (1983) artificial
viscosity leads to an SPH scheme that is both fast and reliable.Comment: 30 pages, 26 figures and 9 tables included. Submitted to MNRAS.
Postscript version available at
ftp://phobos.astro.uwo.ca/pub/etittley/papers/sphtest.ps.g
Peculiar Velocities of Galaxy Clusters
We investigate the peculiar velocities predicted for galaxy clusters by
theories in the cold dark matter family. A widely used hypothesis identifies
rich clusters with high peaks of a suitably smoothed version of the linear
density fluctuation field. Their peculiar velocities are then obtained by
extrapolating the similarly smoothed linear peculiar velocities at the
positions of these peaks. We test these ideas using large high resolution
N-body simulations carried out within the Virgo supercomputing consortium. We
find that at early times the barycentre of the material which ends up in a rich
cluster is generally very close to a high peak of the initial density field.
Furthermore the mean peculiar velocity of this material agrees well with the
linear value at the peak. The late-time growth of peculiar velocities is,
however, systematically underestimated by linear theory. At the time clusters
are identified we find their rms peculiar velocity to be about 40% larger than
predicted. Nonlinear effects are particularly important in superclusters. These
systematics must be borne in mind when using cluster peculiar velocities to
estimate the parameter combination .Comment: 8 pages, 4 figures; submitted to MNRA
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